Marbiolecol_2015

Marbiolecol_2015
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Marine Biology
Philippe Dubois
Master 1 Biologie des organismes et écologie ULB
Master 2 Bioingénieur: Sciences et technologie de l’environnement ULB
(option)
Master Biologie, Aquatic ecosystems VUB
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Introduction
What is marine biology?
Oceanography is a group of disciplines that share the study of the ocean. Physical and
chemical oceanographies are dealing with the abiotic marine environment, biological
oceanography and marine biology are more focused on the living organisms. However, the
latter deeply influence the chemistry of seawater.
Depending on the authors, biological oceanography and marine biology are more or less
equivalent. In general, biological oceanography considers the ecosystem or biosphere levels
of organization, is more global and integrated with chemical oceanography. Marine biology is
more focused on the individual and population levels of organization, dedicated to the marine
aspects of the biology of organisms.
It is noteworthy that marine biology is not a science on itself. It is the application of more
general aspects of biology to the life in the marine environment. That means that it may
include numerous aspects, namely:
- systematics and biogeography
- molecular and general biology
- physiology
- ecology
- ethology
This means that the division between marine biology and marine ecology is of low meaning
and that a significant part of this course will be dedicated to so-called marine ecology.
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Abiotic characteristics of the marine environment
and ocean circulation
1. Abiotic characteristics of the marine environment
1.1. Composition of sea water
Due to the asymmetry of electrical charges in water molecules, water is a polar solvent able to
dissolve numerous ions.
Composition of ocean water at 34.8 ‰
Ion
Concentration (g/kg SW)
(modified from Segar 1998)
% w/w of
ions
(modified from
Nybakken 1993)
H2O
ClNa+
SO4Mg2+
Ca2+
K+
965.31
Main constituents (>100 mg/kg)
19.51
10.81
2.73
55.35
30.61
7.68
1.29
0.41
0.38
3.69
1.16
1.10
Sub-total
99.28
Minor constituents (1-100 mg/kg)
HCO3BrH3BO3
Sr2+
0.142
0.41
0.067
0.025
0.008
0.19
0.07
0.04
Sub-total
Si
F
0.71
0.002
0.0013
Total
<0.01
<0.01
99.99
Trace elements (<1 mg/kg)
N
0.0005
<0.01
4
Li
Ru
P
I
Mo
Others (Fe. Zn. etc)
0.00016
0.00012
0.00006
0.00006
0.00001
<0.000001
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
The general concentration in dissolved ions is expressed by:
Salinity (‰) = weight of inorganic dissolved solids* / 1000 g SW
* when all carbonates are converted in oxides and all bromides and iodides are replaced by
chlorides
This is rather difficult to measure on a regular basis. For this reason a more easy relationship
was derived based on the constant ratio between the most abundant elements in sea water.
This constant ratio is due to the long residence time of these elements in sea water. This
differs according to the considered element. For Na for instance it is 260 millions y. For Fe
and Al it is in between 100 and 140 y. The constant character of the relative composition of
SW means that the measure of the concentration of a major constituent of SW allows to infer
the concentration other constituents and to calculate the salinity. The most abundant
constituent and easiest to measure is Cl-. So, the chlorinity is the concentration of Clexpressed in g/kg SW. As chloride ions accounts for 55% of the dissolved solids,
Salinity (‰) = 1.80655 x chlorinity
Salinity is now routinely measured by electrical conductivity of the analysed sample. Till the
80's the calibration was carried out with a standard sea water reference. Now for practical
reasons (availability of the standard) salinity is expressed as the ratio between the measured
conductivity of the sample and that of a standard solution of KCl chosen such as average sea
water salinity is 35PSU and corresponds to 35g salts/kg SW. As a consequence, salinity is
now a measure without units and is expressed in Practical Salinity Units (PSU).
Salinity in the ocean varies between 33 and 37 PSU. This depends mainly on the balance
between dilution by rainwater and concentration by evaporation. This results in latitudinal
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variations of the mean ocean salinity (see fig.). This general trend may of course be affected
by local processes like river plumes or melting ice. Marginal or semi-enclosed seas may show
enhanced disbalances between evaporation and precipitation or freshwater supply by river.
This is the reason why the Baltic Sea has a salinity of ca 5PSU while the Mediterranean
seawater may be as high as 38PSU.
1.2. Temperature
The hydrogen bonds of water have several consequences, namely:
- a high heat capacity
- high freezing and boiling points
- a high heat of vaporization
These result in a general moderating effect of the ocean on the continental climate.
At low latitudes the sea surface shows a net uptake of heat from solar origin while at high
latitude there is a net loss of heat (see fig). This results in a latitudinal gradient of sea surface
temperature which average 0°C in polar seas and are ≥ 25°C in tropical waters. At medium
latitude marked seasonal variations are observed. Due to the currents (see later), these
differences in sea surface water temperature will result in heat transfers between different
regions (see heading 2).
SW temperature is also varying according to depth. These vertical profiles will be considered
in the next section.
1.3. Density
Both temperature and salinity will influence the density of sea water. Differences in density
will in turn influence the movements of water masses (heading 2.2).
Due to hydrogen bonds, the spacing of water molecules in crystals is larger than that in liquid
water (see fig). As a consequence, ice is floating on water (an aspect of tremendous
importance in the development of life!). Furthermore, this results, in freshwater, in a
maximum density before the freezing point, actually at 4°C in pure water. On the contrary in
sea water whose salinity exceeds 24.7PSU, the maximum density of liquid water is at the
freezing point (see fig). Thus, in liquid sea water, the lower the temperature, the higher the
density.
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The density of seawater is usually included between 1.02300 and 1.03000 g/cm3 .To avoid the
use of numerous non significant ciphers, the value σt is frequently used:
σt = (dSW/dH20 – 1) . 1000
The mean density of sea water is 1.02567, that of σt is thus 25.67.
The relationship between temperature, salinity and density is not linear (see fig.): the same
temperature difference has a much higher effect on density at high temperature that at low
temperature. As a consequence, at low latitudes, the temperature will be the main factor
affecting density, while at high latitudes salinity will have a more pronounced effect.
Vertical density profiles differ according to the latitude (see fig). They principally depend on
temperature profiles except at high latitudes. At the Equator, a rather shallow layer (a few tens
of meters) of low density SW tops a layer where density brutally increases, forming the socalled pycnocline. This constitutes an important barrier against the mixing of superficial and
deep water masses. The pycnocline is principally due to a brutal change in temperature, the
so-called thermocline and to a lesser extent to a change in salinity (halocline). In polar
regions, the pycnocline is missing, the temperature being almost constant all the along the
water column. This results in a less stable water column, without physical barrier, allowing
small variations in salinity linked to ice melting and freezing to be the driving forces of
movements of water masses (see lesson on Antarctica). This will have a tremendous
importance on general water circulation (see heading 2.2).
2. Ocean circulation
2.1. Surface circulation
2.1.1. Surface currents
Surface currents are mostly wind-driven. The principal earth winds are:
- the trade winds blowing from the subtropical high pressures towards the equatorial low
pressures
- the westerlies blowing from the subtropical high pressures towards the low pressures located
at 60° N and S
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Due to the Coriolis effect, the trade winds are blowing from the NE (Northern Hemisphere) or
the SE (Southern Hemisphere) and the westerlies are blowing from the SW (N Hemisph) or
the NW (S Hemisph) (see fig.). Notice that trade winds are not blowing in a narrow band (ca
400km wide) near the Equator (the so-called doldrums, feared by sailors).
The Coriolis effect: any object on the earth's surface moving horizontally through a long
distance for a relatively long period of time will be observed to "turn" to the right in the N
Hemisphere and to the left in the S Hemisphere. It should be noted that this is an effect seen
by an immobile observer. An observer in space would not see any deflection. See:
http://www.mnhn.fr/mnhn/lop/seconde/fiches/Coriolis.html
http://www.mnhn.fr/mnhn/lop/seconde/fiches/Coriolis.mov .
