Final thesis Zahid

Final thesis Zahid
Master of Science
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M.Sc. Thesis in International Studies in Aquatic Tropical Ecolo
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M.Sc. Thesis in International Studies in Aquatic Tropical Ecol
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M.Sc. Thesis in International Studies in Aquatic Tropical Ecology
Baseline Study for the restoration of a formerly oligotrophic,
presently eutrophicated lake in northern Germany
Presented by
Md. Zahidul Islam
University of Bremen, Faculty for Biology and Chemistry
1st supervisor: Dr. Jürgen Laudien, AWI, Bremerhaven, Germany
2nd supervisor: Dr. Eike Rachor, AWI, Bremerhaven, Germany
Bremen, August 2009
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iii
Abstract
Silver Lake (―Silbersee‖) is situated in the sandy Saalian moraine landscape east
of Bremerhaven (Northern Germany), is about 8.0 m deep, and covers
approximately 6.5 hectares. The lake is a nature reserve but, a part is
nevertheless used for bathing and angling. In order to understand the water and
sediment chemistry of the lake water samples were collected monthly from three
different sites, while sediment samples were collected once from three other sites
in the central deep area of the lake. During the summer, surface water is warmer
than the bottom water, thus a thermocline with steep temperature changes is
created. Water analysis indicated that P and N concentrations are too high to
classify the lake as ―oligotrophic‖ as it was in former times. This is further
supported by the oxygen depletion in the deep water body during summer. The
pH of the lake water is weakly acidic (about 6.5). The source of the water is only
precipitation, and almost no water comes from outside the lake. Even in shallow
waters the brownish colorations of the water and its increased turbidity by
phytoplankton provide light conditions that are disadvantageous to rare primary
plants of the Littorelletea-community with the exclusive occurrence of Isoetes
lacustris in Lower Saxonia (northwestern Germany). These plants are further
endangered by the accelerated eutrophication process reflected in the increased
competition of the emergent vegetation as well the coverage (overgrowth) by
filamentous algae. Sediment analyses indicate that C, N and P contents are high
in the near-the-surface layer but decline downwards in the sediment. The
iv
sediments are to be regarded as potential sources especially of P for the water
nutrient regime.
To evaluate the effect of filter-feeding bivalves on water quality,
measurements were taken to estimate the respiration and ingestion rates of the
swan mussel Anodonta cygnea, which is now abundant in Silver Lake. The result
of the respiration rate experiment had a low, stable rate at about 3.07 mg O 2 d-1
per average individual at a temperature of 10°C and ingestion rates were
calculated at 47.5 µg Chl-a d-1. They can ingest only 1.9% of the assumed total
Chl-a present in Silver Lake, which is not significant amount. So, the study
presumes that filter feeding effects have not the sufficient potential for reducing
the Chl-a from the Silver Lake
Lake restorations are attempted to improve water quality and life
conditions, aesthetic and recreational needs. This study advises i) to remove
macrophytes, ii) taking out sediments, iii) to control of runoff from adjacent farm
land, iv) to cut down the deciduous trees from the bank area and to remove live
and dead material from the water edge v) some regulations for bathing and
angling people as well as vi) mechanically ventilate the deep water of the lake
during summer.
Future research should however be undertaken to get insights in the
production, the fluxes of nutrients and practibality of production control by fish as
well as the possible dystrophication influences from the adjacent raised bog and
the emergent vegetation before a very strong effort is put into restoration
measures.
v
Acknowledgement
All praises are credited to ―Almighty Allah‖ who enabled me for successful
completion of this thesis.
I express my abysmal respect, deepest sense of gratitude, sincere
appreciation and ever indebtedness to my honorable 1st supervisor Dr. Jürgen
Laudien, Alfred Wegener Institute for Polar and Marine Research (AWI),
Bremerhaven, Germany for his scholastic guidance throughout the research work
and preparation of this thesis.
I also express my heartfelt and immense indebtedness to my respected 2nd
supervisor Dr. Eike Rachor, Alfred Wegener Institute for Polar and Marine Research
(AWI), Bremerhaven, Germany, for his valuable advice, constructive criticisms and
encouragement throughout the research work and successful completion of this
thesis.
Sincere gratitude to Professor Dr. Rainer Buchwald, University of Oldenburg,
Professor Dr. Micheal Schlüter, Dr. Kuhn and Dr. Eva Nöthig, Alfred Wegener
Institute for Polar and Marine Research (AWI), Bremerhaven, Germany for giving me
laboratory facilities to analyze the water, sediment and Chl-a measurement.
I also wish to take the privilege to express my deepest sense of veneration to
all teachers of the ISATEC program for their teaching, kind co-operation and
encouragement. Thanks are also accorded to the program coordinators for their help
in different steps of the study program.
I am grateful to the authority of the DLRG Wehdel e.V. for providing the
rescue station and boat for field work as well as the Municipality of Schiffdorf for
providing the water sampler.
Thanks and gratitude to all my ISATEC colleagues, many thanks for their
everlasting friendship through which they make these two years unforgettable. I
would like to express my deepest thanks especially to Hasan bhai, Harun, Asad,
Sumon, Elahi, Shuvo, Sharif and Bashir for their good company.
Finally, I would like to express deepest gratitude to my beloved mother,
brothers, sisters and all well-wishers for their understanding, patience inspirations,
sacrifices, and blessing. Special thanks to my wife Salma Begum for her company
and help during the study period in Germany.
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Table of Contents
Abstract
iv
Acknowledgement
vi
Contents
vii
Chapter 1: Introduction
1
1.1
Characteristics of lakes
1
1.2
Plant communities of oligotrophic western and central
1
European lakes
1.3
Freshwater bivalves
5
1.4
Lake restoration
7
1.5
Actual state of Silver Lake
9
Chapter 2: Materials and Methods
11
2.1
Study site
11
2.2
Water analyses
14
2.2.1
Sites
14
2.2.2
Sampling
14
2.2.3
Physico-chemical analyses
15
2.3
Sediment analysis
16
2.3.1
Sites and sampling
16
2.3.2
Chemical sediment analyses
17
2.3.2.1
Total nitrogen
18
2.3.2.2
Total phosphate
18
2.3.2.3.
Total carbon
19
vii
2.4
2.3.2.4
Total calcium
20
2.3.2.5
Total minerals
21
Metabolic rate and feeding of the swan mussel Anodonta
21
cygnea
2.5
2.4.1
Respiration
22
2.4.2
Metabolic rate
24
2.4.3
Quantification of A. cygnea filter feeding
25
Assessment of the lake vegetation
27
2.5.1
Sites
27
2.5.2
Monitoring of the plant assemblage
27
Chapter 3: Results
29
3.1
Description of water body
29
3.1.1
Water chemistry
29
3.1.1.1
Variability of phosphorus
29
3.1.1.2
Variability of nitrogen
31
3.1.1.3
Variability of electrical conductivity
33
3.1.2
3.1.3
Physical parameters
34
3.1.2.1
pH-value
34
3.1.2.2
Variability of oxygen
35
3.1.2.3
Variability of temperature
36
3.1.2.4
Water transparency
37
Hydrology
3.1.3.1
38
Fluctuations in water level
viii
38
3.2
Sediment chemistry
39
3.2.1
Carbon : nitrogen and nitrogen : phosphorus ratio
39
3.2.2
Status of calcium carbonate
40
3.2.3
Status of percentage of total minerals
40
3.3
Primary plants
41
3.4
Metabolic rate of A. cygnea
43
3.4.1
Measurements of test bivalves
43
3.4.2
Size-mass relationship
44
3.4.3
Whole animal metabolic (oxygen consumption) rate
44
3.5
Ingestion rates A. cygnea
Chapter 4: Discussion
4.1
45
46
What is the present feature of the physico-chemical factors
47
of Silver Lake?
4.2
What are the major factors influencing eutrophication of
48
Silver Lake?
4.3
What is the current status of the plants of Silver Lake?
49
4.4
Does the filter-feeding effect of A. cygnea enable to shift the
51
eutrophication status of Silver Lake?
Chapter 5: Conclusion and proposals for restoration actions
52
References
57
ix
List of Figures
Figure 1
Typical Littorelletea-community inhabiting oligotrophic lakes
4
a: Isoetes lacustris (L.) b: Littorella uniflora (L.) c: Lobelia
dortmanna (L.)
Figure 2
Flow chart of nutrient and organic matter decomposition in
6
relation of unionid bivalves in lakes (modified from Vaughn
and Hakenkamp 1988).
Figure 3
Picture of a Swan mussel Anodonta cygnea
7
Figure 4
Location (upper picture) and aerial view of Silver Lake
12
(Silbersee)
Figure 5
Water sampler (Ruttner sampler)
14
Figure 6
KB (Kajak - Brinkhurst) core sampler
16
Figure 7
Bathymetric map from 21.02.2008 (1:1000) showing the three
17
sediment sampling sites
Figure 8
Picture of the Swan mussel Anodonta cygnea in the chember
22
Figure 9
Multi-channel modified intermittent flow system
24
Figure 10
Map is showing the three water sampling sites (X) and nine
27
(1-9) vegetation observation plots
Figure 11
Variability of Total Phosphorus (mg/l) at three different
30
locations (Deep, Angling and DLRG) of Silver Lake from
October 2007 to March 2009.
Figure 12
Variability of PO4-P (mg/l) at three different locations (Deep,
Angling and DLRG) of Silver Lake from October 2007 to
x
30
March 2009.
