Pre-alpine Lake Research Articles

Effects of water-depth and water-level fluctuations on the macroinvertebrate community structure in the littoral zone of Lake Constance

In a 2-year field study, abundance, biomass and community structure of benthic macroinvertebrates and their seasonal dynamics were assessed along the depth gradient in the stony littoral zone of Lake Constance, Central Europe. The macroinvertebrate community patterns differed significantly between the depth zones, partly because of species turnover, but mostly as a result of different dominance structures. These distinct differences should be considered when designing surveys of benthic invertebrates also for applied purposes, since sources of variation should be kept small and particularly on hard substrates, extensive efforts are required to obtain sufficient data for a powerful statistical analysis. A large seasonal variability in the macroinvertebrate communities of the eulittoral zone indicates a strong influence of physical disturbances, particularly of water-level fluctuations. The community pattern of the drift line samples was influenced by the previous development of the water level. The cumulated water-level fluctuations and their net tendency accounted for three quarters of the variation in a PCA. Since Lake Constance is the only large pre-alpine lake whose water level is not extensively regulated, the uniqueness of the littoral community should have implications for nature conservation measures.

Abundance and toxicity of Planktothrix rubescens in the pre-alpine Lake Ammersee, Germany

Regular occurrences of the cyanobacterium Planktothrix rubescens have been observed in several lakes that have undergone recent re-oligotrophication, e.g. Lake Ammersee. Planktothrix species are known to produce microcystins, potent phosphatase inhibitors that have been associated with morbidities and mortalities in humans and animals. The aim of this study was to characterise the temporal and spatial abundance and toxicity of P. rubescens in Lake Ammersee. P. rubescens cell densities and biovolumes were calculated via fluorescence image analyses. P. rubescens was present during the entire observation period from 1999 to 2004, albeit at different cell densities. Maximum biovolumes of 45 cm 3 m −2 were observed in May 2001. Filaments were regularly distributed over the entire water column during winter and stratified in distinct metalimnic layers during summer, reaching maximum cell densities of ≤15,000 (winter) and ≤77,000 cells ml −1 (summer). The results demonstrate that P. rubescens abundance is strongly influenced by water transparency, i.e. illumination in the metalimnion. Moreover, the P. rubescens abundance appears to result from regular phosphate depletion in the epilimnion, possibly additionally benefiting from high nitrogen loads. Microcystin (MC) was detectable in 27 and 38 of 54 seston samples via HPLC and Adda-ELISA measurements, respectively. The main microcystin congeners in the seston samples were [Asp 3]-MC-RR and [Asp 3,Dhb 7]-MC-RR. Microcystin concentrations correlated significantly with the respective phycoerythrin (PE)-concentrations. The variation in the MC/PE-ratios was low suggesting that the microcystin production of P. rubescens in Lake Ammersee is consistent and indicates that the appearance of P. rubescens coincides with measurable microcystin levels. Moreover, the observation of pronounced metalimnic oxygen depletions appears to be causally related to recurring high P. rubescens abundance. In conclusion the results suggest that aquatic organisms such as indigenous fish populations (e.g. coregonids) are regularly confronted with potentially adverse P. rubescens densities, which might provide a possible explanation for the often observed impaired health and growth retardation of coregonid populations in P. rubescens containing pre-alpine lakes.

Determining the Chernobyl impact on sediments of a pre-Alpine lake with a very comprehensive set of data

Man-made and natural radionuclides in Lake Wallersee were determined in the pre-Alpine environment at the northern slope of the Alps, which was heavily affected by the Chernobyl fallout in May 1986. The objective of this study was to get knowledge of location and quantity of man-made radionuclide input (especially 137Cs) generated in the Chernobyl accident to lake sediments. Eleven sediment cores were sampled and activity depth profiles of 137Cs and 210Pb were determined with 5 mm depth-resolution. The Chernobyl fallout produced an extreme 137Cs peak in the sediment cores providing an excellent time marker. The chronological interpretation of deeper sediment layers was done by radiochemical analysis of 90Sr and 239+240Pu, which were released during atmospheric weapons' tests in the 1950s and 1960s. This allowed a complete chronological analysis of the sediment cores with a very compact set of data.