The Coriolis effect is due to the fact that points at different latitude are not rotating at the
same linear speed: this is zero at the poles, 800km/h at 60°, 1400 at 30° and 1600 at the
Equator. So, the Coriolis effect is increasing with latitude and the crucial factor is the time
during which the particle is moving.
The trade winds will induce the movement of the intertropical surface water masses. This will
occur due to Ekman transport: a wind blowing over a water mass for a long time over a long
distance will induce the superficial water layer to move at 45° at the right (N Hemisph) from
the wind direction; this will in turn induce the movement of the immediately below layer at a
lower speed and in a direction deflected to the right and so on till a depth of ca. 100m (Ekman
spiral; see fig). The resulting overall movement of the water mass is at 90° from the direction
of the wind. The Ekman transport induced by the trade winds will cause the piling up of a
water dome at the right (left) side of the wind in the Northern (Southern) Hemisphere. This
piling up of water will in turn generate a pressure gradient in the opposite direction of the
Ekman transport. This results in the establishment of the so-called geostrophic equilibrium.
The results are that the water is flowing in the direction of the wind. Thus, the trade winds are
driving the equatorial currents flowing from E to W. When encountering continental masses,
the equatorial currents are deflected and taken back by the westerlies generating currents from
W to E (e.g. the Gulf Stream in the N Atlantic). The combination of equatorial and westerlies
driven currents results in large ocean gyres centred on 30° of latitude. These are turning
clockwise in the N Hemisph and anti-clockwise in the S. Hemisph. (see figs).
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The westwards movement of water caused by the equatorial currents also results in the piling
up of water on the W side of the ocean basins. This induces an eastwards pressure gradient
which will generate the "small" counter-equatorial currents at the level of the doldrums.
2.1.2. Up- and downwellings
Wind can also drive vertical movements of water in the surface layer (scale of a few tens to a
few hundreds of meters depth).
The geostrophic equilibrium generates a dome of water that makes water layers sink in
(downwelling). Two adjacent currents flowing in opposite direction will cause water to
diverge due to Ekman transport. This will cause an upwelling of deeper water to compensate
for the divergent water (e.g. equatorial or antarctic upwellings). If the wind is blowing parallel
to a coast, Ekman transport will cause water to either diverge from the coast (upwelling) or to
pile up towards the coast (downwelling). In the first case, deeper water from the continental
margin, charged with nutrients, will be brought to the surface. In the latter case, offshore
surface water, poor in nutrients, will move to the coast, making richer coastal waters sink (ex:
Peruvian upwelling, see fig.).
2.1.3. Langmuir circulation
In the centre of gyres, weak , constant winds cause corkscrew motion of the upper few meters
of water. This generates convection cells that result in accumulations of organic material and
living organisms in convergence zones (see fig.). Notice that the scale of this phenomenon is
radically different from the previous ones: we are dealing here with scales of a few meters in
depth.
(from Segar 1998)
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2.1.4. Effects of modifications of the surface circulation: El Nino Southern Oscillation
(ENSO)
The normal climate condition in the Indo-Pacific Ocean is characterized by well-established
trade winds generating a well-developed equatorial current. This pushes warm superficial
waters in the Indian Ocean over Australia and Indonesia, resulting in a low pressure (due to
evaporation of warm water). Concurrently, the subtropical gyre is well established, forming a
northward current along the South American coasts. This current generates an upwelling
along the Peruvian coast, bringing up cold water to the surface. This results in a high pressure
(due to the cold air) in the Eastern subtropical Pacific (see fig.).
Every 2 to 10 years, the trade winds relax, causing a weakening of the equatorial current. The
equatorial counter-current becomes dominant and the warm water masses (and the
accompanying low pressure) are pushed from the Indo-Pacific eastwards to South America
(see fig.). This phenomenon usually occurs around Christmas from which is name is derived
(el Nino in Spanish means the Christ). The oscillation refers to the changing value of the
difference in atmospheric pressure between the Indo-Pacific Ocean and South-America.
The consequences of El Nino are dramatic (see fig.):
- due to the weakening of the equatorial current and dominance of the counter-current, the
coastal current along South America is flowing southwards, causing a downwelling which
brings warm water to the Peruvian coast, killing many cold water organisms, and
tremendously reducing primary production (due to the lack of nutrients) which results in a
crash of the Peruvian anchovy fisheries;
- the low pressure on South America causes storms and massive rains generating floodings
over the W coast of S America and the S of the USA.
- the high pressure over the Indian Ocean causes severe droughts in Indonesia, Australia and
the Philippines but also in S Africa and Zimbabwe. Sometimes, these droughts are inducing
huge forest fires affecting the health of millions of people in SE Asia (this was the case in
1998).
The cost of El Nino in 1982 was assessed to 8 billions US $.
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An El Niño can have impacts on weather at various locations around the globe. Off the east
coast of southern Africa, drought conditions often occur while El Niño is in progress. In
countries such as Zimbabwe, whose economy is critically tied to maize production, the effects
of drought can be devastating. In western South America, farmers can benefit by planting
more rice rather than a normal crop of cotton during an El Niño as they are likely to
experience heavier than normal rainfall. (from http://nsipp.gsfc.nasa.gov/)
2.2. Thermohaline circulation
2.2.1 Mechanisms and conveyor belt
Thermohaline circulation is principally taking place below the pycnocline. It takes its origin
in differences of density of water masses. These differences result in the water masses going
up or down. In turn, this creates horizontal gradients of pressure (for instance when a sinking
mass encounters a denser water mass and is deflected horizontally). The thermohaline
circulation originates at high latitudes where the pycnocline is virtually absent. At these
latitudes, the surface water is cooled by the atmosphere that is making it denser and therefore
sinking (examples of these mechanisms will be studied in the lesson on Antarctica). The main
zones where deep water is forming are:
- the Weddell Sea where Bottom Antarctic Water forms
- the Norway Sea and Iceland Sea where North Atlantic Deep Water (NADW) forms
No deep water is formed in the North Pacific because a rather shallow sill, blocking the
movement of deep water, borders the arctic basin of the Pacific.
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The NADW is flowing southwards till 50°S where it mixing with Deep Antarctic Water and
turning towards the Indian Ocean and the Pacific Ocean. In both oceans the water masses then
flow northwards and towards the surface where they mix with local water masses and warm
up. Then, they enter the wind-driven circulation and the oceanic gyres, going back to the N
Atlantic. This huge "conveyor belt" results in a net transfer of heat from low latitudes to high
latitudes in the N Atlantic. In particular the Gulf Stream is bringing part of this heat towards
the NE (Europe) which is the reason why the N European climate is milder than the East coast
N American climate at the same latitudes.
(from Segar 1998)
2.2.2. Consequences of disruptions of the thermohaline circulation
Models show that if the formation of NADW stops, the air temperature in W Europe would be
reduced by 6°C (see fig). This already occurred in the past (13500 years BP) when the last Ice
Age abruptly stopped in ca. 100y. This caused a brutal thawing of ice shields and mountains
whose freshwater, flowing down the St Laurent River (presently in Canada), covered the NW
Atlantic with a much lighter layer preventing the formation of NADW. In an apparent
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paradox, this corresponded in N Europe with a cold period (despite the end of the Ice Age)
due to the disruption of the conveyor belt, which lasted for several hundreds of years (the
Younger Dryas). During the 20th century, in the 80's, the formation of NADW off Greenland
decreased by 90%. The cause is currently not well understood but could be linked to global
warming due to the increased concentration of greenhouse gas. If this is correct, global
warming could result in lower temperatures in NW Europe.
Changes in surface air temperature caused by a shutdown of North Atlantic Deep Water
(NADW) formation in a current ocean–atmosphere circulation model.
