Lake Belau Research Articles

The quick death of a lake: human impact on Lake Tresssee (N Germany) during the last 6000 years – an approach using pollen, Cladocera and sedimentology

Human-induced fluctuation of lake levels has been a common phenomenon in Europe since Neolithic times. At present, Lake Tresssee is a eutrophic lake covering less than 5 ha in northern Germany, but its sudden shrinking from ~125 ha before 1800 is considered a consequence of anthropogenic lowering of the lake level. We investigated the history of anthropogenic vegetation changes and water level fluctuations by multiproxy studies of a 4 m core from the former lake area. Our analyses of pollen and Cladocera subfossil, chemistry and sedimentological features yielded important conclusions about interactions between land-use history and climate impacts on the lake and its surroundings. The results indicate that the highest lake level persisted until the Late Atlantic. Since the Subboreal there have been several fluctuations, mostly in consequence of climate impacts. Later, different phases of sediment input to the lake from tributary streams and probably also from aeolian processes from an adjacent dune field were observed. At ~2800 BC the sedimentation rate decreased in consequence of fluvial impacts, as the lake basin was nearly filled up with deposits. As a result of greater human impacts, from the Early Bronze Age the macrophyte zone expanded in the lake, the oxygen content of the water continuously decreased, and heathlands developed in the surroundings. From the Late Iron Age and in the Early Medieval, pollutants probably from ironworks are detectable by geochemical analyses in the corresponding segments. In the pollen diagram the Migration Period is clearly visible, but the suggested radiocarbon date is younger than in Lake Belau in the neighboring region of Schleswig-Holstein. Most probably our pollen diagram did not register the absolute maximum values of Fagus related to the Migration Period. From the Early Medieval a clear phase of resettlement occurs. Since the Early Modern period, the lake level has shrunk rapidly in consequence of historically documented human activity.

Retrospective monitoring of mercury in fish from selected European freshwater and estuary sites

Levels and trends of total mercury (THg) and methylmercury (MeHg) compounds in bream (Abramis brama) from different European sites were compared. Bream were collected between 2007 and 2013 in the estuaries of the rivers Scheldt (Netherlands), Rhône (France), Göta älv (Sweden), Tees (United Kingdom), and Mersey (UK), and in Lake Belau (Germany). A direct mercury analyzer was used to determine THg concentrations while MeHg was measured by gas chromatography/inductively coupled plasma-mass spectrometry applying stable isotope dilution. THg and MeHg in annual pool samples of bream ranged between 15.9 and 251μgkg−1 wet weight (ww) with lowest concentrations found at the reference site Lake Belau and highest in samples from the river Rhône. The EU environmental quality standard (EQS) of 20μgkg−1 ww was exceeded at all sites and in all years except at Lake Belau in 2012. Significantly decreasing trends over time were observed only in bream from the Rhône, while THg increased in bream from the Western Scheldt. The MeHg fractions of THg were always >80% and a significant difference between sites was detected only in one case (Rhône vs. Lake Belau).

Lake Belau and its catchment (northern Germany): A key archive of environmental history in northern central Europe since the onset of agriculture