Epidemiology of a Daphnia brood parasite and its implications on host life-history traits

Parasites influence host life-history traits and therefore might crucially shape host populations in natural systems. In a series of laboratory experiments, we studied the impact of an oomycete brood parasite on its Daphnia (waterflea) host. We asked whether Daphnia dump the infected brood and subsequently are able to reproduce again as was occasionally observed in a preliminary study. No viable offspring developed from infected clutches, but 78% of the infected females produced healthy offspring after releasing the infected brood while molting. Neither those offsprings' development success nor their mothers' reproductive potential was affected by the brood parasite. However, infected Daphnia had a reduced life-span and suffered an increased susceptibility to another parasite, an unidentified bacterium. Additionally, we studied the prevalence of this brood parasite and the unidentified bacterium in a natural Daphnia assemblage in a pre-alpine lake, across changing demographic and environmental conditions. The brood parasite epidemic seemed to be host-density dependent. Our results show that the brood parasite's impact on the host population is enhanced when combined with the unidentified bacterium.

Analytical and Functional Characterization of Microcystins [Asp3]MC-RR and [Asp3,Dhb7]MC-RR: Consequences for Risk Assessment?

The microcystin (MC) producing P. rubescens occurs in pre-alpine lakes and may impact fishery success, bathing, and raw water quality. P. rubescens extracts, characterized via LC-MS, contained the two MC-RR variants [Asp3]MC-RR and [Asp3,Dhb7]MC-RR. The protein-phosphatase-inhibition assay (cPPIA with phosphatases 1 and 2A) in its capability to quantify [Asp3]MC-RR, [Asp3,Dhb7]MC-RR, and MC-RR was compared to HPLC-DAD and anti-Adda-ELISA. The IC50 values (PP1 and PP2A) determined for MC-LR, MC-RR, and [Asp3]MC-RR were in the same range (1.9-3.8 and 0.45-0.75 nM). A 50-fold higher concentration of [Asp3,Dhb7]MC-RR (29.8 nM) was necessary to inhibit the PP2A by 50%. The PP1-IC50 of [Asp3,Dhb7]MC-RR was 22-fold higher (56.4 nM) than those of the other MCs, suggesting that specific structural characteristics are responsible for its weaker PPI capacity. Western blots demonstrated that [Asp3,Dhb7]MC-RR does not covalently bind to PP1. [Asp3,Dhb7]MC-RR has comparable in vivo LD50 values to MC-RR, despite a far lower PP-inhibiting capacity, suggesting that toxicodynamic and toxicokinetic characteristics of [Asp3,Dhb7]MC-RR are responsible for its high in vivo toxicity. The data demonstrate that cPPIA analysis of [Asp3,Dhb7]MC-RR-containing samples prevent reliable MC determination and lead to underestimation of potential toxicity.

The influence of eutrophication and population biomass on common whitefish (Coregonus lavaretus) growth — the Lake Constance example revisited

Accelerated growth of freshwater fish during anthropogenic eutrophication has been attributed almost exclusively to the increased nutrient content, while density-dependent effects have been largely neglected. We evaluated the relative importance of these factors by studying the growth of 43 consecutive year classes of common whitefish (Coregonus lavaretus) from Upper Lake Constance. This prealpine lake underwent eutrophication from the 1950s to 1970s, followed by reoligotrophication. Because whitefish are harvested with gill nets in a strongly size-selective way, we used back-calculated lengths of average fast-growing fish to compare growth among cohorts. Standing stock biomass was estimated based upon virtual year-class strengths. Multiple linear regression analysis revealed that growth of whitefish during their second year was most strongly related to standing stock biomass followed by PO4-P content during spring turnover and by calendar year, which was incorporated as a third independent variable (adjusted R2 = 0.84). The negative correlation between whitefish growth rate and calendar year is interpreted as evidence of an evolutionary response to the highly size-selective fishery during at least four decades. We conclude that density-dependent effects on whitefish growth are more important than had been realized previously and that the impact of eutrophication on growth of whitefish needs to be reconsidered.