Note the hemispheric see-saw (Northern Hemisphere cools while the Southern Hemisphere warms) and the maximum
cooling over the northern Atlantic. In this particular model (HadCM3)7, the surface cooling resulting from switching off
NADW formation is up to 6 °C. It is further to the west compared with most models, which tend to put the maximum cooling
near Scandinavia. This probably depends on the exact location of deep-water formation (an aspect not well represented in
current coarse-resolution models) and on the sea-ice distribution in the models, as ice-margin shifts act to amplify the
cooling. The largest air temperature cooling is thus greater than the largest sea surface temperature (SST) cooling. The latter
is typically around 5 °C and roughly corresponds to the observed SST difference between the northern Atlantic and Pacific at
a given latitude. In most models, maximum air temperature cooling ranges from 6 °C to 11 °C in annual mean; the effect is
generally stronger in winter. (from Rahmstorf 2002)
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Pelagic biological processes
1. Divisions of the pelagic environment
The pelagic domain is spited in neritic (above the continental shelf) and ocean provinces
(above the shelf slope and abyssal plains).
It is also divided in zones according to available light and depth.
Depth (m) Zone
0
Epipelagic
Characteristics
- euphotic (available light is able to support
photosynthesis
- mixing zone
- surface water masses
- nutrient concentrations are low
- O2 concentration high
200
Mesopelagic
1000
Bathypelagic
- disphotic (available light is unable to support
photosynthesis
- pycnocline zone
- intermediate water masses
- O2 conc decreases towards a minimum at 7001000m due to bacterial activity
- nutrient conc. increase
- base of pycnocline
- aphotic
- deep water masses
- O2 conc increases (deep water is usually
originating from rapidly sinking surface water,
cf NADW and diffusion from bottom water of
similar origin)
- high nutrient conc.
4000
Abyssopelagic
- aphotic
- bottom water masses (originating at high
latitude)
- high O2 conc.
- high nutrient conc.
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2. Biological processes in the pelagic domain
2.1. Definitions
Pelagos: Organisms living in the water column without any contact with the bottom and
which do not depend on the benthos for feeding
Plankton: pelagic organisms unable to swim against the currents
Nekton: pelagic organisms able to swim against the currents
Tripton: Particulate Organic Matter (POM)
Seston: plankton + tripton (i.e. material retained on a 0.45 or 0.22µm membrane filter)
Functional classification of the plankton
Classically the plankton is divided in phyto- and zooplankton, i.e. in autotroph and
heterotroph planktonts. However, there is no clear-cut barrier between these two groups for
several reasons:
- mixoplankton: these are planktonts that are principally autotroph but can be heterotroph or
vice versa. Autotrophs (like some dinoflagellates) can be phagotrophic (mainly by
ingestion of bacteria) or osmotrophic (absorption of dissolved compounds to get extra
nutrients or vitamins). Heterotrophs (e.g. ciliates and amoeba) may either trap
functional chloroplasts from their preys or include endosymbiotic algae
- importance of the bacterioplankton: this can be the main heterotrophic component in some
communities but it also includes several autotrophic groups (e.g. cyanobacteria).
- there is no systematic divide between the phyto- and the zooplankton: most groups include
autotroph and heterotroph planktonts
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Classification of the plankton according to size
Ultraplankton
Nanoplankton
Microplankton
Macroplankton
Megaloplankton
< 2µm
2 – 20 µm
20 – 200 µm
200 – 2000 µm
> 2000 µm
Mesoplankton
200 – 20000 µm
1000 – 5000 µm
(according to authors)
2.2. Primary production
2.2.1. Limiting factors
Light and mixing
Photosynthesis is related to light intensity (see fig). This relationship is characterized by a
first linear increasing phase corresponding to limiting light intensities. This phase is possibly
followed by a plateau that corresponds to the saturating light intensity. The latter is followed
by a decreasing phase due to the deleterious effects of high light intensities (especially UV),
increased respiration (inducing a reduced net production), and increased leakage of organic
molecules by the cell (also reducing the net production).
This relationship between light intensity and photosynthesis is qualitatively the same for most
planktonts but quantitatively differs according to the considered group or species (see fig);
therefore, according to light regime, different groups will be favoured (e.g. diatoms against
flagellates).
As water absorbs light, light intensity across the water column follows an inverse exponential
profile with depth (see fig):
Iz = Io e-kz
where k is the extinction coefficient and z depth.
This will induce a very general relationship between photosynthesis and depth:
- photoinhibition will occur at the very surface (a few meters; inhibition threshold is usually
170Wm-2)
- except in this superficial layer, light is limiting (saturation level: 120Wm-2) and
photosynthesis follows an inverse exponential relationship with depth.
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This allows to use a very general formula to calculate the net primary production per surface
unit of the ocean:
ΣiP = ni Pmaxi d
where ΣiP is the net primary production of the population of the ith organism, ni is the density
of this population, Pmaxi is the photosynthetic saturation rate of the ith organism and d is the
depth at which the intensity of the most penetrating radiation is reduced to 10% of the surface
value. This relation makes it usual to express the plankton primary production per surface
unit.
Respiration is independent of depth. Therefore, there is a depth at which the respiration of the
considered primary producer is equal to its gross photosynthesis. This depth is the
compensation depth. Of course, it varies according to the considered organism. When the
primary producer is above its compensation depth, its net primary production is positive.
When it is below, it needs to use stocked energy to survive.
Now, according to mixing conditions determined by hydrodynamism and wind, the primary
producer community is spending different amounts of time above and below the respective
compensation depths of the different species. The depth at which the gross primary
production of the whole community is equal to the total respiration of this community is
called the critical depth. If the mixing depth (i.e. the depth above which the water column is
mixed) is shallower than the critical depth, the net primary production of the community will
be positive. If it is deeper, the net primary production will be negative and the primary
producer community will be reduced.
Nutrients
Three types of nutrients can be recognized:
- Major nutrients (building blocks of the phytoplanktonts): C, N, P, O, Si, Mg, K, Ca
- Trace nutrients: Fe, Cu, V, (Cd)
- Organic nutrients: vitamins
The uptake of nutrients may be described by a Michaelis-Menten type equation (notice that
this is just a mathematical description, it is not implying that the uptake of nutrients is
depending on enzyme kinetics following Michaelis-Menten processes) (see fig):
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V = Vmax . C / Ks + C
Where Vmax is the velocity at saturation, C is the concentration of the nutrient in SW and Ks
is a constant corresponding to the concentration of nutrient at which V = Vmax/2.
Thus Ks is a measure of the nutrient concentration necessary to reach half the maximum
uptake rate. Primary producers with a low Ks will be favoured in oligotrophic waters (because
they reach high uptake rate at lower nutrient concentrations) while those with a high Ks will
be more efficient in eutrophic waters (because they will take up higher concentrations of
nutrients).
Ks is varying according to the species and groups. As a very general rule (with several
exceptions!), flagellates show lower Ks than diatoms. This is in part due to their higher
surface/volume ratio: smaller cells are taking up (on a weight basis) nutrients faster because
their absorption surface is proportionally higher (see fig.).
For all these reasons, nutrient availability will be a major factor controlling the composition
of the primary producer community.
Four nutrients have been demonstrated to be possibly limiting for the marine primary
producers: nitrogen, phosphorus, silicon, and iron.
Nitrogen and phosphorus were demonstrated by enrichment experiments to be limiting in
numerous locations (see figs). Usually, N is the primary limiting factor while P is only
secondarily limiting (once N limitation is removed; like for instance in eutrophicated waters
as in the Southern North Sea).
Nitrogen is necessary for protein synthesis. It occurs in the sea under 3 principal inorganic
forms: NH4+, NO3-, NO2-.The ammonium ion is the most favourable for primary producers
because it does not need reduction prior to its incorporation in proteins. Nitrate and nitrite
need a previous reduction by nitrate reductase. However, most dissolved N in the sea is
nitrate, at concentrations usually around 1µM, rarely >25µM. It is noteworthy that N is much
less concentrated in SW than in freshwater (see fig.) where P is usually the limiting nutrient
for primary producers. There are three main sources of N in the marine environment:
(1) sediments and deep waters: usually as nitrate; the primary production based on these
sources is called "new production" because it is based on N coming from outside the
considered water mass
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(2) excretion and decomposition from or by consumers: usually producing ammonium; the
primary production based on these sources is called "regenerated production" because it is
based on N coming from inside the considered water mass
(3) fixation of atmospheric N2: carried out by yeasts and principally by cyanobacteria, some
of them being symbionts of diatoms in oligotrophic waters; fixation of gaseous nitrogen may
account for up to 20% of N supply in some zones
Phosphorus is necessary in energy transfer processes (ATP), in the phosphorylation of
enzymes, and in the synthesis of nucleic acids. In the ocean it is present as dissolved
inorganic phosphate, dissolved organic phosphate, and particulate phosphorous. Phosphate is
the most favourable. It is usually very quickly recycled.