The environmental history since the onset of agriculture was reconstructed from sediments and soils in the catchment area of Lake Belau (Schleswig-Holstein, northern Germany). The established chronologies are based on varve counts, radiocarbon data, embedded Icelandic tephra (lake sediments), radiocarbon dates, and embedded artifacts (slope deposits). Neolithic land use triggered small-scale erosion. Lake trophic indicators and pollen data clearly reflect the onset of agriculture and the middle Neolithic expansion of agricultural activity. Despite numerous archaeological findings, less intensive field use and no soil erosion occurred during the older and younger Bronze Age. Perhaps, animal husbandry was more important than cereal cultivation. Between 2000 and 500 cal. bc, Aeolian input from a distant source can be observed. During the late Bronze Age and Pre-Roman Iron Age, intensified field use enabled severe soil erosion (gullies c. 200 cal. bc). The degradation of soils started in this period, influenced by excessive land use, climatic variability, and/or the reduction in Aeolian deposition. The Roman Iron Age and Migration Period were phases of reduced human impact and soil formation. During medieval times, the intensity of field use increased again and another phase of soil erosion (gully in the 14th century, probably reflecting the ‘Magdalenenflut’ 1342) and soil degradation started. After the late medieval crisis caused by the ‘black death’ and several armed conflicts, another increase in soil erosion may reflect the rearrangement of estates (18th−19th century) and introduction of industrial agriculture (20th century). A recurrence of high-intensity erosion events is indicated at c. 1540, 1710, and probably 1939 ad. Whereas the lake trophic status is influenced by human impact on the lake catchment, wetter and warmer climatic phases are reflected in increased carbonate precipitation modulated by solar activity, human impact, and the long-term evolution of vegetation and soils in the lake catchment.

Spatial and temporal trends of metals and arsenic in German freshwater compartments

Cadmium, lead, mercury, copper, nickel, zinc, and arsenic were analyzed in suspended particulate matter (SPM), zebra mussels, and bream sampled yearly under the program of the German Environmental Specimen Bank (ESB) in the rivers Rhine, Elbe, Danube, Saar, Mulde, and Saale and in Lake Belau. Temporal and spatial trends were analyzed, correlations between metal levels in different specimen types assessed, and sampling sites ranked according to their metal levels by calculating a Multi-Metal Index (MMI) for every specimen type and site. SPM: Highest metal loads were detected in Mulde, Saale, and Elbe right downstream of the Saale confluence. In the Elbe, metal loads in SPM were mostly highest in the upper and middle section of the river while in Rhine and Saar concentrations increased downstream. Temporal trends since 2005 were detected only at three sites. Zebra mussel: MMIs were highest in the tidal section of the Elbe and the lower Rhine and lowest in Lake Belau and the upper Danube. Different temporal trends were detected since the early 1990s depending on site and metal. Bream: As, Pb, Cu, and Hg were analyzed in muscle tissue and Pb, Cd, Cu, and Zn in liver. For both tissues, MMIs were highest in Mulde and Saale and the lower and middle Elbe. Since the early 1990s, Hg, Pb, and Cu decreased in bream muscle at many sites while As increased at 6 of the 17 sites. The findings indicate that Hg, Pb, and Cu have obviously decreased in many freshwater ecosystems in recent years, whereas As and Ni levels have increased at several sites. Metal levels and temporal trends mostly differed between the specimen types under investigation and only few correlations between specimen types were detected. This underlines the importance of including different components of an ecosystem when assessing its environmental quality.

Evidence of the North Atlantic Oscillation in varve composition and diatom assemblages from recent, annually laminated sediments of Lake Belau, northern Germany

The recent sediment record of Lake Belau (Schleswig–Holstein, Germany), deposited in the period 1945–2002, was compared with instrumental meteorological and limnological data. The sediments deposited during this period are annually laminated. A varve chronology was established and supported by 137Cs measurements. Micro-facies and diatom assemblage composition analyses were confirmed in thin sections and compared statistically with limnological and meteorological data. Comparison of phytoplankton data with diatom assemblage data from the sediment for the time interval from 1988 to 1999 proved that the sediments reflect limnological processes in the lake and record seasonal changes in the primary producer communities. Among the climatological data, the number of contiguous ice days (days with maximal temperatures ≤0 °C) and the state of the winter NAO are strong predictors for micro-facies development and diatom assemblage composition. Furthermore, solar and local (nutrient input) influences are visible in the diatom assemblage compositions. Our study illustrates the high potential for using analyses of micro-facies and diatom assemblages to reconstruct past weather conditions in varved sediments of Lake Belau.