Figure 13
Variability of total Nitrogen (mg/l) at three different locations
32
(Deep, Angling and DLRG) of Silver Lake from October 2007
to March 2009.
Figure 14
Variability of NH4-N (mg/l) at three different locations (Deep,
32
Angling and DLRG) of Silver Lake from October 2007 to
March 2009
Figure 15
Variability of NO3-N (mg/l) at three different locations (Deep,
33
Angling and DLRG) of Silver Lake from October 2007 to
March 2009
Figure 16
Variability of Electrical conductivity at three different locations
34
(Deep, Angling and DLRG) of Silver Lake from October 2007
to March 2009
Figure 17
Variability of the pH at three different locations (Deep, Angling
35
and DLRG) of Silver Lake from October 2007 to March 2009.
Figure 18
Oxygen saturation [%] at three different depths of the water
36
column (1, 5 and 8m) of Silver Lake from August 2008 to July
2009
Figure 19
Variability of water temperature at three different depths of the
37
water column ( 1, 5 and 8 m) of Silver Lake from August 2008
to July 2009
Figure 20
Secchi disk depth and water transparency indicating the
degradation of the light conditions in Silver Lake from
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38
September 2008 to July 2009.
Figure 21
Water level fluctuations in Silver Lake from October 2008 to
39
June 2009.
Figure 22
Distribution pattern of vascular plants in Silver Lake (
42
Buchwald and Hilbich 2008)
Figure 23
Logarithmic relationship between the ash free dry mass
44
(AFDM) [g] of the soft body and height [mm] of the swan
mussel Anodonta cygnea
Figure 24
Oxygen consumption rate (mg O2 d-1) of 15 A. cygnea and
45
their mean value at a temperature of 10°C
Figure 25
Clearance rate (CR) (µg Chl-a g DM-1min-1) of A. cygnea at a
temperature of 10°C (N = 15).
xii
46
List of Tables
Table 1
Status of C:N, N:P ratio, CaCO3 and % total minerals
40
(burning) of Silver Lake collected from three different sites
(P1,P2 and P3) and depths (A: 0.-10. B:>10 – 20 and C:> 20
– 30 cm )
Table 2
List of vascular plants and mosses in the Silver Lake
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Table 3
Size and mass parameters of the animals for the respiration
43
and ingestion measurements: N =15
Table 4
Rough calculation of Chl-a content of Silver Lake
xiii
52
1 Introduction
1.1 Characteristics of lakes
On a larger scale, natural lakes are not only part of human‘s quality of life, but
also increase the biodiversity and functional properties of the surrounding
ecosystems. The environment and conservation value of lakes include
biodiversity, heritage and visual values as well as values for water uses. In terms
of nutrient concentration, lakes are generally classified into three categories: i)
oligotrophic, ii) mesotrophic, and iii) eutrophic (Owens and Chiras 1990).
Oligotrophic lakes are characterised by low nutrient concentrations often
associated with low pH and low CO2, reflected in low macrofloral abundances
and
meagre
phytoplankton
production.
These
lakes
have
phosphorus
concentrations less than 1 microgram per litre (Klapper 1991). In contrast,
eutrophic lakes are characterized by high nutrient levels (e.g. phosphorus
concentrations can be up to 1 milligram per litre; Klapper 1991), turbid water, and
abundant macrophyte populations (Owen and Chiras 1990), as well as strong pH
variation and (deep water) oxygen depletion. Mesotrophic lakes form
intermediate stages.
1.2 Plant communities of oligotrophic western and central European lakes
Western and central European lowland oligotrophic freshwater lakes are
inhabited by a typical plant community characterised by Littorelletea (Schaminee
et al. 1992, 1995). The Littorelletea-community is characterised by small
limnophytic and amphiphytic plants, growing in the littoral zone of oligotrophic to
slightly mesotrophic tarns, lakes and pools. These communities are relatively
1
poor in species. In addition, the characteristic species have a low competitive
ability. These communities have disappeared in many parts of their range due to
natural succession and/or human impact. Thus, the characteristic species are
among the rarest plants of the native flora in Germany, and their communities are
elements of the most endangered ecosystems (Dierssen 1981). The Littorelletea
include ―isoetid‖ plant species, such as Littorella uniflora, Lobelia dortmanna and
Isoetes spp. (Den Hartog and Segal 1964, Schoof-Van Pelt 1973, Wittig 1982)
(Fig. 1 a-c). These plant species can only survive in stagnant, extremely weakly
buffered oligotrophic waters with carbon dioxide levels of generally below
40 pmol l-1. Many other submersed species depending on CO2 uptake through
their leaves are unable to absorb enough CO2 for net photosynthesis in such
lakes, because the diffusion rate of CO2 is very low, e.g. in stagnant water 104
times lower compared to the diffusion rate in air (Madsen et al. 1993). However,
pore water of fresh water-body sediments may hold CO2 levels 10-100 times
higher compared to the water layer and can reach values up to 4,000 pmol l-1
(Roelofs
1983).
Isoetid
plant
species
have
several
physiological
and
morphological adaptations to survive under those conditions, such as root uptake
of CO2 (Wium-Anderson 1971, Sand-Jensen and Sondergaard 1979), recapture
of photo respired CO2 in their lacunal system (Sondergaard 1979), and high
oxygen release by their roots (Sand-Jensen et al. 1982, Roelofs et al. 1984,
1994). Besides carbon dioxide, the availability of nutrients such as phosphorus
and nitrogen, is very low in oligotrophic lake systems. Several isoetid plants have
mycorrhiza symbionts, which support them in ―nutrition‖. As a result, a quite
stable ecosystem, with low productivity, is sustained for decades or even
2
centuries. Through pollen extracted from sediments from a Danish lake, it is
reported that there have been hardly any changes in the abundances of isoetid
species between 6,000 and 100 years ago (Müller and Kleinmann 1998). The
pollen record of Lake Wollingst, Northern Germany, also shows that this lake was
oligotrophic since its origin at the end of the Pleniglacial. After medieval forest
clearing the lake has changed its quality, and its sediments exhibit altered pollen
composition (Müller and Kleinmann 1998). The numbers of isoetids decreased
drastically and the abundances of species inhabiting eutrophic environments
increased in the last century, (B. Van Geel, personal communication).
Eutrophication and the decline of isoetid species of similar lake systems in
Germany and the Netherlands were observed in the last century (Schoff-Van Pelt
1973, Westhoff 1979, Wittig 1982, Arts 1990). Roelofs (1983) revealed that in 12
out of 53 lake systems, from which isoetid species had disappeared since 1950,
the water changed to more or less turbid conditions, reflected in the presence of
more eutraphent species such as Riccia fluitans in combination with
mesotraphent species such as Myriophyllum atterniflorum and Ranunculus
peltatus (Roelofs 1996). As a result of the altered light condition, the occurrences
of Lobelia dortmanna, Isoetes lacustris and Litorella uniflora became restricted to
the very shallow shore waters, while they were found down to 5 m depths in
cooler waters before eutrophication. Thus, these plants became more exposed to
critical influences e.g. overgrowth and competition by other plants, disturbances
by bathing people, ice and wave forces and the global and local warming
(Rachor, lecture in Oldenburg, Feb. 2009, Vahle 1990).
3
a
b
c
Fig. 1: Typical Littorelletea community inhabiting oligotrophic lakes a: Isoetes
lacustris ( L.) b: Littorella uniflora (L.) c: Lobelia dortmanna (L.)
4
1.3 Freshwater bivalves
Benthic suspension feeder communities are considered among the most efficient
assemblages in extracting and processing energy-rich organic matter from
aquatic ecosystems (Gili and Coma 1998). Suspension feeding bivalves directly
control phytoplankton biomass in lake ecosystems (Cahoon and Owen 1996,
Strayer et al. 1999). They are capable of cycling a significant amount of nutrients
(Lewandowski and Stanczykowska 1975, Stanczykowska and Planter 1985,
Kasprzak 1986, Vanderploeg et al. 1995).
Bivalves of the family Unionidae are key components of freshwater
ecosystems. Being primary consumers, they occupy an intermediate position in
the food web, passing energy from the primary producer to other animals and to
micro-organism. Thus, their filtering activity may contribute to maintain lake, river
and stream ecosystems (Müller and Patzner 1996).
The size and composition of unionid communities may affect the primary
producer community structure and indirectly other grazers as fresh water
bivalves may filter phytoplankton, bacteria and particulate organic debris from the
water column (Paterson 1986, Leff et al. 1990). Filtration rates vary with bivalve
species and size, temperature, particle size, nutrition concentration, oxygen
conditions, water turbulence and currents (Vaughn and Hakenkamp 1988). On
the other side, the bivalves may modulate nutrient and organic matter dynamics
through excretion as well as biodeposition of faeces and pseudofaeces.
Excretion rates are both, size and species dependent, influenced by the
reproductive stage, and vary largely with temperature and food availability.
Bioturbation of sediments through bivalve movements increases sediment water
5
Fig. 2: Flow chart of nutrient and organic matter decomposition in relation of
unionid bivalves in lakes (modified from Vaughn and Hakenkamp 1988)
and oxygen contents and releases nutrients from the sediment to the water
column (Vaughn and Hakenkamp 1988, Fig. 2). The ‗Swan mussel‘
(―Teichmuschel‖) Anodonta cygnea (L.,1758) inhabits large ponds, lakes and
slow moving water, such as canals and (small) rivers with muddy bottoms.