Physiological stress and pathology in European whitefish ( Coregonus lavaretus) induced by subchronic exposure to environmentally relevant densities of Planktothrix rubescens

Planktothrix rubescens belongs to the most ubiquitous cyanobacterial species in mesotrophic and oligotrophic lakes in the pre-alpine regions. In most of these lakes, coregonids are among the dominant species of the ichthyofauna with great importance for the professional fishery. A possible link between the occurrence of toxic Planktothrix blooms and the recurrent slumps in coregonid yields has been suggested. Indeed, acute toxic effects of microcystins and other cyanobacterial toxins have been shown for various fish species. However, chronic exposure scenarios appear to be more common and thus more environmentally realistic than acute intoxications. The aim of this study was therefore to investigate the physiological stress response and organ pathology in coregonids sub-chronically exposed to ambient water containing low, medium and high P. rubescens densities, known to be typical of pre-alpine lakes. Coregonid hatchlings were exposed in four tanks containing 0 (sham-control) and approximately 1500 (low), 15,000 (medium) and 55,000 (high) P. rubescens cells/ml for up to 28 days. Temperature, oxygen concentration, pH-value, P. rubescens cell density and microcystin concentration were recorded and the fish were observed for behavioural changes and examined for parasite infestations. Gill ventilation rates, general condition factors and mortalities were determined and liver, kidney, gut and gill were assessed histopathologically and immunhistologically. Depending on the cell density, exposed fish showed behavioural changes, including increased ventilation rates possibly representing a physiological stress response. Susceptibility to ectoparasitic infestation and increased mortality in exposed fish suggested P. rubescens associated effects on fish fitness. Histopathological alterations in liver, gastrointestinal tract and kidney, which were also immunopositive for microcystin suggested causality of tissue damage and the presence of microcystins. In contrast, observed gill pathology appeared to result primarily from mechanical abrasion and irritation due to ectoparasitic infestation. The current exposure experiment confirmed the hypothesis that subchronic and chronic exposure to low cyanobacterial cell densities and hence microcystins can exacerbate physiological stress and sustained pathological alterations in exposed coregonids. The study therefore supports the theory that P. rubescens blooms may be causal to the observed weight reduction and hence fitness of coregonids in pre-alpine lakes such as Lake Ammersee (Germany).

Circulation in Stratified Lakes due to Flood-Induced Turbidity Currents

The river inflow in a natural lake with important suspended sediment load during floods, can impact water quality by mobilizing dissolved matters like phosphorous from deep to surface waters. Generally due to thermal stratification in prealpine lakes, the water column is stable. It does not mix vertically unless acted on by outside forces, for example, currents or winds. Since Lake Lugano has a strong thermal stratification, river inflow exhibits different modes of density currents, from surface flows and thermocline intrusion to bottom currents. Turbidity currents are the direct cause of the downward water flow, and at the same time at the origin of upward directed flow. In this study, the impact of river born turbidity currents in Lake Lugano under varying ambient conditions was investigated using field measurements at the inflow river and inside the lake, together with a full three-dimensional numerical model of the entire lake. The paper characterizes the induced circulation of the turbidity plume and gives some indications on the relevance of turbidity currents on the lake.

Seasonal dynamics of pelagic and benthic (littoral and profundal) bacterial abundances and activities in a deep prealpine lake (L. Constance)