Most primary producers require a rather constant ratio C/N/P of 106/16/1 that is called the
Redfield ratio (according to the name of Redfield who described it for the first time).
Departures from this ratio often indicate the limitation by a nutrient.
Silicate is the main building block of diatom shells. Its depletion may terminate diatom
blooms although limitation by other factors is often involved.
Iron
is a prominent part of ferredoxin, which is involved in electron transfer from
photosystem I to NADP+). Its concentration in oceanic waters is usually below 1nM but it is
enriched (1-3nM) in neritic waters because its main sources are continental either from river
or airborne. Iron is limiting in some oceanic waters where high concentrations of other
nutrients are present. These zones were called High Nitrate Low Chlorophyll (HNLC) zones.
Limitation by Fe in these zones was demonstrated by elegant mesoscale enrichment
experiments in the Equatorial Pacific and in the Southern Ocean. In all these experiments, the
Fe enrichment induced an important increase in production and biomass (up to 27 times for
chlorophyll a, a measure of biomass) going together with a depletion of nitrate (see figs). This
clearly indicates that Fe is the factor limiting the use of N by primary producers. The principal
group that respond to the enrichment was diatoms. Limitation by Fe is further evidenced by
measures up- and downstream islands (Galapagos) or capes (Drake passage). Around the
Galapagos for instance, Fe and chlorophyll a concentrations up- and downstream the islands
are, respectively 0.06nM Fe, 0.25µg chla/l and 1.3 to 3nM Fe, 0.7µg chla/l.
HNLC ecosystems were recognized in offshore upwelling zones(Equatorial Pacific, Antarctic
circumpolar current -see lesson on the Antarctic Ocean- but also in coastal Southern
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California where supply of Fe is very low due to the very dry climate resulting in very low
river discharges.
Grazing
Grazing is able to control primary production as evidenced by respective spatial distributions
of producers and grazers (see fig.). However, this control may be of very different magnitudes
(see table). This is due to differences in generation times between primary producers (hours,
days) and grazers (weeks). Food preferences of grazers depending on size or chemical
composition of producers are also important. In particular they may modify the producer
community by systematically eliminating a given size class. It is noteworthy that the
interaction between primary producers and grazers is for a part mutualistic. Indeed, grazing is
bringing back nutrients in the environment, which are the base for regenerated primary
production. This is particularly important in regions where nutrient supply is poor.
2.2.2. Variations of primary production in space and time
Spatial variations
Large scale variations in primary productions (see fig.) are principally linked to hydrographic
processes. High production is recorded in coastal upwellings where nutrient-rich waters are
brought to the surface (Peru, NW and SW Africa, E India, W coast of N America). Coastal
zones with large nutrient supply of land origin and with high mixing conditions also support
high productions (e.g. the Southern North Sea). Offshore upwellings (equatorial and antarctic
divergences) bring superficial waters to the surface, allowing the recycling of nutrients and
support medium productions; however, these are often limited by iron. Eddies ensure mixing
of the water column, supporting a medium to low regenerated production. On the contrary,
the centre of oceanic gyres is the place of a downwelling with a permanent pycnocline so that
recycling of nutrients is very low and primary production is among the lowest of the oceans.
Finally, the primary production in the Arctic Ocean is limited by the low light experienced by
this region of the world. Actually this is the only region, on a global scale, where production
is limited by light. In all other regions large scale primary production is limited by nutrient
supply depending on hydrographic conditions.
20
The map displays the composite of all Nimbus-7 Coastal Zone Color Scanner data acquired
between November 1978 and June 1986. Approximately 66,000 individual 2 minutes scenes
were processed to produce this image.
(From http://seawifs.gsfc.nasa.gov/SEAWIFS/CZCS_DATA/global_full.html)
At small scale a patchy distribution of primary production is evidenced due to local
phenomenons like Langmuir circulation, grazing (see above) or reproduction (daughter cells
are necessarily close to each other, so blooms are localized).
Seasonal variations
Seasonal variations of primary production differ according to latitude and oceans.
In the boreo-temperate North-Atlantic, the seasonal succession is particularly outstanding (see
fig.). In winter, the water column is not stratified and the mixing depth is deeper than the
critical depth. Primary production is limited by light and nutrient concentrations in water are
high. In spring, temperature increases and the water column becomes stratified. Mixing depth
21
becomes shallower and light increases. As a consequence, the primary production increases
exponentially and a spring bloom, usually of diatoms develops. As a result, nutrient
concentrations (mainly nitrate and silicate) dramatically decrease which eventually terminates
the bloom. Due to their heavy shell, dead diatoms rapidly sink and are exported below the
thermocline. Simultaneously, the consumer biomass increases, being limited by the
termination of the bloom. In summer, the water column is totally stratified and the water layer
above the thermocline is poor in nutrients (which were exported below the latter by sinking
diatoms). Primary production is based on nutrients recycled by heterotrophs (regenerated
production). Primary producers are principally nanoflagellates controlled by grazers. The
system is limited by nutrients. At the end of the summer and in the fall, the storms break the
stratification allowing a mixing of the water column bringing up nutrients. A small bloom
develops, immediately controlled by grazers. Due to a rather low concentration in silicate and
still reduced nutrient concentrations, this bloom is dominated by flagellates. The decreasing
light eventually terminates this bloom.
In polar regions, a single short bloom develops in summer, followed by a peak in consumer
biomass. The system is principally limited by light. In tropical regions, no seasonality occurs;
primary production is low all around the year due to a permanent thermocline and controlled
by grazers. In boreo-temperate North Pacific, seasonality is, surprisingly, not the dominant
factor. The dominant grazers (copepods of the genus Neocalanus) "anticipate" the bloom:
they reproduce before the bloom using food reserves and prevents the development of the
bloom.
2.3. Consumers
2.3.1. The microbial loop
Until the mid-70's, the typical pelagic food chain was considered to be:
Dinoflagellates + diatoms -> copepods ->herring -> mackerel ->tuna.
(the so-called "linear chain")
That means that considered primary producers were only those retained on plankton nets
(therefore called "net plankton"). The development of membrane filters (Millipore) allowing
to filter much smaller organisms in the mid-70's allowed to realize that more than half of the
22
world primary production was actually the fact of nanoflagellates (2-10µm in diameter) and
cyanobacteria. At the same period, the marketing of DAPI, a fluorescent dye of DNA allowed
to directly count bacteria in sea water instead of tedious culturing on Agar plates which only
reveal 1-10% of the bacteria present in the sample. This reveals bacteria concentrations of
1million/ml, accounting for 200µgC/l, meaning that bacterial biomass may equal the biomass
of primary producers. Eighty to ninety % of these bacteria are free in the water column. Their
secondary production was measured to range between 0-500 µgC/l. day in coastal waters and
0.5 – 5 µg C /l day in oceanic waters. This corresponds to 5-30% of the primary production in
the same waters. Taking into account a maximum efficiency of 0.5 for bacteria, this means
that 10 to 60% of the primary production may be consumed by bacteria. These consumed
dissolved organic matter leaked by primary producers (this may account for as much as 30%
of the primary production) as well as fragments (particulate organic matter) of dead or preyed
primary producers (generated by sloppy feeding of macrograzers).
Bacteria populations are limited by several factors: availability of food (in spring, when
nutrients are available), nutrients (bacteria are limited in summer by N and P; see fig.) and
predation, mainly by heterotrophic nanoflagellates. The latter was evidenced in incubations of
pelagic microbial communities where micro- and macroplankton components were removed
(to avoid control by higher levels). In these incubations, oscillations of bacteria and
heterotrophic nanoplankton population sizes, typical of predator-prey interactions were
observed (see fig). When the nanoplankton was also removed, bacteria showed first an
exponential increase in population size, which later reached a plateau due to limitation in food
and/or nutrients. These oscillations are at a short-term scale for two reasons. First the
generation time of the nanoplankton is of the same order of magnitude as that of bacteria (324h vs. 6h for bacteria). Second, the heterotrophic nanoplankton has a very high clearing rate.