6
Fig.3: Picture of a Swan mussel Anodonta cygnea
Its distribution is limited down to 10 m depth. Highest abundances (> 10 ind./m 2)
are common between 2.5 and 6 m depth (Patzner et al. 1993). A. cygnea prefers
nutrient rich waters and is one of the most common freshwater bivalve species
widespread across central Europe and the United Kingdom. They burrow into the
substrate normally with just the siphon tips exposed to filter particles from the
water, pumping each up to 30 litres of water a day (Müller and Patzner 1996).
1.4 Lake Restoration
A lake is seen as part of an interdependent system of surface and subsurface
water and of plant and animal habitats. These components are related to, and
interact with each other. Thus, lake restoration requires general and special
knowledge of the specific lake ecology, the causes of changes in water quality
and species composition, as well as the techniques for restoring and protecting
such lakes. Additionally, the legal and financial realities are to be considered, and
7
the administrative and technical resources available. Lake restoration begins with
ecological awareness. Ecosystem-based restoration efforts typically involve the
establishment of restoration targets. Ideally, these targets should be reflective of
historical conditions (Lichatowich et al. 1995, Shuter and Mason 2001), although
in reality, the relevant information on historical states is rarely directly available or
obtainable, leaving managers often to speculate as to the historical state of
ecosystems. The lack of baseline information on physical, chemical and
biological interactions is a major obstacle to efforts to characterize the ecological
changes. Such information may be a critical element in the evaluation of the
restoration potential. However, old historical records, comparative studies (of
similar systems) and lake sediments can help to understand the lake history.
New approaches based on the reconstruction of historical ecosystems may thus
make a substantial contribution to ecosystem-based restoration efforts.
However, ecosystem-based restoration can be limited by a number of
constraints, especially drastic changes in the surrounding landscape and in land
use and drainage, and, nowadays, by drastic climatic alterations, and the
presence of exotic species in freshwater lake systems (e.g. Coblentz 1990,
Lodge 1993, Mills et al. 1994, Ricciardi and MacIsaac 2000). In many cases,
limnic ecosystems have lost part of their native species assemblage and may
host a variety of introduced and invasive species, many of which dramatically
alter the structure and function of these systems (Ludyanskiy et al. 1993, Mills et
al. 1994, MacIsaac 1996).
8
1.5 Actual state of Silver Lake
The morphology and hydrology of Silver Lake with its slow water renewal and an
unfavourable relation of the epi- and hypolimnion, make the lake ecosystem very
sensitive, especially to nutrient burdening (Rachor 1998). Eutrophication is the
natural aging process for most lakes, which involves an increase in nutrient
concentrations in the water body, as well as rising sedimentation. The actual
trophic stage of Silver Lake is indicated by oxygen depletion and H2S formation
in the deep-water body during each summer and the lack of any profundal fauna.
It is now eutrophicated mainly by P- and N-compounds. Atmospheric imission of
nutrients into the lake is almost sufficient today, to keep it in critical trophic stage,
with a total phosphate concentration between 0.06 and 0.08 mg P per litre in
1996 and 1998 (Rachor 1998, in Lake Wollingst). According to near-by imission
measurements, 11.5 kg of ammonium- and nitrate-N and 0.25 kg of phosphate-P
per hectare are annually deposited from the atmosphere (Rachor 1998).
Accordingly, and considering the poor water renewal rates of Silver Lake, the
amount of dissolved inorganic N may be renewed within about 3-5 years, while P
may be replenished within a few more years, not considering temporary sinks in
the sediments.
Additional causes for nutrient richness may be:
- remainders of uncontrolled bathing activities in the 1950s to 60s, especially Pcompounds,
- introduction of new nutrients by bathing and angling
- the (P and N rich) runoff/emissions from the adjacent farming and holiday
lodging areas,
9
- the surrounding dominating emergent and terrestrial plant species and their
leave litter,
- influences from the adjacent raised bogs nutrients
The resulting brownish colouration of the water and its increased turbidity by
phytoplankton provide even in the shallow waters light conditions that are
disadvantageous to the rare Lobelion-association (Rachor 1998, Vahle 1990).
Additionally, the mentioned plants are covered by fouling organisms (algae,
fungi, etc.) especially in spring, which is also a light inhibiting and even a
burdening obstacle to sensitive plants. But the lake condition in terms of the
physical, chemical and biological parameters are not fully understood and
relevant scientific studies and reports are lacking yet.
Therefore the objectives of the present research for restoration measures are:
a) to describe the recent physico-chemical and biological properties of Silver
Lake (oxygen, temperature, pH, nutrients i.e. P, N) in order to determine the
actual status
b) to monitor the surviving primary plants
c) to investigate the metabolic rate and feeding of the Swan mussel Anodonta
cygnea and estimate its possible influence on the nutrient regime.
d) to describe the restoration proposal for future actions.
10
2 Materials and Methods
2.1 Study site
Silver Lake (Silbersee, Fig. 4) is located approximately 14 km east of
Bremerhaven (northern Germany) near the little village Wehdel in the community
of Schiffdorf. It is situated at an altitude of approximately 10 meters above the
sea level of the Beverstedt Moorgeest (Pleistocene Saalian old moraine
landscape with bogs in depressions). The lake was recorded to be 11 meters
deep in the past; in recent years just 8 meters were measured; and it covers
approximately 6.5 hectares. Silver Lake was originally oligotrophic, but the
feature on its exact origin is still unclear and the origin of the lake is under
discussion. According to Merkt and Kleinmann (1998), a Pingo genesis (or
transformation) during the last glaciation period as suggested for the near-by
Lake Wollingst may be most plausible. The lake is situated in the sandy to
sometimes loamy Saalian moraine landscape. The slope of Silver Lake shore is
about 2 meter higher in the North, while a degenerated raised bog in the West is
not much elevated from the landscape.
11
Fig. 4: Location (upper picture) and aerial view of Silver Lake (Silbersee),
Source of upper picture: (http://cuxland-gis.landkreis-cuxhaven.de/gis/schutznature/viewer.htm) Source of aerial view DLRG Wehdel e.V.
12
There are no visible inflows and only poor groundwater supplies to the lake,
which is partly adjacent to degenerated raised bogs. According to Vahle (1990),
a relict oligotraphent vascular plant association was still existent in the shallows
in 1990: Isoetes lacustris (Fig. 1a) and Littorella uniflora (Fig. 1b). Mainly to
protect these rare, in Germany almost extinct assemblage and associated
organisms, the lake and its direct surroundings (32.7 ha) is a nature reserve
since 1932. Nevertheless, recreational activities such as swimming and angling
are tolerated. The history of the vegetation in the near-by Lake Wollingst
indicates that in former times most of the shallow lake bottom was covered by an
Isoeto-Lobelietum community; but in the last century, especially in the seventies,
a dramatic decline of this assemblage has been observed (extinction of Lobelia
dortmanna and reduction of the growing belt of Isoetes lacustris and Littorella
uniflora lo less than 0,7 m water depth). By the accelerated eutrophication
process the emergent vegetation has rapidly increased since the fifties, with
Sphagnum and other paludal plants as well as Phragmites australis and others
dominating (Rachor 1998, for Lake Wollingst). Similarly, in Silver Lake, species
of Typha angustifolia, Nuphar leatum, Nymphaea alba, Potamogeton spp, reed,
mosses and others have taken over. This indicates an acceleration of the
dystrophication process, which might be part of a natural succession together
with anthropogenic impact in such types of lakes (Rachor 1998).
13
2.2 Water analyses
2.2.1 Sites
Water samples were collected from three different sites namely, ―DLRG-station‖,
―Deep‖ and ―Angling‖ (Fig. 9). Water samples were analysed for NH4+, NO3-,
NO2-, total N, PO4+, total P, pH, in situ temperature, and electrical conductivity
(EC). The water level and Secchi disc visibility depths were additionally
measured.
2.2.2 Sampling
Samples were collected monthly from August 2007 to March 2009. At ―DLRGstation‖ and ―Angling‖ replicated water samples were taken by filling two 100ml
polyethylene bottles at 0.2-0.5 m depth, while the deep area was sampled at
about 8 m depth using a Ruttner sampler (Hydro-Bios, Germany). All samples
were transported to the laboratory immediately and stored at -80°C prior to
analyses of nutrients and electrical conductivity (EC).
Fig. 5: Water sampler (Ruttner sampler, Hydro-Bios, Germany) used to sample
water from 8 m depth of Silver Lake, in this case from the ice in January 2009.
14
2.2.3 Physico-chemical analyses
Water samples were analysed in the laboratory of the section vegetation and
nature conservation of the Institute of Biology and environmental science, Carl
von Ossietzky University of Oldenburg to quantify reactive orthophosphate using
the molybdenum blue method (Grasshoff et al. 1983). Ammoniacal nitrogen (NNH3 + N-NH4+, hereafter referred to as ammonia) was determined by indophenol
(Parsons et al. 1992). Nitrite was quantified by the diazotation method (Grasshoff
et al. 1983), and Nitrate was determined by the reduction in a Cd-Cu column
followed by diazotation (Grasshoff et al. 1983). Dissolved oxygen and
temperature were measured by a diffusion O2 meter (YSI Model-57, EQUIPO,
USA); pH was measured by a pH meter (Multi 340i, WTW, Germany) from at
least three different depths (1, 5 and 8 m) in the water column of the deep
station.