In order to obtain a first insight into the characteristics of pelagic, littoral and profundal bacterial communities and their potential role for the degradation of organic matter in the deep prealpine Lake Constance, we compared the seasonal dynamics of bacterial abundances and activities during the annual cycle in the epilim- netic water of a central station as well as in surface layers of sediments at a central pro- fundal and a littoral site. For this purpose, bacterial abundances (DAPI counts), rates of leucine incorporation, respiration of 14 C-labeled substrates (glucose, phenol) and ectoenzymatic activities were measured biweekly-monthly at the different sampling sites. Bacterial densities in lake sediments generally exceeded those of epilimnetic waters by at least two orders of magnitude with higher values in profundal sediments. A seasonal pattern was well expressed for bacterioplankton, less pronounced for ben- thic littoral bacteria and hardly visible for profundal bacteria. Compared to pelagic bacteria, benthic bacteria exhibited lower specific rates of leucine incorporation, espe- cially during the warmer periods. For profundal benthic communities specific rates of glucose respiration and of enzyme activities were permanently low with little seasonal fluctuation. In contrast, littoral benthic communities showed the highest values for these specific metabolic activities among the three sites tested. Additionally, the capac- ity of metabolism of phenol (used here as a representative of more recalcitrant aro- matic substances) was by far highest respired for littoral communities, and almost lacking for pelagic communities. Evidence from the literature suggests that the qualita- tively and quantitatively enhanced level of degradation capacities observed for littoral bacteriobenthos may be predominantly a result of the combined effect of increased supply of additional organic matter (e. g. allochthonous matter and macrophytes), an enriched bacterial gene reservoir in sediments, and frequent resuspension. Littoral ben- thic communities showed the highest specific metabolic activities exceeding those of bacterioplankton and profundal bacteriobenthos by an order of magnitude. As a whole,

Stoichiometric mismatch between littoral invertebrates and their periphyton food

Abstract: Ecological stoichiometry is considered a key concept in understanding con-straints in energy transfer at the plant-herbivore interface. However, whether this con-cept is relevant for benthic freshwater ecosystems is not fully known. Therefore, afield survey was conducted in 2003 during the growing season in the littoral zone ofLake Constance, a large pre-alpine lake in central Europe. The aim was to assess tem-poral variation in the elemental stoichiometric composition in both herbivorous macro-invertebrates and their food resource, the periphyton in two different lakes. The peri-phyton showed large temporal and spatial variation in carbon, nitrogen, and phospho-rus content, with particularly high molar C:P ratios of up to 1225:1. Periphyton C:Pand C:N ratios were often high and constantly above the Redfield ratio that is consid-ered optimal for autotrophic growth. In contrast to the pronounced fluctuations in thenutrient ratios of their food resource, the herbivorous macroinvertebrates showed onlyvery little variation in their nutrient ratios, which indicated that they are homeostatic,i.e., physiologically restricted to a comparatively narrow range of C:P and C:N ratios.Distinct species-specific C : P and C : N ratios were found for different taxonomicgroups of macroinvertebrates, which indicated different requirements of optimal die-tary C:P and C:N ratios and which might influence the ability of the taxa to competefor limiting elemental nutrients. Considering the temporally very high C:P and C:Nratios of the periphytic resource and the very low ratios of the consumer body tissue,this stoichiometric mismatch is likely to constrain growth and reproduction of these lit-toral invertebrates. Therefore, the concept of stoichiometric food quality limitationmight also be applicable to the littoral food web in lakes.Key words: benthos, C:N:P ratio, ecological stoichiometry, herbivory, homeostasis,lake littoral, macroinvertebrates, mismatch, nutrient ratios, phosphorus.

Behavioral Responses of Wintering Great Crested Grebes to Dissuasion Experiments: Implications for Management

Great Crested Grebes (Podiceps cristatus) wintering on prealpine lakes of north Italy are considered to cause economic damage to fish, reducing the stock of native Bleak (Alburnus alburnus). We experimentally investigated the effectiveness of three deterring actions (gas-cannon, crackers and shooting) in four study sites at Lake Como, by comparing the time-budget and space-use of wintering grebes in absence and presence of disturbance. All dissuasion methods affected grebe behavior and position on the lake, but some resulted more effective than others. The gas-cannon detonations had little effect, but, crackers and shooting caused significant changes in grebe behavior. Grebes spent less time feeding, resting and preening, but more time moving/swimming during dissuasion than in the control situation. Moreover, birds also moved away from the areas with bleak shoals, resulting in a significant decrease in grebe numbers with either dissuasion methods. Although dissuasion reduced feeding of grebes in areas with shoals of Bleak (Alburnus alburnus), and probably reduced grebe predation on immature fish at localized areas, it involves intensive effort, in terms of man-days.