High-resolution reconstruction of climate and environ-mental history during Funnel-Beaker Culture Time based on annually laminated lake sediments of Lake Belau and Lake Poggensee (northern Germany)

High-resolution reconstruction of climate and environ-mental history during Funnel-Beaker Culture Time based on annually laminated lake sediments of Lake Belau and Lake Poggensee (northern Germany)

A high-quality annually laminated sequence from Lake Belau, Northern Germany: Revised chronology and its implications for palynological and tephrochronological studies

The annually laminated record of Lake Belau offers an exceptional opportunity to investigate with high temporal resolution Holocene environmental change, aspects of climate history and human impact on the landscape. A new chronology based on varve counts, 14C-datings and heavy metal history has been established, covering the last 9400 years. Based on multiple varve counting on two core sequences, the easily countable laminated section spans about 7850 varve years (modelled age range c. 9430 to 1630 cal. BP). Not all of the record is of the same quality but approximately 69% of the varves sequence is classified to be of high quality and only c. 5% of low quality. The new chronology suggests dates generally c. 260 years older than previously assumed for the laminated section of the record. The implications for the vegetation and land-use history of the region as well as revised datings for pollen stratigraphical events are discussed. Tephra analysis allowed the identification of several cryptotephra layers. New dates for volcanic eruptions are presented for the Lairg B event ( c. 6848 cal. BP, 2s range 6930–6713 cal. BP), the Hekla 4 event ( c. 4396 cal. BP, 2s range 4417–4266 cal. BP), and Hekla 3 eruption ( c. 3095 cal. BP, 2s range 3120–3068 cal. BP).

Mercury levels and trends (1993–2009) in bream ( Abramis brama L.) and zebra mussels ( Dreissena polymorpha) from German surface waters

Mercury concentrations have been analysed in bream ( Abramis brama L.) and zebra mussels ( Dreissena polymorpha) collected at 17 freshwater sites in Germany from 1993–2009 and 1994–2009, respectively, within the German Environmental Specimen programme. Mercury concentrations in bream ranged from 21 to 881 ng g −1 wet weight with lowest concentrations found at the reference site Lake Belau and highest in fish from the river Elbe and its tributaries. Statistical analysis revealed site-specific differences and significant decreasing temporal trends in mercury concentrations at most of the sampling sites. The decrease in mercury levels in bream was most pronounced in fish from the river Elbe and its tributary Mulde, while in fish from the river Saale mercury levels increased. Temporal trends seem to level off in recent years. Mercury concentrations in zebra mussels were much lower than those in bream according to their lower trophic position and varied by one order of magnitude from 4.1 to 42 ng g −1 wet weight (33–336 ng g −1 dry weight). For zebra mussels, trend analyses were performed for seven sampling sites at the rivers Saar and Elbe of which three showed significant downward trends. There was a significant correlation of the geometric mean concentrations in bream and zebra mussel over the entire study period at each sampling site (Pearson’s correlation coefficient = 0.892, p = 0.00002). A comparison of the concentrations in bream with the environmental quality standard (EQS) of 20 ng g −1 wet weight set for mercury in biota by the EU showed that not a single result was in compliance with this limit value, not even those from the reference site. Current mercury levels in bream from German rivers exceed the EQS by a factor 4.5–20. Thus, piscivorous top predators are still at risk of secondary poisoning by mercury exposure via the food chain. It was suggested focusing monitoring of mercury in forage fish (trophic level 3 or 4) for compliance checking with the EQS for biota and considering the age dependency of mercury concentrations in fish in the monitoring strategy.