15
2.3 Sediment analysis
2.3.1 Sites and sampling
Sediment samples were collected from three selected sites (P1, P2 and P3, Fig.
7) using a KB (Kajak - Brinkhurst) core sampler (Model: 603 – 034, Rickly
hydrological company, USA). Cores were sliced into 3 horizons where possible:
0-10 cm, >10-20 and >20-30 cm, respectively. After collection, samples were air
dried; roots and gravel were eliminated prior to analysis.
Fig. 6: KB (Kajak - Brinkhurst) core sampler (Model: 603 – 034, Rickly
hydrological company, USA).
16
N
Fig. 7: Bathymetric map from 21.02.2008 (1:1000) showing the three sediment
sampling sites (P1, P2 and P3). Source: Hydrographic Service GmbH, Schessel.
2.3.2 Chemical sediment analyses
Sediment samples were analysed in the laboratories of the sections of Geology
and Geochemistry, Alfred Wegner Institute for Polar and Marine Research (AWI).
Before the chemical analyses started, sediment samples were air dried, ground
and passed through sieves (2mm mesh size) to get rid of larger particles and
17
stones. The samples were then analysed for total nitrogen, total phosphate, total
sulphur, total carbon, total calcium (%CaCO3) and total minerals as explained in
the following paragraphs.
2.3.2.1 Total nitrogen
Total nitrogen content of sediments was determined by the Kjeldahl digestion
method (Kjeldahl 1883). A catalyst mixture (K2SO4: CuSO4* 5H2O: Se = 10: 1:
0.1), 30% H2O2 and concentrated H2SO4 were used to digest the soil samples.
Nitrogen was estimated by distillation with 40% NaOH followed by titration of the
distillate, trapped in H3BO3 with 0.01 N H2SO4 (Page et al. 1982).
2.3.2.2 Total phosphate
100 mg soil samples (from the three sites
three horizons) were placed in a
50 ml boiling flask before 3 ml of sodium hypobromite (NaOBr) solution was
added, and the flask was swirled for a few seconds to mix the contents. The flask
was allowed to stand for 5 min., before it was swirled again and placed in a sand
bath adjusted to 260 to 280°C. The sand bath was situated in a hood. The flask
was heated until the contents evaporated to dryness (10 to 15 min). After
evaporation, the flask was continuously heated for additional 30 min. Thereafter
the flask was removed from the sand bath, and allowed to cool down for 5 min, 4
ml of distilled water and 1 ml of formic acid were added. The flask was shaken
and 25 ml of 0.5 M H2SO4 was added. The mixture was transferred to a 50 ml
plastic centrifuge tube and centrifuged at 12,000 rpm for 1 min. For analyzing
total P, 2 ml of centrifuge sample was transferred into a 25 ml volumetric flask.
Thereafter 4 ml of ascorbic acid reagent was added and field up to volume with
18
distilled water. The solution was mixed and placed for 30 min for color
development. Optical density of sample was measured at a wavelength of 720
nm (Dick and Tabatabai 1977).
2.3.2.3 Total carbon
Equipments used in sample processing were combusted at 400°C for at least 4
hours to get rid of all combustable organic matter. The soil samples of Silver
Lake remained frozen at -20°C until processing. Sediment samples are thawed,
homogenized and dried in an oven at 40°C. 10g of the sample was removed,
ground and homogenized. Dried and homogenized samples were placed in an
aluminum-weighing pan and dried at 105°C. The LECO CR-412 Carbon Analyzer
was calibrated prior to the analyses of samples. Different amounts of high purity
calcium carbonate standard (99.95% purity, carbon content of 12.0%) were used
to calibrate the instrument. The approximate amounts of calcium carbonate used
for the six-point calibration were 0.01 g, 0.05 g, 0.10 g, 0.25 g and 0.50 g. An
empty carbon-free combustion boat was analyzed as a blank for the calibration
curve. Total carbon was analyzed by placing approximately 0.350 g of the dried,
ground and homogenized sample into a clean, carbon-free combustion boat. The
sample boat was placed on the autosampler rack assembly and loaded onto the
LECO Carbon Analyzer. Each sample boat was treated with phosphoric acid
drop by drop until the sample stopped ―bubbling‖ and the sample was completely
moist with acid to remove the calcium carbonate from the sample. The sample
was placed into an oven set at 40°C for 24 hours and then transferred to an oven
set at 105°C.
19
Calculations:
Carbon content:
Carbon [ g ]
(1)
(b) * (A) a
Where:
b = the slope of the linear calibration curve (g per unit area)
A = the area under the sample curve
a = the intercept of the calibration curve (g)
Percent total Carbon
%TC
Carbon [ g ]
W[ g ]
(2)
Where W (g) = dry sediment analysis mass (g)
Percent Total Organic Carbon content (TOC)
(%)TOC
Organic Carbon [ g ]
W[ g ]
(3)
Note: When sample had been acidified, organic carbon (g) replaces carbon (g) in
the above equation.
Percent Total Inorganic Carbon Content (TIC)
(%)TIC
(4)
(%)TC (%)TOC
2.3.2.4 Total calcium
Percentage of calcium carbonate, CaCO3 [%], is defined as the total calcium.
Percent calcium carbonate [%CaCO3] was determined by mass of total inorganic
carbon (TIC) [%] multiplied by 8.33 (mass of CaCO3/mass of carbon =100/8).
20
To express TIC as a percent calcium carbonate (CaCO3), use the following
equation.
CaCO 3 [%]
(5)
(TC TOC) * 8.33
2.3.2.5 Total minerals
Empty crucibles were weighed by digital balance (AC211S, Sartorious,
Germany), and labeled by a pencil. Then the sample (crucible +sediment) was
dried at 60°C (Memmert, Germany) for 24 hours. Thereafter the dried sample is
combusted in a Muffle Kiln (Heraeus, Germany) at 600°C for 10 hours. Total
minerals were calculated in the following equations:
Total minerals
(N T)
* 100
(B T)
(6)
Where,
N = mass of crucible with sample after burning at 600°C
T = mass of empty crucible
B = mass of crucible with dried sample before burning.
2.4 Metabolic rate and feeding of the swan mussel Anodonta cygnea
In September 2007, 15 Swan mussels (Anodonta cygnea) were transported alive
to the Alfred Wegener Institute for Polar and Marine Research (AWI, Germany)
and were kept for 4 - 6 weeks in aerated aquaria with natural freshwater and
sediments from Silver Lake (about 8cm thick layer) before starting the
experiments. Bivalves were fed once a week with a live algal suspension
maintained in the laboratory.
21
2.4.1 Respiration
Respiration was measured in a multi-channel modified intermittent flow system
as described by Heilmayer and Brey (2003). Prior to respiration measurements,
A. cygnea were maintained without food for three days, in order to eliminate the
impact of specific dynamic action (SDA) on respiration (Bayne et al. 1976).
Bivalves were allowed to accommodate to the respiration chambers
Fig 8: Picture of the Swan mussel Anodonta cygnea in the chamber
22
overnight; and oxygen consumption of only actively respiring animals that had
their siphons open to the surrounding water were measured. Respiration
chambers consisted of small Perspex cylinders with a movable lid to adjust
chamber volume between 600 ml and 1450 ml to animal size (Heilmayer and
Brey 2003). Experimental temperature was maintained stable (10°C) by placing
the chambers in a water bath set in a within jacketed container, that was
connected to a thermo circulator (Julabo FP 40, USA). Three respiration
chambers with one animal each (of similar size) and a control chamber (without
animal) were measured simultaneously per experimental run. Total two runs
were taken for each animal. Oxygen content in the chambers was monitored
continuously with oxygen microoptodes connected to a MICROX TX3 array
(PreSens, Neuweiler, Germany). Optodes were calibrated to 100% oxygen
solubility in saturated air and to 0% in N2-saturated freshwater (technical gas with
99.996% N2) at experimental temperatures. All measured data were stored in a
PC (Program Excel, Microsoft office, 2003, USA). After the measurements,
animals were dissected immediately. Soft tissue wet mass (WM) was determined
to 0.001g precision after careful blotting with blotting paper. The soft tissue was
dried at 60°C for at least 48h to get a dry mass value (DM). Thereafter dried
tissues were combusted at 500°C for 24h and ash free dry mass (AFDM = DM ash) was calculated.
23
Fig. 9: Multi-channel modified intermittent flow system, K) control, 1-3)
Chambers with animals, 4) Optodes, 5) Peristaltic pump, 6) TX-3 Boxes, 7)
Recording PC, 8) Opening structure
2.4.2 Metabolic rate
Standard metabolic rate (SMR, mol O2 ind-1 h-1) was calculated from the slope
of the oxygen saturation curve after subtraction of the microbial oxygen demand,
determined as post-measurement blank. Percent O2 saturation was transformed
to VO2 (i.e. micromoles of dissolved oxygen) using known values of oxygen
solubility (
VO 2
where
O2
satt o
satt 60
O2
, mol dm-3, Benson and Krause 1984) by:
O2
(7)
Vchamber
is the oxygen solubility in freshwater (µmol* dm -3), VChamber is the
volume of the respiration chamber and tubing (dm 3), sat t0 is the oxygen
saturation (%) at the beginning of the experiment and sat t60 is the oxygen
saturation (%) after 60 min as calculated from linear regression. Individual
metabolic rates were corrected (SMR= Standard metabolic rates) with the
24
oxygen consumption of control chambers (no animal) and converted to milligram
O2 by 44.66 µmol O2 = 1 mg O2 (Brey 2001).