It is able to clear bacteria of 105 times their volume every hour. In coastal waters, one
considers that the heterotrophic nanoplankton filters the whole water column every day.
Nanoflagellates are in turn controlled by ciliates, which usually prevents the occurrence in the
field of the oscillations in population sizes observed during incubation experiments.
These different discoveries led to a new view of the pelagic food chain including bacteria and
the nanoplankton (see fig). This added a whole network to the linear chain, called the
"microbial loop".
The occurrence of the microbial loop has several implications in our understanding of pelagic
food networks.
23
(1) Much more trophic levels are involved than previously assumed; this means that the
energy that was believed to be available for the linear food was overestimated. Indeed the
energy available for the next trophic level is equal to:
P = B En
where P is the production of trophic level n, B is the annual primary production of the
ecosystem, E is the ecological efficiency, i.e. the ratio between the energy effectively
incorporated in growth by level n and the energy ingested by level n, and n is the number of
trophic levels calculated from primary producers.
Usually E equals 10-50%, meaning that the highest n the lowest P. Actually, in most pelagic
ecosystems, the main part of chemical energy and carbon is dissipated in the microbial loop
(this is the reason why it is called a loop, as carbon and energy are cycling inside this
compartment of the ecosystem). Therefore, the transfer of C between the microbial loop and
the linear food chain is pretty low or even not significant in some cases. Only the primary
production from microplankton (mainly diatoms) is available in significant amount to the
linear food chain.
(2) Bacteria were, for a long time, considered as the main recyclers of nutrients. Actually the
bacterial biomass is a sink for nutrients (as illustrated by their C:N ratio; see table). However,
they are very important in the modification of nutrient redox potential, being involved in their
reduction. Now, protozoans and other nano and microconsumers are considered as the main
actors of nutrient regeneration. Zooplanktonts excrete 2-10% of their N charge and 5-25% of
their P charge every day, ensuring a rapid recycling of nutrients in the water column (turnover
of phosphate in temperate waters is 1.5 day, a little bit more fro N).
(3) In the microbial loop, organisms of different size classes (up to three orders of magnitude)
occur at the same trophic level. This means that the classical separation of trophic levels
according to size is no more valid, making calculation of the energy available for the next
trophic level difficult. This also implies that there is no simple trophic chain but that a trophic
network has to be considered.
The importance of the microbial loop was further emphasized by the assessment, in the 90's,
of the importance of virus in the water column. Surface waters count 106-107 up to 108 (in
coastal waters) virus particles/ml. These are responsible for 8-26% of bacterial mortality and
may terminate some blooms like those of coccolithophorids.
24
2.3.2 Linear food chain
Plankton
Pelagic food webs are relatively "unstructured" (if compared with benthic ecosystems) due to
several factors:
- the complexity of trophic relationships in the water column (see above)
- most predaceous species are themselves controlled by their predators; so, no predator may
become dominant
- "gelatinous" species (jellyfishes and salps –planktonic tunicates-) may control in some
ecosystems but not in other most zooplankton preys, consuming 10-59% of the zooplankton
daily (Nova Scotia)
- some planktonts show a density-dependent control of abundance in some instances but not
in other (see fig.).
It results from these factors that large differences occur between ecosystems in the prevalence
of carnivorous or herbivorous species (see table).
Nekton
Most data deal with fishes, cetaceans, and birds.
Teleostean fishes are the more studied due to the economical importance of fisheries. Most
Teleosteans are opportunist species that follow an r strategy. They produce huge numbers of
small-sized planktonic larvae (≤ 1 mm). These larvae are submitted to heavy predation by
numerous trophic levels during their development and are dependent on currents. The survival
till maturity is very low (10/million). This survival is generally density-dependent, indicating
food limitation in larvae (bottom-up control) (see fig). This is combined with very variable
recruitments into the adult population according to years (in most cases this is controlled by
hydrodynamic factors). Interestingly, both the recruitment success and the growth rate of
juveniles are independent of adult population density indicating that there is probably no
food-limitation in adult populations (see fig.). This also indicates a disconnection between
larval and adult ecologies. Actually, teleostean fishes are mainly controlled by predators (topdown control) among which man is the most important for numerous species.
Birds and mammals
Contrary to teleosteans, birds and mammals follow a K strategy. They produce few offspring,
take care of youngsters, in some case for protracted periods of time, which reduces juvenile
25
mortality, age at first reproduction is delayed, and life is usually long. Their role in the
structure of pelagic communities is rather poorly known. However, the effects of whaling in
the Antarctic Ocean provide a full-scale experiment. Industrial whaling lasted from 1920 till
1970, until most whale populations crashed. This resulted, in Antarctica in a reduction of prey
(mainly krill) consumption by whales of 75%. Interestingly, the four krill-eating penguin
species increased in parallel to the reduction of whale populations (see table). Similarly, the
population of two seal species also increased. This strongly suggests that these species were
limited by food through competition with whales (bottom-up control).
Further evidence indicates that whale themselves were competing for food. The modal size of
fin whales landed just after the Second World War was smaller than those catched later (see
fig). The arrest of whaling during the war allowed an increase in the fin whale population,
which induced an increased competition for food and a reduced growth.
These data show that pelagic mammals and birds are controlled by bottom-up mechanisms.
This suggests that they have a strong influence on the structure of the communities to which
belong their preys. As they are K species, they are particularly prone to overexploitation by
predators, in this case mainly man. This anthropic overexploitation is so important that it is,
from the ecological point of view, very difficult to separate the impact of fisheries from other
control mechanisms.
26
Case study: the Antarctic Ocean
1. Physico-chemical environment
1.1. Water masses and circulation
The Antarctic continent has a surface of 14 millions km2. It is surrounded by a continental
shelf which is four times deeper (500-900m) than usual, due to isostasic subsidence caused by
the weight of the ice that caps most of the continent (24.1015 T).This shelf is also narrower
(30-200 km) than usual (with the exception of Ross and Weddell Seas). These characteristics
favour water exchange between offshore and coastal zones. The basins are also usually deeper
(≥ 3000m).
The Antarctic Ocean totally encircles the continent, which ensures some homogeneity. Two
main currents flow around Antarctica (see fig.). The East Wind Drift is located between the
continent and 60°S; it is generated by polar east winds and flows anti-clockwise around the
continent. The east Wind Drift is not continuous: it is mainly developed east from the
Weddell Sea and at the level of the Ross Sea. The West Wind Drift or Antarctic Circumpolar
Current (ACC) is located between 60 and 40°S. It is generated by westerlies and flows
clockwise. ACC is the main circulation system of Antarctic water masses and the largest
current in the world (flow rate is 130 millions m3/sec). The two currents flowing in opposite
directions generate a divergence, the so-called Antarctic divergence that results in an oceanic
upwelling. Upwelled water is the Circumpolar Deep Water (CDP) that arises from the North
Atlantic deep water through the conveyor belt. CDP has a high salinity (>34.7PSU, a rather
high temperature (1.6 – 2.5°C) and is poor in oxygen (4-5ml/l). CDW will mix with Ice Shelf
Water (ISW) on the one hand and with Antarctic Surface Water (ASW). ISW is a coastal cold
water mass which, once mixed with CDW, will sink and form the Antarctic Bottom Water
(ABW), whose salinity is 34.65PSU and temperature –0.9°C. This is one of the main sources
of bottom water in the global ocean. Its signature can be recognized as far as 5°S (see fig).
Winter ASW, formed during the freezing of ice has a rather low salinity (34.2 – 34.5 PSU)
and a very low temperature (-1.8°C). Summer ASW, formed during ice melting has,
therefore, a lower salinity (<34 PSU). As a consequence, winter ASW flows below summer
ASW, at depths between 50 and 300m.