Monitoring of hexabromocyclododecane diastereomers in fish from European freshwaters and estuaries

Hexabromocyclododecane (HBCD) is a brominated flame retardant used mainly in polystyrene foam as well as in textile applications. In recent years, measures were taken to reduce HBCD emissions during its production and use. To evaluate the efficacy of these measures, a monitoring project was initiated with fish as bioaccumulation indicators. Since 2007 bream was sampled annually at several European freshwater sites applying standardized methods. Muscle samples were analyzed by a validated liquid chromatography-tandem mass spectrometry (LC-MS/MS) method which allows the quantification of α-, β-, and γ-diastereomers of HBCD. In most samples, α-HBCD was predominant. The lowest HBCD levels (11 μg/kg lipid weight (lw), sum of three HBCD diastereomers) were detected in bream sampled in 2009 from Lake Belau which is situated in a rural area of Northern Germany. During the 4-year study period, statistical significant decreases of HBCD concentrations were detected in bream from the rivers Rhone (France; -85%, level 2010, 205 μg/kg lw) and Western Scheldt (the Netherlands; -60%, level 2010, 36 μg/kg lw). High HBCD concentrations (9,480-14,500 μg/kg lw) without a clear time trend were observed in bream from the river Tees (UK). These high levels are in line with published data for fish from contaminated regions. Overall, the results indicate that environmental burdens of HBCD are decreasing at those locations where point sources and more diffuse emissions are expected.

Retrospective monitoring of organotin compounds in freshwater fish from 1988 to 2003: Results from the German environmental specimen bank

In archived samples from the German environmental specimen bank (ESB) organotin compounds including tributyltin (TBT) and triphenyltin (TPT) as well as their potential degradation products were quantified. Muscles of bream ( Abramis brama) sampled in the period 1993–2003 from the rivers Rhine, Elbe, Saale, Mulde, Saar, and from Lake Belau (period 1988–2003) were analyzed by gas chromatography/atomic emission detection-coupling after extraction and derivatization. TBT was detected in nearly all samples and a decrease in levels was observed at all sampling sites. At most sites, the reduction seemed to be a result of the ban on the use of TBT-based antifoulants for the application on small boats, which became effective in Germany in 1989. Highest TBT levels were found in fish from the Elbe near Blankenese (470 ng TBT cation per g fresh weight; in 1995) and lowest in bream from Lake Belau (<1 ng g −1; in 2001 and 2003). Highest TPT levels (253 ng g −1 in 1993) were also found in bream caught near Blankenese where the occurrence seemed to be correlated to the former use of TPT as co-toxicant in antifoulants. At other sites TPT levels seemed to be correlated to its use as fungicide (e.g. 9 ± 2 ng g −1 in bream from Lake Belau in 2001).

A spatiotemporal individual-based fish model to investigate emergent properties at the organismal and the population level

A spatiotemporal individual-based model (IBM) of roach ( Rutilus rutilus) including bioenergetic principles was used to study emergent properties at the individual and the population level which appeared as a result of resource allocation and time scheduling of activities related to maintenance, growth, and reproduction in a seasonally changing environment. The model was used as a virtual laboratory to consider the impact of various parameters on vital rates and spatial behaviour of roach, which is particularly difficult to study in the field. The parameterisation based on field studies performed at Lake Belau (Schleswig-Holstein, Germany) and laboratory measurements. This allowed us to explore the relationship between population structure and environmental heterogeneity on an integrated base. At the individual level emergent properties such as spatial behaviour, growth, and food consumption could be identified. Emergent properties at the population level resulted as phenotypic consequences of a trophic bottleneck, the influence of lake morphology on the phenotype as well as size-dependent winter mortality rates and post-reproductive mortality rates.

Effects of the flow field on small scale phytoplankton patchiness

We present an example of small scale (10–100 m) horizontal, subsurface patchiness of phytoplankton (Ceratium) during an intensive bloom in August 1993 and link it to the flow field. In the small Lake Belau (1.1 km2) in northern Germany large areas of the water surface are sheltered from wind, due to vegetation. Wind sheltering effects decrease with wind speed and below 2 m/s a spatially homogeneous wind field is observed. Under weak wind conditions near-surface Ceratium patches with local chlorophyll-concentrations up to 200 mg/m3 were observed in the south bay (0.3 m depth) as well as in parts of the central basin (1 m depth). Detailed flow simulations show very good agreement between location and size of current gyres and phytoplankton patches. Inside the gyres we find low flow velocities with low vertical turbulence. This allows Ceratium to form distinct vertical layers with high densities close to the water surface, according to the light gradient. Especially in the south bay flow eddies are determined by the course of the coastline and their location and intensity is, to a large degree, independent of prevailing wind directions.