2.4.3 Quantification of A. cygnea filter feeding
Swan mussels A. cygnea were placed in the above described chambers and the
chamber volume (600-1450 ml) adjusted to animal size. Three chambers with
one test bivalve each (of similar size) and a control chamber (without animal)
were used simultaneously per experimental run (one run for each animal).
Oxygen content in the chambers was monitored continuously with oxygen
microoptodes as described above. After 30 minutes acclimatization time a fresh
algal suspension was added to the chambers. After two hours the chambers
were disconnected without losing water. The water of each chamber was drained
into separate plastic bottles and homogenized by shaking three times. Three
replicate 100 ml water samples per chamber were filtered through a precombusted Whatman GF/F filter, and the filters stored at -80°C in 2 ml cryo vials.
Thereafter, filters were ground in the dark after 4 ml of aqueous acetone solution
had been added and kept at 4°C for 12 hours before the filter slurry was
centrifuged at 675 g for 15 min to clarify the solution. An aliquot of the
supernatant was transferred to a glass cuvette, and florescence was measured
before and after acidification by adding 0.1N HCl. Then the solution was
transferred to a glass cuvette, and concentration of Chlorophyll a (Chl-a) was
determined by fluorometry (TD-700, Turner Designs Inc., USA). After that 2 drop
of 0.1N HCl were added into the glass cuvette to determine Pheophytin a by
fluorometery (TD 700). Sensitivity calibration factors, which have been previously
25
determined on solutions of pure Chl-a of known concentration were used to
calculate the concentration of Chl-a and Pheophytin a in the sample extract as
Chl a
K(
Pheo a
Fm
v
) (Fb - Fa) ( )
Fm - 1
V
K(
(10)
Fm
v
) Fm (Fa -F b ) ( )
Fm - 1
V
(11)
Where K is the sensitivity coefficient, Fm is the maximum acid ratio Fb/Fa of pure
Chl a standard, Fb is the fluorescence before acidification, Fa is the fluorescence
after acidification, v is the extract volume and V is the filtered water volume.
Thereafter corrected Chl-a concentrations were calculated according to the
equation:
Corrected Chl a
C
* 1000
V1
(12)
the Ingestion coefficient was estimated as:
I
(13)
Ce Cc
Where, Ce and Cc is the concentration of corrected Chl a with test animal and
without animal (blank).
Finally the clearance rate CR (volume cleared per biomass and time) was
computed as:
CR
I
b
(14)
Where b is the biomass of the tested bivalve within the chamber.
26
2.5 Assessment of the lake vegetation
2.5.1 Sites
In order to assess the vegetation inhabiting the shallow water of Silver Lake,
different observation plots (from north-east of the DLRG-station to south-east of
the angling pier) were employed in water depths down to 75 cm (Fig. 22).
Fig. 10: Map showing the three water sampling sites (X) and nine (1-9)
vegetation observation plots. (Modified from DLRG Wehdel e.V.).
2.5.2 Monitoring of the plant assemblage
Plant assemblage monitoring as well as quantification of the submerged
vegetation was carried out from March to May (spring to summer) at each plot.
27
Species identification followed books (Cordes et al. 2006, Vahle 1990) and the
personal communication with experts. For Littorella uniflora the size of the
covered area was additionally measured by hand scale (Zollstock) .
28
3 Results
This chapter presents the results of the study of different aspects of Silver Lake.
3.1 Description of water body
3.1.1 Water chemistry
3.1.1.1 Variability of phosphorus
The chemistry of the three sampling sites showed considerable similarities in
their seasonal changes (Fig. 11 and 12). Maximum total phosphate (TP)
concentration was measured in the two winter seasons, the highest value
amounted 0.0412 mgl-1 at the deep site in January 2009. Total phosphorus
concentrations were lowest (0.0233 – 0.0231 mgl-1) at ―Angling pier‖ in spring
and throughout most of the summer. Phosphate phosphorus showed a maximum
(0.0151 – 0.0153 mgl-1) at the deep site in winter and a minimum (0.0091 –
0.0093 mgl-1) at ―Angling pier‖ in summer (Fig. 12). Highest concentrations of
total phosphorus (0.0233 mgl-1) and phosphate phosphorus (0.0153 mgl-1) were
recorded at the deep site compared to ―Angling pier‖ and ―DLRG station‖.
Whereas total phosphorus and phosphate phosphorus concentrations were
similar at the two latter sites.
29
Total Phosphate (mg/l)
0.045
0.040
0.035
0.030
Deep Total P
0.025
Angling Total P
0.020
DLRG Total P
0.015
0.010
0.005
0.000
8
8
9
8
7
8
8
07
08
, 0 g, 0
,0
,0
,0
, 0 b, 0
,
,
r
t
t
n
c
c
b
Ap
Ju
Oc
Fe
Oc
Fe
De
Au
De
Month
Fig. 11: Variability of Total Phosphate (mg/l) at three different locations (Deep,
Angling and DLRG) of Silver Lake from October 2007 to March 2009.
0.018
0.016
PO4-P(mg/l)
0.014
0.012
Deep PO4-P
0.010
Angling PO4-P
0.008
DLRG PO4-P
0.006
0.004
0.002
O
ct
,0
7
De
c,
07
Fe
b,
08
Ap
r,
08
Ju
n,
08
Au
g,
08
O
ct
,0
8
De
c,
08
Fe
b,
09
0.000
Month
Fig. 12: Variability of PO4-P (mg/l) at three different locations (Deep, Angling and
DLRG) of Silver Lake from October 2007 to March 2009.
30
3.1.1.2 Variability of nitrogen
Figure 13 shows that the total nitrogen concentration was maximum in fall and
winter at all three sites. The highest concentration was recorded 1.43 mg l -1 in
January 2009 at the deep site. Total nitrogen concentrations were relatively low
in spring and in summer at all three stations, total nitrogen concentrations were
observed highest at the deep site (1.1500 – 1.4285 mg l-1) compared to the other
two stations (Angling: 1.0530 – 1.3475 mg l-1 and DLRG: 1.1100 – 1.3215 mg l-1)
(Fig. 14). Total nitrogen concentrations were similar at the angling pier and the
DLRG station. Overall, NH4-N was more important than NO3-N (Fig. 14 and 15).
In summer, NH4-N concentration was much higher (0.197 mg l-1, June 2009) with
a very strong smell of H2S at the deep site compared to the angling pier and
DLRG station. At the same time NH4-N concentrations at the angling pier and
DLRG station were very low (<0.030 mg l-1); without any smell of H2S in the
shallow water of these two sites.
Figure 15 shows, that NO3-N was absent at the deep site in summer, but
NO3-N was present at the angling pier and DLRG station at that time. NO3-N was
higher in late fall to summer at the angling pier and DLRG station compared to
the deep site.
31
Total Nitrogen (mg/l)
1.5
1.4
1.3
Deep TN
1.2
Angling TN
1.1
DLRG TN
1.0
0.9
O
ct
,0
7
D
ec
,0
7
Fe
b,
08
A
pr
,0
8
Ju
n,
0
A 8
ug
,0
8
O
ct
,0
D 8
ec
,0
8
Fe
b,
09
0.8
Month
Fig. 13: Variability of total Nitrogen (mg/l) at three different locations (Deep,
Angling and DLRG) of Silver Lake from October 2007 to March 2009.
0.25
NH4-N (mg/l)
0.20
Deep NH4-N
0.15
Angling NH4-N
0.10
DLRG NH4-N
0.05
O
ct
,0
7
De
c,
07
Fe
b,
08
Ap
r,
08
Ju
n,
08
Au
g,
08
O
ct
,0
8
De
c,
08
Fe
b,
09
0.00
Month
Fig.14: Variability of NH4-N (mg/l) at three different locations (Deep, Angling and
DLRG) of Silver Lake from October 2007 to March 2009.
32
0.60
NO3-N (mg/l)
0.50
0.40
Deep NO3-N
0.30
Angling NO3-N
DLRG NO3-N
0.20
0.10
O
ct
,0
7
De
c,
07
Fe
b,
08
Ap
r,
08
Ju
n,
08
Au
g,
08
O
ct
,0
8
De
c,
08
Fe
b,
09
0.00
Month
Fig. 15: Variability of NO3-N (mg/l) at three different locations (Deep, Angling and
DLRG) of Silver Lake from October 2007 to March 2009.
3.1.1.3 Variability of electrical conductivity
Electrical conductivity (EC) ranged between 67.0 – 77.1 μScm-1 at the DLRG
station, 67.0 - 76.6 μScm-1 at the angling pier and 63.7 - 77.0 μScm-1 at the deep
site (Fig. 16) with highest values in winter and lowest in summer. Figure 16 also
shows that EC starts to increase from mid fall to winter and decreases from
spring to summer and shows almost identical values at all three sites throughout
the study period.
33
Electrical Conductivity (µS/cm)
80
75
70
Deep
Angling
65
DLRG
60
55
Fe
b,
09
8
Au
g,
0
O
ct
,0
8
De
c,
08
8
Ju
n,
0
Ap
r,
08
Fe
b,
08
O
ct
,0
7
De
c,
07
50
Month
Fig. 16: Variability of Electrical Conductivity of water from three different
locations (Deep, Angling and DLRG) of Silver Lake from October 2007 to March
2009
3.1.2 Physical parameters
3.1.2.1 pH-value
The pH-value of Silver Lake water ranged from 6.3 to 6.7 (Figure 17). Figure 17
shows that highest pH was measured during winter (6.7) at all three stations.