27
Between 50 and 60°S, "warm" Subantarctic Surface Water (SSW) meets ASW. The latter, 2
to 4°C colder sinks and forms the Antarctic Intermediate Water (AIW) of rather low salinity
(34.2PSU) and rich in oxygen (5-7ml/l). AIW can be recognized until 20°N. The zone where
the two water masses (SSW and ASW) meet is the so-called Antarctic convergence or polar
front. This corresponds to the northern limit of the Antarctic Ocean. The area between the
Antarctic and Subtropical (40°S) convergences is the Subantarctic zone. The Southern Ocean
extends from the Subtropical convergence to the Antarctic continent. (Notice that Englishspeaking authors no more speak o the Antarctic ocean but only of the Southern Ocean despite
the fact that the polar front is a true hydrological and biological barrier, especially for
plankton.
1.2. Light
Antarctica experiences a particularly marked day length cycle. At 75°S for instance, there are
100 days of continuous night (see fig). A second characteristic very influential on light
availability in the water column is the extent of sae ice. This is maximum from June to
September, covering 56% of he Southern Ocean and minimum from January to March
(covering 17.5% of the Southern Ocean) (see fig). Of course, it directly influences available
photosynthetic active radiations (PAR) (see fig.).
28
2. Primary production
2.1. Producers
The main primary producers of the Antarctic Ocean of the nanophytoplankton are flagellates
(Prasinophyceae, Cryptophyceae, Prymnesiophyceae, Cryptomonas being the most abundant
genus). Among microphytoplanktonts, centric diatoms (Bacillariophyceae) of the genus
Corethron, Thalassiosira, Rhizosolenia, and Fragilariopsis are the most frequent together with
Phaeocystis colonies (Prymnesiophyceae) (see figs).
2.2. Factors controlling primary production
2.2.1. Light
Due to the photoperiod and ice cover, primary production only occurs in the spring and
summer with a single peak of production (see fig.). If ice cover is higher than 20%, no bloom
occurs and nanophytoplanktonic communities develop.
2.2.2. Wind
Strong winds are frequent over the Antarctic Ocean due to its continuity (winds may "turn"
around Antarctica without continent to stop them). They are also fluctuating a lot over time
scales of weeks. They directly influence the mixing depth (see fig.). If wind speed is higher
than 8m/sec no bloom develops and nanophytoplanktonic communities are favoured.
2.2.3. Nutrients
Macronutrients
Due to the Antarctic divergence, the Antarctic Ocean surface waters are rich in nitrate
(32.5µM), phosphate (2.5µM) and silicate (100µM) (see fig for comparison with other
regions of the world).
29
Micronutrients
On the contrary, iron is generally present at low concentration (<1nM) except in neritic zones
(Weddell and Ross Seas), in the plume of Drake passage, and in the marginal ice zones (see
table). This prevents diatom blooming in most regions of the Antarctic Ocean: these
microphytoplanktonts require 2nM dissolved Fe to bloom (see fig). The shortage in Fe is the
reason for the offshore Antarctic Ocean being a HNLC ecosystem. This was clearly
demonstrated by mesoscale enrichment experiments (over 200km2) that brought dissolved Fe
to concentrations of 1-3nM for a transient period. This enrichment resulted in a first response
of the system (increased photosynthetic competency) after 24h and in increases of biomass
(x6) and primary production (x3) after 3 to 4 days (see fig). Picoeucaryotes and
prymnesiophytes were the first to increase but a shift in the community occurred after 6 days
when the diatom Fragilariopsis kerguelensis bloomed.
Due to this limitation in iron, primary production in the Antarctic Ocean is usually below
1gC/m2.day (mean: 0.3 gC/m2.day). In comparison the Peruvian upwelling shows production
of 2.7 gC/m2.day. Based on available nitrate and without Fe limitation, primary production in
the Antarctic Ocean could reach 2.2 gC/m2.day.
3. Consumers
3.1. The microbial loop
The microbial loop is, as usual, initiated by autotrophic nanoflagellates consumed by
microflagellates and whose dissolved and particulate organic material is used by bacteria,
themselves consumed by heterotrophic nanoflagellates which are also consumed by
microflagellates (see fig). The microflagellates control the production of autotrophic
nanoflagellates preventing them to bloom.
The budget of the marginal ice zone (integrated on the 70 days of biological activity) show
that 88% of the net primary production (29 gC/m2) is assimilated by the microbial loop (see
fig). The net secondary production of the microbial loop is 8 gC/m2 (which corresponds to an
efficiency of 25%) and the primary production not ingested by the microbial loop is 4 gC/m2.
So, a maximum of 12 gC/m2 is available for the linear food chain. This means that the
possible export of this system is low. This is even more pronounced in offshore waters where
production available for the linear food chain does not exceed 8.5 gC/m2.
30
Thus, conditions favouring the nanophytoplanktonic communities will favour the microbial
loop which in turn control these communities, preventing their blooming.
3.2. The linear food chain
The linear food chain is initiated by microphytoplankton blooms whose main consumer is
krill (see fig.).
3.2.1. Krill
The most abundant species is Euphausia superba. It belongs to the class Malacostraceans,
order Euphausiacea. Adults are 6.5 cm long and weight ca. 1gFW. Its life span is 4 to 7 years.
E.superba is able to swim at ca 1km/h, which makes it able of autonomous movements in the
water column. Krill is thus at the limit between plankton and nekton. They form swarms
which may reach several millions tons. Krill shows an aggregative distribution that
corresponds to zones of microphytoplankton blooms (see fig.) This distribution makes it
difficult to establish budgets for this species. Knox tentatively estimated total krill biomass in
the Antarctic Ocean to be in the order of 108 T. Taking 500 106 T as mean estimate, and a
supposed production-biomass ratio between 0.8 and 2.77, he reached an estimated secondary
production for krill of 400 to 1385 106 T/year.
The gregarious behaviour of krill allows them to consume rapidly microphytoplankton
blooms and to actually terminate them (see fig). As a consequence, these blooms are most
often short-lived. Swarm grazing eradicates the whole microplankton community.
Microphytoplankton does not recover as iron is exported together with the grazed algae.
Microplanktonic heterotrophs are also seriously depleted by krill. Therefore, the
nanophytoplankton is able to bloom before being progressively controlled by the slowly
recovering microprotozoans (see fig.).
Thus, microphytoplankton may bloom in iron-rich zones if light and mixing conditions are
appropriate. These blooms are usually short-lived due to grazing by krill swarms. After such
grazing, a nanophytoplankton bloom may develop until it is eventually controlled by grazing
microplanktonts.
31
Controls on primary production in the Antarctic Ocean
Ice cover
> 20%
< 20%
Mixing conditions
High
Low
< 1nM
> 2nM
Fe concentration
Nanoplankton community
controlled by fast-growing
microprotozoans
Microbial
food web
krill swarm
Diatom bloom
Linear food chain
32
3.2.2. Higher rank consumers
Krill is the main prey of most higher rank consumers. Therefore, it is the principal link
between primary producers and other consumers, playing a pivotal role in the Antarctic
Ocean ecosystem. Krill consumers include cetaceans, seals, cephalopods, birds, and fishes.
Krill consumption (106 T/year)
Taxa
Lower estimate
Cetaceans (baleen whales)
Higher estimate
34
43
Lobodon
64
129
Cephalopods (principally squids of the
30
50
25
50
10 ?
20 ?
163
292
Seals
(crabeater
seal
carcinophagus)
order Oegopsidea)
Birds (penguins accounting for 90% of
the biomass of and 86% of the food
consumed by Antarctic birds)
Fishes
(Champsocephalus gunnari
Notothenia rossii)
Total
Using Knox's minimum and maximum estimates of krill production (see 3.2.1), consumption
of krill ranges between 11. 8% (163/1385) and 73% (292/400) of the annual krill production.
This indicates that these consumers are possibly food-limited (bottom-up control). This is
further emphasized by the estimated krill consumption by baleen whales before industrial
whaling: 190 106 T/year, meaning 13.7 to 47.5% of the krill production.