Is there a linkage between bioaccumulation and the effects of alkylphenols on male breams (Abramis brama)?

There was some evidence from a previous study that estrogenic disruptors, like alkylphenols, could effect fish in the small River Saar of Southwestern Germany. Concentrations of 4NP and 4NP1EO found in breams (Abramis brama) in the Saar River were much higher than those found in other sampling sites of the German Environmental Specimen Bank, including those from sampling sites in the Rivers Elbe, Rhine, Mulde, and Saale and in Lake Belau. We studied the relationship between accumulation and effect using vitellogenin (vtg) and a hepatosomatic index (HSI) of estrogenic effects and by measuring concentrations of AP and APE accumulated in breams caught at six sampling sites in the River Saar and one in the River Mosel. To link these results with those of the previous study we standardized our sampling efforts to obtain comparable data. Elevated vtg levels were found in the breams at all sampling sites near to or downstream of sewage plant discharges, whereas low vtg levels corresponded to sampling sites not influenced by municipal waste water. While HSI values did not correspond to the location of sampling sites, there was a weak but statistically significant correlation to vtg concentrations. Concentrations of four AP and APE were much more lower, as in the previous study, and were neither linked with sewage treatment plant discharges nor correlated with vtg levels. In conclusion, a linkage between accumulation and the effects of AP and APE could not be established, but the relationship between elevated vtg concentrations and municipal waste water, which contains other important endocrine disruptors, was clear.

Retrospective monitoring of alkylphenols and alkylphenol monoethoxylates in aquatic biota from 1985 to 2001: results from the German Environmental Specimen Bank.

Breams (Abramis brama) and zebra mussels (Dreissena polymorpha) from freshwater, and common mussels (Mytilus edulis) from marine ecosystems, archived in the German Environmental Specimen Bankwere analyzed for the presence of 4-nonylphenol (NP), 4-tert-octylphenol (OP), nonylphenol monoethoxylate (NP1EO), and octylphenol monoethoxylate (OP1EO). The samples were collected in the German rivers Elbe, Rhine, and Saar, and in Lake Belau between 1992 and 2001, as well as in the North Sea and Baltic Sea between 1985 and 2001. The main purpose of the study was to investigate the effectiveness of imposed reduction measures regarding the use of alkylphenol ethoxylates. NP1EO and OP were detected in all breams. NP was predominantly above the limit of quantification (LOQ, 2 ng/g; all data on a wet weight basis), and OP1EO was mostly below the LOQ (0.2 ng/g). Maximal concentrations of 112 ng/g NP, 259 ng/g NP1EO, 5.5 ng/g OP, and 2.6 ng/g OP1EO were found in Saar breams from 1994. NP was detected in all zebra mussels from the river Elbe (up to 41 ng/g), whereas in rather few samples OP and NP1EO were found at low levels. OP1EO was not detected in any sample. Concentrations in mussels and breams from the reference site Lake Belau were below the LOQ for all compounds. In marine biota NP was found until 1997 with maximum concentrations up to 9.7 ng/g, whereas NP1EO was detected at levels between 1.7 and 12.9 ng/g in very few samples collected at the end of the 1980s. A tendency of the concentrations to decrease was obvious for all sampling sites; it was most pronounced for NP1EO and NP after 1996/1997. The effectiveness of the reduction measures is most evident at the Saar sampling site Güdingen and the North Sea sampling site Eckwarderhörne.