While lowest pH values were recorded in summer at all three stations.
34
6.8
6.7
6.6
pH
Deep
Angling
6.5
DLRG
6.4
6.3
O
ct
,0
7
De
c,
07
Fe
b,
08
Ap
r,
08
Ju
n,
08
Au
g,
08
O
ct
,0
8
De
c,
08
Fe
b,
09
6.2
Month
Fig. 17: Variability of the pH at three different locations (Deep, Angling and
DLRG) of Silver Lake from October 2007 to March 2009.
3.1.2.2 Variability of oxygen
The upper water layer (about 1m depth) of Silver Lake was well saturated (101 to
104%) with dissolved oxygen around the year. A considerable depletion (0%)
was observed at the deep water body (5 and 8 m) in summer (Fig. 18).
35
Oxygen saturation [%]
120
100
80
1m
60
5m
8m
40
20
0
8
9
9
8
8
9
9
9
9
8
9
, 0 p, 0 t, 0 v, 0 n, 0 b, 0 r, 0 r, 0 y, 0 e, 0 ly, 0
g
a
Ja
Ap Ma Jun
Oc
No
Au
Se
Fe
M
Ju
Month
Fig. 18: Oxygen saturation [%] at three different depths of the water column (1, 5
and 8 m) of Silver Lake from August 2008 to July 2009.
3.1.2.3 Variability of temperature
Result on temperature variability show that water temperature was more or less
stable (about 3°C) during the winter at all three depths (1, 5 and 8 m) of the water
column (Fig. 19). During summer, temperature was ≥20°C in the epilimnion
(measured at 1m depth, June 2009), the temperature of the hypolimnion (8m
depth) varied between 7.5°C (June 2009) and 7.9°C (July 2009), respectively.
36
Temperature (°C)
25
20
1m
15
5m
10
8m
5
S
A
ug
,0
8
ep
,0
O 8
ct
,0
8
N
ov
,0
8
Ja
n,
0
Fe 9
b,
0
M 9
ar
,0
9
A
pr
,0
9
M
ay
,0
Ju 9
ne
,0
Ju 9
ly
,0
9
0
Month
Fig. 19: Variability of water temperature at three different depths of the water
column (1, 5 and 8 m) of Silver Lake from August 2008 to July 2009.
3.1.2.4 Water transparency
Secchi disk visibility depth ranged between 1.62 – 1.75 m in Silver Lake from
September 2008 to July 2009 (Fig. 20). The maximum depth was measured in
November 2008, while the minimum depth was recorded in June 2009. This
indicates that water turbidity is higher in summer compared to winter months.
37
Se
pt
,0
8
O
ct
,0
8
No
v,
08
Fe
b,
09
M
ar
,0
9
Ap
r,0
9
M
ay
,0
9
Ju
ne
,0
9
Ju
ly,
09
Secchi depth (m)
1.50
1.55
1.60
1.65
1.70
1.75
1.80
Month
Fig. 20: Secchi disk depth and water transparency indicating the degradation of
the light conditions in the Silver Lake from September 2008 to July 2009.
3.1.3 Hydrology
3.1.3.1 Fluctuations in water level
The water level increased by 14.5 cm from October 2008 to January 2009 (Fig.
21) and was highest from January (maximum) to March. It decreased from April
2009 to June 2009, and thus the lowest water level was recorded for the latter
month (Fig. 21).
38
16
Water level [cm]
14
12
10
8
6
4
2
09
Ju
ne
,
M
ay
,
09
09
Ap
ril
,
ar
,0
9
M
Fe
b,
09
9
Ja
n,
0
8
De
c,
0
8
No
v,
0
O
ct
,0
8
Se
pt
,
08
0
Month
Fig. 21: Water level fluctuations in Silver Lake from October 2008 to June 2009.
3.2 Sediment chemistry
3.2.1 Carbon : nitrogen and nitrogen : phosphorus ratio
The highest C (20.442 – 26.599%), N (1.518 – 1.856%) and P (0.111 – 0.183%)
contents of the lake sediment were observed at the bottom surface, which
declined towards deeper horizons of the sediment (Table 1). For the sediment
surface (0-10 cm) a C:N of 13.47 and a N:P ratio of 9.03 were measured,
respectively. Both ratios increased gradually (≥15) to deeper horizons of the
sediment (10-30 cm).
39
Table 1 Status of C:N, N:P ratio, CaCO3 and % total minerals (burning) of Silver
Lake collected from three different sites (P1, P2 and P3) and depths (A: 0-10,
B: >10-20 and C >20-30 cm ), for site locations refer to Figure 7. (note that the
core of site P2 was only 20 cm, thus horizon C could not be taken).
Sampling
site
%N
%C
%P
C:N
ratio
N:P ratio %CaCO3
P1A
1.616
24.342 0.111
15.06
14.56
12.847
Total
minerals
(burned)
[%]
47.338
P1B
0.857
14.219 0.055
16.59
15.58
6.820
71.362
P1C
0.309
06.535 0.017
21.17
18.16
2.573
82.961
P2A
1.856
26.599 0.183
14.33
10.14
9.407
44.235
P2B
0.977
16.893 0.076
17.28
12.86
6.522
66.055
P3A
1.518
20.442 0.168
13.47
9.03
11.513
55.261
P3B
1.111
17.072 0.121
15.37
9.17
7.183
64.185
P3C
0.207
03.188 0.019
15.39
10.90
2.142
91.083
3.2.2 Status of calcium carbonate
CaCO3 concentrations of Silver Lake (Table 1) were highest (12.847, 9.407 and
11.513%) in the upper sediment horizon (0 -10 cm), values declined with depth to
6.802, 6.522 and 7.183% (>10-20 cm), and 2.142 – 2.573% (>20-30 cm),
respectively.
3.2.3 Status of percentage of total minerals
Table 1 show that the percentage of total minerals was lowest in the upper
sediment layer and increased with depth. The percentage of total minerals
ranged from 44.24 to 55.26 in the surface sediment of the three sites. In the
middle horizon and the lower horizon percentage of total minerals ranged
between 64.19 and 71.36, and 82.96 to 91.08, respectively.
40
3.3 Primary plants
The shoreline vegetation was divided into nine plots from north-east near the
DLRG station to the south-east of the angling pier. Except for plot no. nine,
Littorella uniflora dominated at all plots across the shoreline. The area covered
by Littorella uniflora was estimated to be approximately 262 m 2 (Table 2). This
primary plant grew in water depths down to 40 cm. Isoetes lacustris was found in
plot 7 and 9, on both sides of the angling pier; and counted a total 420 of
individuals. This plant grows individually and was found at a depth range
between 25 to 50 cm (Fig. 22). Lobelia dortmanna was completely absent from
the observed area with presence of some other species e.g. Carex rostrata,
Eleocharis palustris, Hydrocotyle vulgaris and Lysimachia thyrsiflora also
Menyanthes trifoliata, Potentilla palustris, Typha angustifolia. This indicates a
presence of other invasive plants and no longer a mere occurrence of Littorella
uniflora. However, the boundary between the occurrences of Littorella uniflora
and Isoetes lacustris is quite sharp; there is only a very narrow strip, in which
both species occur together (Fig. 22). On the other side, Littorella unifloria and
Eleocharis paulistris, Isoetes lacustris occur together with Typha angustifolia, the
latter stock is sometimes very dense with no presence of Isoetes lacustris. The
substrate of Isoetes lacustris is sandy-gravelly with single larger stones, in
contrast to the more (silty-) sandy areas, where Littorella unifloria occurs (Fig.
22). There are many other plant species present in this lake, e.g. Phragmites
australis, Nuphar luteaum. Nymphaea alba, and Potamogeton species.
Nymphaea alba are very densely found in the north-west part of the lake.
41
W
A
T
E
R
D
E
P
T
H
Littorella uniflora
Eleocharis plaustris
Isoetes lacustris
Typha angustifolia
Typha angustifolia
Fig. 22: Distribution pattern of vascular plants in Silver Lake (Buchwald and
Hilbich 2008).
Table 2 List of vascular plants and mosses in Silver Lake (Buchwald and Hilbich
2008).
Experimental plot
Name of species
Isoetes lacustris
Littorella uniflora
1
2
3
4
5
6
7
8
9
X
X
X
X
X
X
x
X
X
X
X
(area covered in m )
1
1
4.5
0.9
128
24
84
18
1
Agrostis canina
X
Carex rostrata
X
X
X
X
X
X
X
X
X
Eleocharis palustris
X
X
X
X
X
X
X
X
Hydrocotyle vulgaris
X
X
X
X
X
X
X
Juncus articulatus
X
Juncus bulbosuss
X
Juncus effusus
X
Lycopus europaeus
X
Lysimachia thyrsiflora
X
X
X
X
2
X
X
X
X
X
Menyanthes trifoliata
X
Myrica gale
X
Potentila palustris
X
Ranunculus flammula
X
Sphagnum spp.
X
X
X
X
X
X
Typha angustifolia
X
Others, mainly outside the plots: Phragmites australis, Nuphar luteaum. Nymphaea alba, Potamogeton spp.
and several mosses
42
3.4 Metabolic rate of Anodonta cygnea
3.4.1 Measurements of test bivalves
The size range of the measured test bivalves varied from 73 - 160 mm (Table 3).