33
Benthic biological processes
1. Divisions of the benthic environment
The benthic environment is divided into zones according to their position on the shore
(intertidal zones), available light and depth.
Tide level or depth
Zone
Supralittoral
Characteristics
Saline moistening
Continuous emersion except at
extreme high waters of spring tide
Mesolittoral
Daily cycles of immersion and
emersion
Infralittoral
Continuous immersion except at
low
waters of spring tide
Mean high water of spring tide
Mean low water of neap tide
Compensation depth of seagrasses
or photophilic algae
15-20m at high latitudes
30-40 m Mediterranean
80 m intertropical regions
Circalittoral
Compensation depth of the algae
tolerating the lowest light
intensities (150-200m)
Bathyal
Continental slope and its foothills
Abyssal
Abyssal plains
Hydrothermal vents
Hadal
Deep trenches
2500-3000m
6000-6500m
34
2. Primary producers
2.1. Main taxa
Domain Bacteria
Cyanobacteria make multicellular assemblages. They are able to fix atmospheric nitrogen if
they are in anaerobic conditions. Therefore, they build multicellular mats, often on mudflats
(but these also occur on rocky substrates) whose interstitial water is anoxic. They can be
symbionts of marine angiosperm rhizomes (which benefit from the N2 fixed by the bacteria.
Main pigments include chlorophyll a (absorbs red), phycocyanin (absorbs blue) and
phycoerythrin (absorbs green).
Chemosynthetic bacteria do not obtain their energy from light but from reduced substrates,
originally fixed by photosynthesis; therefore this is usually regarded as regenerated
production. It is noteworthy that these bacteria need both reduced compounds and oxygen for
their metabolism. For this reason they often live at the boundary between oxic and anoxic
zones. A particular case is the chemosynthetic bacteria from hydrothermal vents that are using
reduced sulphur compounds of geothermal origin; this results in true new production.
Domain Eukarya
Reign Chromista (chlorophyll a and c)
The microphytobenthos principally includes pennate diatoms (Ph Heterokonta, Cl
Bacillariophyceae) that are often mixotrophic. They form algal mats on mudflats.
The Phaeophyceae (Ph Heterokonta; brown algae) contain fucoxanthi, xantophyll and
carotene (the last two groups of pigments being responsible for their colour). They absorb
green and yellow. They are the dominant algae of intertidal zones and rocky infralittoral,
whatever the latitude.
Reign Protozoa (chlorophyll a and c)
Coral reefs include endosymbiotic zooxanthellae (Ph Dinophyta, genus Symbiodinium) that
make them important primary producers.
35
Reign Plantae (chlorophyll a and b)
The Chlorophyta (green algae) absorb red and blue. They generally have a low capacity for
storing nutrients. For this reason, they mainly occur in nutrient-rich habitats, in particular
eutrophicated areas.
The Rhodophyta (red algae) contain chlorophyll d, phycoerythrin (responsible for their red
colour, absorbs green), and phycocyanin (absorbs blue)
The marine angiosperms (Ph. Spermatophyta; flowering plants) (see fig.) are generally
encountered on soft substrates. They include seagrasses which are living submerged most of
the time (genus Zostera, Posidonia, Thalassia), saltmarsh plants (Spartina, Salicornia) and
mangrove trees that replace saltmarsh plants in sheltered habitat with soft substrate in tropical
environments (Avicennia, Rhizophora).
Notice that macroalgae belong to very different taxonomic groups!
2.2. Factors controlling benthic primary production
2.2.1. Light
As for pelagic primary producers, the production of benthic primary producers is a function of
light intensity (see fig.), including photoinhibition at high intensities. However, the latter is
less pronounced that for pelagic producers due to shading by fronds or leaf tissues. Due to
light absorption by water (and possibly particles), benthic autotrophs are limited in depth as
pelagic producers. However, due to their sessile character, their compensation depth is the
same as the critical depth.
Different light wavelengths are not absorbed by water in the same way (see fig.). As different
taxa do not contain the same pigments, their vertical distribution will differ. Seagrasses and
green algae will be limited to shallow depths. On the contrary, red algae that are able to
absorb blue and green lights and will be the deepest encountered algae. Moreover, their
pigments are adapted to low light intensities.
2.2.2. Nutrients
Algae are limited by nutrients, especially nitrogen, when these are low in the water column
as demonstrated by enrichment experiments(see figs). Angiosperms have roots and are
therefore able to take up nutrients from the interstitial water of sediments where their
concentrations are usually high. However in some periods of the year uptake of nutrients by
36
competing primary producers (microphytobenthos, epiphytes) may render these limiting (see
course on the Posidonia ecosystem). Also, if sediments are anoxic, transport mechanisms are
inhibited by sulphide and nutrients may become unavailable and therefore limiting.
2.2.3. Emersion
In the intertidal zone, emersion results in desiccation, warming and salinity fluctuations, all
factors that affect production and vertical zonation. These aspects will not be addressed in the
present course.
2.2.4. Substrate
The nature of substrate (soft vs. rocky) will influence the primary producers that will develop.
Macroalgae are more frequent on rocky substrates on which they settle thanks to holdfasts
while most angiosperms are rooted in soft substrates.
Notice that there is no clear-cut difference between soft and rocky substrates. The functional
limit is linked to the mobility of particles. This will depend on their size and weight but also
on hydrodynamism in the considered zone. In general particles larger than 2cm and richly
colonized by sessile organisms will be considered as a rocky substrate.
2.2.5. Exposure
Hydrodynamic forces will influence the composition of communities and/or the growth of the
primary producers. Different species or morphologies will be encountered on exposed or
sheltered shores.
2.2.6. Biotic interactions
The structure of communities is deeply influenced by biotic interactions. These include intraand interspecific competition and grazing. Two classical examples are the tide pools on rocky
shores of New England and the kelp forests of the NE Pacific.
In tide pools of New England, available algae are Enteromorpha intestinalis (a green alga)
and Chondrus crispus (a red alga). The main grazer is the common periwinkle Littorina
littorea whose preferred food is E.intestinalis. The latter is abundant in tide pools where there
are few periwinkles and Chondrus is the dominant specie sin pools with numerous snails. To
determine if differences in seaweed species composition were caused by the different grazer
abundances, experimental alterations were carried out by Lubchenco. All periwinkles were
37
removed from a pool where Chondrus was dominant. This caused Enteromorpha to quickly
settle on Chondrus (together with ephemeral algae) and to outgrow the latter that disappeared
after one summer (see fig.). In another pool dominated by Enteromorpha, Littorina were
added. This severely reduced the abundance of the green alga and some ephemeral became
abundant in winter, when snails were less active. Chondrus did not recover due to the low
recruitment rate of this species. In an untreated control pool, the abundance of Chondrus
remained high trough the 1.5 year of the study. These experiments show that the grazer
controls the composition of the producer community, allowing the less competitive species to
become dominant. The latter is then determined by other factors like light and nutrients.
Kelp forests off California consist in the canopy-forming giant kelp Macrocystis pyrifera and
two understory kelp species Laminaria dentigera and Pterygophora californica. The main
grazer is the sea urchin Strongylocentrotus franciscanus whose preferred food is the giant
kelp, then other kelps and, if those are not available, detritus and algal turf. In 1976, a disease
decimated the population of sea urchins. Soon after, the density of Macrocystis increased
markedly (see fig.) and by 1977 only about 1% of the light at the surface reached the bottom.
This caused intraspecific competition for light due to self-shading by the giant kelp and
eventually resulted in a decline in the numbers of Macrocystis. However, the number of
fronds on the surviving kelp increased, so that the total biomass of giant kelp remained
significantly higher after the mass mortality of sea urchins. The two understory kelp species
increased rapidly after sea urchins died but they decreased in abundance to almost zero in
subsequent years due to shading by Macrocystis. Here we have an example of control of the
producer by grazing with subsequent competition for light between primary producers. In
some documented cases this control may go as far as to graze out the kelp bed. In this case, it
is rather difficult for the kelp community to re-establish, since the urchins remain, feeding on
detritus, algae and benthic fauna. This results in a new totally different equilibrium (barren
grounds) that may persist for many years.