Nutrient Budgets, Dynamics and Storm Effects in a Eutrophic, Stratified Baltic Lake

AbstractBetween 1989 and 1998 the small eutrophic stratified Lake Belau was investigated intensively and multidisciplinarily. This article is a short, comprehensive summary and re‐evaluation of the hydrochemistry of the lake, with focus on nitrogen and phosphorus. In several aspects the lake can be regarded as a typical example of the glacial north German lakes. The 1960's and 1970's are characterised by heavy nutrient inputs and fast eutrophication. During the last two decades the external nutrient load, especially the phosphorus load into Lake Belau was significantly reduced. But phosphorus‐rich sediments and large areas with summerly anoxic sediment surface conditions cause intensive release of phosphorus from older deeper sediment layers. Annual budgets reveal that despite an average sediment accumulation of 3 mm a−1 the lake has lost its function as net phosphorus sink and it is very likely that internal eutrophication by the sediments will keep the lake in its eutrophic state during the next decades. Despite that, monthly budgets of five vertical layers show that the main phosphorus supplier for the phosphorus depleted epilimnion during summer is the creek Alte Schwentine.The annual nitrogen budget indicates groundwater and interflow water as well as atmospheric input as additional important nitrogen sources. 36% (98 μmol m −2 h−1 N) of all nitrogen input is lost to atmosphere mainly due to denitrification. The example of a heavy storm shows that about 10% of the annual nitrogen loss to the atmosphere can take place during a single day and in form of ammonia. The storm further made obvious that these unpredictable events can have strong impact on nutrient cycling and ecology in Lake Belau and the lake can become an unexpected nutrient source for downstream systems.

Influence of activity in a heterogeneous environment on the dynamics of fish growth: an individual‐based model of roacl

A spatio‐temporal individual‐based model (IBM), including bioenergetic principles, is used to investigate how energy costs coupled to activity represent a variable fraction in a heterogeneous environment and how this is related to another energetic criterion, the potential individual growth, under the conditions of a structured environment. With this approach, it is possible to connect simulated spatio‐temporal activity patterns with the energetic needs required for these activities. By using simple foraging rules and a modified random walk model it is possible to reproduce spatial distributions and length frequency distributions. The simulated spatial distribution of roach Rutilus rutilus in Lake Belau, Germany, results in a mean weekly activity multiplier [(standard metabolic rate + activity costs) (standard metabolic rate)−1] of 1–9 with deviations &gt;100% during a simulated year. These deviations are of key importance to differences in the growth rate of individual simulated roach.

FEUWAnet: a multi-box water level and lateral exchange model for riparian wetlands

Riparian wetlands as typical aquatic-terrestrial interfaces control, in a very specific way, nonpoint water and related chemical fluxes exchanging between catchment areas to their respective water systems (streams, lakes). The existing groundwater and soilwater flow models reveal gaps in dealing with the complex behaviour of processes and the considerable spatial and temporal heterogeneity of riparian wetlands. Based on long-term experience gained through field observations and the interpretation of model produced data, a multi-box aggregation of processes which determines lateral as well as vertical flows and, as a whole, water balance, is used to discretise a generic riparian wetland transect situated between an upland aquifer and a receiving water body. The resulting mathematical model, FEUWAnet, endowed also with an original methodology to adapt parameters, has been applied to a riparian alder wetland adjacent to Lake Belau (northern Germany). Results of simulations illustrate a good fit between calculated water levels and observed values and an accordance of calculated water balance to previous independent evaluations. This confirms that the sound simplifications of real situations performed by the FEUWAnet mathematical model are a promising way to deal with hydrological complexity of riparian zones. Moreover, FEUWAnet permits, to a certain extent, one to unravel the spatial heterogeneity and temporal variation of lateral (from catchment area to water systems) and vertical (from canopy to groundwater zone) water fluxes typical of riparian ecosystems: this is the necessary step to undertake when developing integrated models capable of assessing the effectiveness of riparian systems in controlling the fluxes of nonpoint pollution discharging in the open water bodies.