All animals were dissected after the respiration and ingestion experiments,
respectively. Accordingly, shell length, shell height, shell dry mass, shell free dry
mass (DM) and ash free dry mass (AFDM) were measured.
Table 3 Size and mass parameters of the animals for the respiration and
ingestion measurements: N =15,
Animal ID
Shell length Shell height
[mm]
[mm]
57
Shell dry
mass
[g]
18.798
Shell free
dry mass
[g]
1.690
Ash free
dry mass
[g]
1.3589
1
96.
2
98
59
21.978
1.470
1.1946
3
126
74
51.774
8.717
7.2230
4
147
80
71.318
6.443
5.8638
5
90
54
17.306
1.148
0.9208
6
149
92
92.80
14.626
13.3070
7
160
90
103.787
7.333
5.8472
8
145
83
81.213
8.736
7.9016
9
149
96
89.892
8.278
7.0844
10
95
59
24.943
2.124
1.8034
11
155
98
83.524
10.654
9.0865
12
124
73
61.285
7.599
6.2518
13
73
45
15.127
1.447
1.1843
14
88.5
56
21.306
2.046
1.7284
15
153
95
103.340
9.344
8.4632
Mean
123.233
74.067
52.226
6.110
5.2810
43
3.4.2 Size- mass relationship
Figure 23 indicates the logarithmic relationship of ash free dry mass against shell
height of Anodonta cygnea. The regression line describes the following
relationship: AFDM = -12.934*height3.338, R2 = 0.8594, indicating that AFDM is
linearly related with height.
3.0
2.5
ln (AFDM)
2.0
1.5
1.0
0.5
0.0
3.8
4
4.2
4.4
ln (height)
4.6
4.8
Fig. 23: Logarithmic relationship between the ash free dry mass (AFDM [g])
of the soft body and height [mm] of the Swan mussel Anodonta cygnea.
3.4.3 Whole animal metabolic (oxygen consumption) rate
Whole animals (N = 15) standard respiration rate was measured at 10°C. Figure
24 shows that oxygen consumption was 9.19 mg O2 d-1 by animal no. 6 and shell
free dry mass was 14.63 g. (Table 3). The lowest oxygen consumption was 3.03
mg O2 d-1 by animal no. 13 and its shell free dry mass was 1.45 g. The average
oxygen consumption was 3.07 mg O2 d-1 (Fig. 24).
44
Fig. 24: Oxygen consumption rate (mg O2 d-1) of 15 Anodonta cygnea and their
mean value at a temperature of 10°C. Bar is showing the standard deviation of
the mean value.
3.5 Ingestion rates of A. cygnea
Ingestion rates of Anodonta cygnea were measured as an estimation of bivalve
feeding. The bivalves clearly depressed phytoplankton concentrations in the
experimental chambers compared to the control chamber (Fig. 25). Maximum
and minimum ingestion rates of the Swan mussels were recorded as 0.0067 and
0.0046 µg Chl-a g DM-1min-1, respectively; and average ingestion rate was
0.0054 µg Chl-a g DM-1min-1. Average dry mass (N = 15) of Anodonta cygnea
was recorded as 6.11 g (Table 3). So, mean individual ingestion rate of a
standard individual was estimated to be 47.5 µg Chl-a ind-1d-1.
45
M
ea
15
14
13
C
10
11
12
C
8
7
5
C
9
1
3
C
6
4
2
C
n
Animal ID
CR [µg Chl-a mg DM-1min-1]
0.000
-0.001
-0.002
-0.003
-0.004
-0.005
-0.006
-0.007
-0.008
Fig. 25: Clearance rate (CR) (µg Chl-a mg DM-1min-1) of Anodonta cygnea at a
temperature of 10°C (N = 15). Bar is showing the standard deviation of the mean
value.
4 Discussions
The present research assessed physic-chemical and biological conditions of a
typical central to western European, originally oligotropic lake, namely Silver
Lake east of Bremerhaven. Based on existing data, this study clearly points that
Silver Lake has changed its typical characteristics from an almost steady
oligotrophic state towards an eutropicated phase. However, long term detailed
investigations concerning the typical lake water regime are scarce (Herbichowa
1979, Dierssen 1981). First, for a better understanding of the lake processes, it is
recommended to measure also the surface water qualities from the deep site to
find out, whether there are influences of the shore plant belts on the local values
(which sometimes seem to indicate low water nutrient conditions). The results of
this study can be further helpful to restore the typical northern Germany lake
46
ecosystems. In the following I will discuss the feasibility of the restoration of
Silver Lake with respect to specific questions.
4.1 What is the present feature of the physico-chemical factors of Silver
Lake?
Taking into account the depth range up to 8 m, a considerable variation in
temperature as well as oxygen content was observed for Silver Lake due to the
summer stratification from June to August (Fig. 18 and 19, see also Buchwald
and Hilbich. 2008). Only the warm top layer circulates, and it does not mix with
the more viscous colder water, creating a transition zone called thermocline
(Odum 1971). Hence, in depths downward from 4 m the lake becomes anoxic
(Fig. 18) during summer, which supports also the results obtained by Buchwald
and Hilbich 2008 and Rachor in earlier years (pers. comm.). During winter low
water temperature and reduced light result in low photosynthesis; and the
accumulated and regenerated nutrients remain unused (e.g. Odum 1971, Gupta
and Gupta 2006). Therefore electrical conductivity tended to increase in winter
(Fig. 16) compared with summer (Buchwald and Rath 2007). Increased
conductivity is a result of high ionic concentrations, and decreased conductivity is
mainly a result of low ionic concentrations in the water. In summer, a
considerably higher amount of turbidity recorded in the lake water body may be
allocated to a certain extent to the phytoplankton abundance. Accordingly, in this
study, Secchi depth visibility (Fig. 20) showed a considerable decrease on a
yearly cycle, which is in good agreement with the result obtained by Rachor
(1998) in the nearby Lake Wollingst. The author found a gradual degradation of
47
the light condition from 1932 (about 4.5 m) to 1998 (1.5 m) Secchi depth for the
similar lake type.
4.2 What are the major factors influencing eutrophication of Silver Lake?
Dissolved nitrogen and phosphorous levels frequently exceed the reported
limiting values for an oligotrophic lake type (Fig. 12 and 13). The concentration of
these nutrients tends to be higher in winter resulting from the eutrophication
processes in summer and a general weak pH (e.g. Dierssen 1981). The relative
concentrations of nutrients (N and P) in aquatic systems have been suggested to
control primary producer community structure and biomass (e.g., Smith 1986,
McCauley et al. 1989, Elser et al. 1990, Downing and McCauley 1992, Urabe
1993). Algae typically grow well at N : P-rates near 16 : 1 (Redfield 1958). N and
P values of harvested algal biomass are within the range of 6–9% and 1–2%,
respectively, of nutrient-rich freshwater (Adey and Loveland 1998). Absence of
NO3-N in the deep station in summer (Fig. 15) is well explained by the fact that
during the summer, surface water becomes warmer than the bottom water. As a
result a thermocline with a steep temperature gradient is created; and O 2 cannot
enter into the deep water. Therefore, O2 is depleted and denitrification starts; and
NO3-N is converted to N2O, NO and finally N2 gas or NH4. But; in winter NO3-N is
higher than in summer because stratification is eroded in the late autumn time by
cooling, strong wind and rainfall. On the other hand, in shallow areas such as at
the angling pier or the DLRG station, NO3-N concentration is much higher in
summer than at the deep station due to the presence of O2.
The C:N and N:P ratios found increasing with increasing sediment depths (Table
1) can be an outcome of nutrient load from the dead phytoplankton. Fresh
48
organic matter always deposits in the upper layer of the sediment, decomposes
and mineralization of organic matter with organic nitrogen takes place; the soil
microorganisms consume the mineralized nitrogen for their sustenance (Sharma
and Biswas 2006). So, the share of nitrogen is higher in the upper layer and as a
result C:N and N:P rations are low in the upper layer of the sediment. On the
other hand humus materials with increased C shares increase with sediment
depth, and the percentage of nitrogen is relatively low in deeper sediment layers.
The increase in the C:N ratio with increasing depth is expected because of
bacterial preference for degradation of N-rich compounds (Fenchel et al. 1998).
In line, newly deposited sediment contains easily degradable material from algae
and has a C:N ratio of 6–8 (Meyers and Ishiwatari 1993, Meyers and Teranes
2001, Talbot 2001), and when this material is degraded, the relative influence of
more resistant terrestrial and decomposed material with a higher ratio becomes
larger, leading to a higher C:N ratio of the sediment with depth.
4.3 What is the current status of the plants of Silver Lake?
Concerning the nutrient level, the originally oligotrophic Silver Lake was
characterized by the indicator plant community of Littorelletea species (Isoetes
lacustris, Littorella uniflora, Lobelia dortmanna), adapted to the litoral habitat.
Some of them need a terrestrial phase to fulfill their full generative cycle
(Littorella uniflora). Lobelia dortmanna are reported to set flowers and fruits
mainly in a submerged status (Dierssen 1981). But, this species has already
disappeared from Silver Lake, presumably caused by the pressure of the reed
swamp species, of benthic algae as well as by the turbidity load during the
eutrophication (E. Rachor, pers. comm.) and by overgrowing Sphagnum-species
49
across the littoral areas of the lake (own investigation). Under these influences
Isoetes lacustris suffers, too. It is to be expected that this last occurrence of
Isoetes lacustris in Lower Saxonia will vanish, if no deep-water (1-3 m) subpopulations can be re-established in the near future (E. Rachor, pers. comm.).