In both examples, the food preference of the grazer (due to the absence of efficient
phytochemical defence of some producer) results in the overgrazing of this producer, the less
favoured producer species being controlled by competitive interactions. In both cases, the
control process is top-down.
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3. Benthic consumers
3.1. Classification
3.1.1. According to localization
- epifauna: the whole organism or most of it are localized above the water-substrate interface
- endofauna: organisms living below this interface; these can be burrowers, perforators (in
rocky substrates) or interstitial (living and moving in interstitial water between
sediment particles.
3.1.2. According to size
Size limits
(depending on authors)
Macrofauna
2 – 0.5 mm
Meiofauna
100 – 40 µm
Microfauna
3.1.3. According to diet
- suspensivorous: feeding on particles caught in the water column
- depositivorous: feeding on sediment
- herbivorous: feeding on primary producers
- carnivorous: feeding on consumers
- detritivorous: feeding detritus
Notice that the different categories are not all mutually exclusive.
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3.2. Factors controlling benthic consumers
3.2.1. Emersion
The factors affecting primary producers in the intertidal zone are also controlling the
consumer communities that are also presenting a zonation depending on physiological
adaptations of the members of the community. This is particularly obvious on rocky shores.
3.2.2. Substrate
On rocky shores, the main problem for consumers is to resist to hydrodynamic constraints that
will depend on the exposure mode, the slope of the substrate and microtopography (cracks,
caves etc.)
Physical and chemical characteristics of sediments will affect their associated fauna. Physical
characteristics include the particle size distribution and porosity. Particle size will directly
influence the benthic life habits. For instance, very fine sediments will be too unstable for
large size organism that will be unable to maintain their position. Gravels will contain very
few organic fine particles and will therefore be unsuitable for depositivorous organisms.
Sediments may also be homogenous ("well-sorted") or inhomogeneous ("poorly sorted"). The
porosity is the ratio between interstitial volume and total volume. It will directly control the
oxygenation of the sediment through diffusion rate. It will also influence the meiofauna living
in the interstitial volume.
Chemical characteristics are dependent on oxygen that is diffusing from the water-sediment
interface and on the action of bacteria. Aerobic bacteria consume oxygen in the upper layers
of the sediment. Therefore, oxygen concentration in interstitial water progressively decreases
with depth in the sediment. This causes a modification of the redox potential that is usually
brutal (see fig.), the so-called redox potential discontinuity (RPD). The RPD zone is the
interface between oxic and anoxic layers of the sediment. The depth of the RPD will depend
on water motion above the sediment (favouring oxygen diffusion) and the porosity of the
sediment. This gradient in redox potential induces a vertical zonation of sediment
microorganisms. In the oxic layer, aerobic microorganisms, possibly photosynthetic occur.
The RPD zone is occupied by chemosynthetic sulphur bacteria that need both H2S and
oxygen. Below the RPD zone the general vertical succession is as follows. Fermenting
bacteria, which use organic compounds and produce fatty acids and alcohol are the first
encountered. Then, come sulphate-reducing bacteria that reduce sulphate to H2S (which will
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diffuse upwards and be used by sulphur bacteria). Methanogenic bacteria are the deepest; they
break down organic substrates and produce methane.
Eukaryotes will be mainly restricted to the oxic zone. However, some ciliates
(Odontostomatida) containing endosymbiotic anaerobic bacteria are able to live permanently
below the RPD. Some Metazoa are also found below the RPD, namely nematodes,
turbellarians, gnathostomulida, rotifera, and gastrotriches (see fig.). However, it is not clear if
they are living permanently below the RPD and if they are able to live only on an anaerobic
metabolism. The whole community found below the RPD was called by Fenchel and Riedl
the thiobios.
It is noteworthy that the RPD is not always horizontal due to the activity of bioturbators (see
fig.). These will not only affect the oxygen distribution in the sediment but they will also
increase the heterogeneity of the sediment due to, for instance, selective feeding on some
particle sizes. They will also structure the sediment (tube building) and take part in the
transfer of particulate organic matter to deeper layer through burying or mixing (see fig.).
3.2.3. Biotic interactions
Rocky substrates
In heading 2.2.6 we considered the role of sea urchins in the control of kelp forests. The main
predator of sea urchins was the sea otter (Enhydra lutris) that was distributed from Japan and
Aleutian Islands to Southern California. The species was quasi extirpated by hunting for fur in
the 19th and first half of the 20th centuries. At present, the sea otter occurs mainly in certain
islands off Alaska and as a remnant population in Central California. On average, an otter
weighs 23kg and consumes 20 to 30% of its body weight in food every day (sea otters have
no blubber and need to have a very high metabolism to maintain their temperature). In the
Aleutian islands some islands still have a sea otter population (Amchitka) while others do not
(Shemya). In Amchitka, the population is dense (20-30 individuals/km2). Algae almost
completely cover the substratum, density of sea urchins is low with small size classes being
dominant and found in cracks (see fig.). In Shemya, on the contrary, there is no subtidal algal
cover and the density of sea urchins is high (up to >400ind/m2) with large size classes being
well developed (see fig.). This study was extended to many other locations in Alaska, clearly
showing that the absence of otters is always highly correlated with a low cover of kelp (see
41
fig.). Follows up of the recolonization of some islands also showed shifts from low to high
algal covers with some variability due to the frequency of sea urchin recruitment.
These studies clearly demonstrated that sea otters do control sea urchin populations by
predation and, therefore, indirectly, the primary production. This system is actually one of the
best studied examples of trophic cascade in the benthic domain. Otter, in this case, is a socalled "keystone-species" (sensu Paine), that means a species whose impact on the ecosystem
is not proportional to its biomass. Notice that this is different from a "dominant species"
whose importance is immediately proportional to its biomass (kelp are dominant species of
the kelp forests).
In the tide pools of New England (heading 2.2.6), the main predators are the dog whelk
(Nucella lapillus) and the starfish Asterias forbesi. Their main preys are the barnacle Balanus
balanoides and the blue mussel Mytilus edulis. The two latter compete for space with the red
alga Chondrus crispus. To understand the interactions between these species, Lubchenco and
Menge carried out different experiments. In a first set of experiments, plots were cleared of all
sessile organisms (see fig.). In cleared areas with no further manipulations ("control" in the
fig), the mussel settled but did not survive predation for very long, and Chondrus became the
dominant species. Where stainless-steel cages excluding the dog whelk and starfish were
installed on cleared plots, Mytilus survived and was able to outcompete the barnacle in a
short time. In other cleared plots with cages, the mussels were removed; this allowed Balanus
to settle and eventually to grow into the most abundant species. In cages where mussels were
removed and barnacles reduced in density, Chondrus, after some initial coexistence with
Balanus, became dominant. Thus, at least as colonizers of bare substrates, the competitive
hierarchy is (1) Mytilus, (2) Balanus, (3) Chondrus. In places where predators are naturally
absent, such as in sites exposed to severe wave action, Mytilus is the most abundant species.
In protected sites, predators are not swept away or damaged by waves, and their presence
prevents their preferred preys from monopolizing space. Chondrus can then colonize.
Another set of experiments was carried out in plots where stands of Chondrus were already
established (see fig.).Cages excluding predators allowed the recruitment of Mytilus and
growth of these eventually resulted in the exclusion of Chondrus from the plot.
So, in both experiment series, if predators are present (as on sheltered shores), they control
the abundance of the dominant competitors and colonization by algae is possible. This
corresponds to a top-down control. If predators are absent (as on exposed shores), the
dominant competitor occupies the space available.
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Soft bottoms
On soft bottoms, the main predators are usually fishes and crabs. If these are prevented to
enter plots by cages, the biomass of macroinvertebrates in the cage remains high during all
the growing season while in the absence of cage, it rapidly decreases (see fig.). Contrary to
the situation on rocky shores, exclusion of the predators on soft bottoms does not have much
impact on the competition between preys probably due to the fact that these are not sessile.
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