An individual-based approach to depict the influence of the feeding strategy on the population structure of roach ( Rutilus rutilus L.)

We used an individual-based modelling strategy to simulate the growth of a roach population. Individual growth is based on food utilization, bioenergetics, spatial distribution, and seasonal influences. With this approach, it is possible to connect the animals' activity pattern with the energetic needs required for this activity. This allows investigating the role of spatial heterogeneity and individual variability for the dynamics of fish growth. The parameterisation is based on laboratory measurements and field studies performed at Lake Belau (Schleswig-Holstein, Northern Germany). The interactions of the behavioural repertoire, growth processes, and food preferences emerge in the context of a decreasing proportion of zooplankton in the food composition of simulated roach. Roach feeding on molluscs grow faster. The ontogenetic shift is related to the necessity to switch to larger prey as a consequence of the increase of energetic demands with body size.

Benthic secondary production and biomass of insects emerging from a northern German temperate stream

1. Secondary production and emergence of aquatic insects were examined in the outlet of Lake Belau, Northern Germany, by means of benthic samples and emergence traps. 2. At three stream sections annual larval secondary production varied between 4.9 and 10.8 gDM (dry mass) m -2 year -1 . Insects contributed with 3.4, 8.9, and 8.7% to the total macroinvertebrate production that varied between 56.5 and 215.1 gDM m -2 year -1 . Emerged biomass was between 1.0 and 2.0 gDM m -2 year -1 . At all three stream sections Diptera dominated with a larval production of 3.0-6.1 gDM m -2 year -1 , followed by Trichoptera with 1.0-2.1 and Ephemeroptera with up to 0.9 gDM m -2 year -1 . 3. Average larval production amounted to 9.0 gDM m -2 year -1 and emerged biomass to 1.7 gDM m -2 year -1 . Larvae of insects amounted to 7.0% of total macroinvertebrate production. 4. The ratio of annual emerged biomass to secondary production (E/P) varied among individual taxa. At the stream sections the ratio ranged from 15.9% to 20.1% with an average of 18.3% for the stream. 5. Relative constancy of the E/P ratio suggests that the determination of emerged biomass can be used as a method for estimating the secondary production of aquatic insects. The composition of functional feeding groups clearly differs between emerged biomass and total macroinvertebrate production. Therefore, the method does not allow conclusions on the level of production of the whole benthic community. However, composition of functional feeding groups at emergence roughly reflects composition of these groups in the benthic insect community.

The significance of photoautotrophic picoplankton in the eutrophic Lake Belau (Bornhöveder Seenkette, North Germany)

The significance of photoautotrophic picoplankton (APP) was investigated for three years in the eutrophic Lake Belau. Using epifluorescence microscopy with green excitation, the abundance of APP was found to vary seasonally with maximum values reaching 1.7* 10 6 cells ml −1 in June and July. Above a threshold of 12°C, cell numbers of APP in the epilimnion were strongly correlated with the water temperature (r = 0.837 > r (34;0.001) = 0.526). The horizontal distribution varied by a factor of 1.1 to 3.3. All stations in the different parts of the lake, including the littoral zone, exhibited a similar seasonal pattern. Photoautotrophic picoplankton accounted for up to 54% of the total picoplankton carbon content (bacteria and photoautotrophic picoplankton) and averaged 8% over the entire year. It contributed <1–29% to the total phytoplankton carbon in the pelagic zone and up to 47% in the littoral. Size fractionated primary productivity determinations revealed that APP contributed between 13% and 17% to the total primary production in the pelagic zone during the period of its peak abundance and between 6% and 37% in the littoral zone. The annual averages were 6% and 7%, respectively. These results support the assumption that the average contributions of APP to phytoplankton biomass and primary production decrease as the trophic state of the lake increases. However, the individual timepoints show that APP sometimes contributes significantly to the primary producers in eutrophic systems, particularly in the littoral zone.