The study confirms the presence of a considerable number of ―new plant
associations‖ (Table 2) that may be adapted to the present higher nutrient level
of the lake and succeed in the course of eutropication, whereas the above
mentioned indicator species are gradually decreasing (Buchwald and Hilbich
2008, Rachor 1998). It is also reported that the numbers of isoetids decreased
drastically; and the abundances of species inhabiting eutrophic environments
increased in many lakes in the last century (B. Van Geel, pers. comm.).
Eutrophication and the decline of isoetid species of similar lake types in Germany
and the Netherlands were observed in the last century (Schoff-Van Pelt 1973,
Westhoff 1979, Wittig 1982, Arts 1990).
The result also point towards the changes of the hydrodynamic (e.g.
seasonal water level, Fig. 21) and unfavourable conditions (temperature, light,
nutrients) for the germination and seedlings of the indicator plant species (e.g.
Wittig 1982, Schoff-Van Pelt 1973).
50
4.4 Does the filter-feeding effect of A. cygnea enable to shift the
eutrophication status of Silver Lake?
The filter feeding Swan mussel (―Teichmuschel‖) Anodonta cygnea (L.) inhabits
large ponds, lakes and slow moving waters, such as canals and (small) rivers
with muddy bottoms. Its distribution is limited down to 10 m depth. Highest
abundances (> 10 ind./m2) are common between 2.5 and 6 m depth in southern
Germany (Patzner et al. 1993). Taking in account its ecology and biological
requirements, the average (mean) oxygen consumption of this bivalve as
measured during this study is 3.071 mg d-1 at 10°C (Fig. 24) (Mean shell length
123.2 mm and shell free dry mass 6.1 g, Table 3). But, it should be noted that
this bivalve can tolerate a wide range of temperatures and oxygen conditions
(Müller and Patzner 1996). Therefore, in terms of temperature and other physical
factors, which influence the average oxygen requirement of this bivalve, the
current in vitro experiments (Fig. 24) may be easily used in the Silver Lake. A.
cygnea prefers nutrient rich waters and is one of the most common freshwater
bivalve species widespread across central Europe and the United Kingdom. They
burrow into the substrate, normally with just the siphon tips exposed to filter
particles from the water (Müller and Patzner 1996). The results of this study show
that A. cygnea cannot ingest a significant amount of Chl-a (47.5 µg ind-1d-1)
compared to the lake conditions. Here I want to show a rough calculation that is
shown in Table 4.
51
Table 4 Rough calculation of Chl-a content of Silver Lake (Total water surface
area = 6.3 ha)
Criteria
Average
Area
depth
covered
[ha]
Volume
[m3]
i) From
shore to
3m depth
1.5m
30000
ii) From
3m depth
to rest
3.0m
2.0
4.3
Total
volume
(i +ii) m3
Average conc.
of Chl-a
[mg/m3]
Total Chl-a
present in
Silver Lake
[g]
159000
14.3
2273.7
129000
Total volume of the lake surface water down to 3 m depth is 159000 m 3 (Area
from shore to 3 m depth is 2 ha and assuming that average depth is 1.5 m
(<30.000 m3), area of the rest part of the lake is 4.3 ha and assuming that
phytoplankton can grow down to 3 m depth, so, the total volume is 30000 m 3 +
129000 m3 = 159000 m3). Wetzel (1983) shows that the average concentration of
Chl-a in an eutrophicatic lake is 14.3 mg/m3. So, total Chl-a present in Silver
Lake surface water may be 2273.7 g. If we release 5000 A. cygnea then they can
ingest 42.75 g Chl-a during 6 month. So they can ingest only 1.88% of the total
Chl-a present in Silver Lake, which is not a significant amount. Therefore the
study presumes that the filter feeding effects have not the sufficient potential for
reducing the relatively high nutrient load under the present conditions.
4.5 Conclusions and proposals for restoration actions
From the discussion of water qualities, nutrient loading to the habitat, and the
environmental conditions observed, the eutrophication apparently arise from a
variety of natural and anthropogenic influences. Presumably the information
52
presented here suggests that some restoration of the naturally oligotrophic lake
with its typical relict plant species can be achieved if it is managed in a well
integrated system. That includes man awareness and biological methods to
reduce the undesired nutrient load of the lake.
Direct precipitation, surface and plant evaporation are the principal source
and loss of lake water respectively. There is no direct strong inflow to the lake.
Thus, water level increases during the winter season and decreases during the
summer season. The main external sources of nutrients to the lake may be
surface runoff from the adjacent agricultural land, holiday lodging area and the
surrounding area of the lake and inputs via the atmosphere. The lake and its
surrounding area is a nature reserve, but still now it‘s used for holiday lodging,
bathing and angling. Such kind of activities increases the nutrient concentrations
and also disturbs the habitats of Littorelletea communities in the shallow area.
At the end of summer, leaves from trees from embankments add few
nutrients by decomposition. According to the present results it is clear that the
nutrient concentration has been increasing in Silver Lake. It‘s also clear that
nitrogen and phosphorus concentrations are above the limits for an oligotrophic
lake. As a result, the light dependent Isoetes lacustris and Littorella uniflora are
declining, which are much endangered species in Germany. Typha spp. grow
very densely in many sections of the shore and outcompete Isoetes lacustris.
The present study concludes that despite the nutrient load and apparently
eutrophicated water, however, it is still possible to measure the detailed
eclological end biological conditions and further restore the lake as natural
conservator to save the traditional ecosystem. To manage a good water
53
condition for a typical northern German lake, the proposals for restoration actions
are as follows (not further considering the use of Anodonta cygnea):
Pumping/sucking sediment
Carbon and nutrient contents are higher in the deep area, and accumulated
sediment thickness including nutrients tends to increase in Silver Lake. Removal
of accumulated sediments and nutrients from the lake bottom can increase the
depth by taking out the sediment with a suction pipe (or dredging) and at the
same time remove nutrients. This may also influence rooted aquatic vegetation,
deepen the water body, increase the lake volume and improve the water quality,
and reduce and control especially phosphorus levels in the contaminated
sediments.
Harvest macrophyte populations
Harvesting specific macrophyte populations from/around the lake, e.g Typha
angustifolia,
Nuphar
luteaum,
Phragmites
australis,
Potamogeton
spp.,
Nymphaea alba and others may be a choice. Typha angustifolia is very abundant
in the north-east to south-east part of Silver Lake. Its roots and rhizomes take up
nutrients from the surrounding area and store them. When their roots decompose
they also increase the nutrient level in the lake again. It is suggested to remove
them and other species like Nuphar luteaum, Phragmites australis and
Nymphaea alba with their roots in late summer; this will reduce the internal
productivity of the water body and remove phosphorus that is stored in the plant.
54
Need to be aware of bathing and holiday logging people
Every summer bathing and holiday logging people come for recreation. They also
increase the nutrient level in the lake in the following ways
-
through excess food and any other organic waste materials into the lake,
-
cleaning of the skin in the water,
-
urine (from children) can add ammonia etc. in the lake system,
-
destruction of plants.
So it is essential to setup some regulations concerning the awareness for bathing
and holiday people on a bill board, e.g., not to throw food or waste materials into
the lake and not to destroy littoral plants
Need to be aware of angling people
Angling people release different fish of fingerling size such as pike, (Esox lucius
―Hecht‖), pike-perch, (Lucioperca sandra, ―Zander‖), perch, (Perca fluviatilis,
“Barsch‖), Carps (Cyprinus carpio, ―Karpfen‖) etc. in the lake every year. Among
them pike, pike-perch and perch are top predators. They feed other fish, which
consume zooplankton. Thus zooplankton may increase and better control the
phytoplankton, Carps are polyphagous and they feed or at least destroy Littorella
uniflora, Isoetes lacustris from the shore area. Anglers import excess food for fish
bait.
Thus, the Silver Lake authority is suggested to set up the following rules and
regulations of the angling people:
-
allow to release some selected fingerlings such as pike, pike-perch and
perch but not allow to release carps
55
-
not allow to throw excess fish food into the lake
-
to prohibit to damage the vegetation e.g. by trampling or rowing boat
Hypolimnic aeration
Oxygen (or air) may be pumped into the deep water (oxygen depleted layers)
during summer to maintain oxygen in this layer to limit phosphorus release from
sediments and generally improve the water conditions. A windmill or photovoltaic
elements may be used to drive the pump.
Decidious trees and plant materials
To cut down the decidious trees from most of the bank area and remove live and
dead material from the water edge. When leaves and other plant materials
decompose, they also add nutrients into the lake.
Controlling fertilizer use
Farmers may use high doses of fertilizer and pesticides on their land, which is
almost adjacent to the lake. So, it is recommended to control the fertilizer and
pesticides use by the farmers up to 150 m from the lake. The authority may need
to pay them a compensation for not use or reduce fertilizers.
Finally, it is recommended to continue and expand the research on the
Lake e.g. to better understand water renewal, the influences of littoral plants on
the nutrient cycling, the competitive role of algae as well as the overgrowth ,
possible dystrophication effects from the adjacent raised bog, role of top predator
fish on the nutrient regime.
56
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