Studying the spatiotemporal variation of the littoral fish community in a large prealpine lake, using self-organizing mapping

One of the most fundamental feature of freshwater systems is the spatiotemporal structure of their communities. In the present study, we used an artificial neural network model, i.e., self-organizing mapping, together with a likelihood ratio χ2statistic for proportions to investigate the influence of each factor of a complex sampling scheme (i.e., site, year, month, and time of day) on the littoral fish community of Lake Constance (south Germany). Based on self-organizing mapping, four clusters of samples were defined characterized by distinct fish communities. The samples gathered in clusters 1 and 2 were significantly related to the factors month and time of the day, while those in cluster 3 were related to the factors month and site and those in cluster 4 to each of the four factors. The results are discussed with regard to the temporal patterns of species succession in lakes and their similarities with the spatial patterns observable in streams, the importance of plasticity with regard to the fish nycthemeral preferences, the partitioning of habitat at a large spatial scale and its importance for the coexistence of species, and the effects of the reoligotro phica tion process in lakes.

Zooplankton communities in a large prealpine lake, Lake Constance: comparison between the Upper and the Lower Lake

The zooplankton communities of two basins of a large lake, Lake Constance, were compared during the years 2002 and 2003. The two basins differ in morphology, physical and chemical conditions. The Upper Lake basin has a surface area of 470 km2, a mean depth of 100 and a maximum depth of 250 m; the Lower Lake basin has a surface area of 62 km2, a mean depth of only 13 and a maximum depth of 40 m. Nutrient, chlorophyll-a concentrations and mean temperatures are somewhat higher in the Lower than in the Upper Lake. Total abundance of rotifers (number per m2 lake surface) was higher and rotifer development started earlier in the year in the Lower than in the Upper Lake. Total abundance of crustaceans was higher in the Upper Lake in the year 2002; in the year 2003 no difference in abundance could be detected between the lake basins, although in summer crustacean abundance was higher in the Lower than in the Upper Lake. Crustacean communities differed significantly between lake basins while there was no apparent difference in rotifer communities. In the Lower Lake small crustaceans, like Bosmina spp., Ceriodaphnia pulchella and Thermocyclops oithonoides prevailed. Abundance (number per m2 lake surface) of predatory cladocerans, large daphnids and large copepods was much lower in the Lower than in the Upper Lake, in particular during the summer months. Ordination with nonmetric multidimensional scaling (NMS) separated communities of both lakes along gradients that correlated with temperature and chlorophyll a concentration. Clutches of copepods were larger in the Lower than in the Upper Lake. No difference could be detected in clutch size of large daphnids between lake basins. Our results show that zooplankton communities in different basins of Lake Constance can be very different. They further suggest that the lack of large crustaceans in particular the lack of large predatory cladocerans in the Lower Lake can have negative effects on growth and reproduction of zooplanktivorous European whitefish, Coregonus lavaretus, which feeds highly selectively on large cladocerans and which is of great economic significance for the whole region. Another possibility could be that the lack of large Cladocera in the Lower Lake is a result of strong fish predation which could be a consequence of lake morphology.

The impact of predation by burbot (Lota lota L.) on the macroinvertebrate community in the littoral zone of a large lake

We studied the effects of predation by juvenile burbot (Lota lota) on the macroinvertebrate community in mesocosm experiments in the stony littoral zone of Lake Constance, a large prealpine lake in Central Europe. Although the growth data of the burbot suggest that the benthivorous fish exerts strong predation pressure on the invertebrate community, the predicted level of consumption is only poorly reflected by changes in biomass and abundance of most invertebrate prey taxa. High exchange rates of the prey between the mesocosms and the ambient littoral environment apparently masked the true predation effects of fish. Also, life-history events such as hatching or synchronised emergence of larvae led to temporal effects that obscured the impact of predation. However, for the dominant prey organism, the freshwater amphipod Gammarus roeseli, direct lethal effects appeared when its migration was limited. When exchange with the ambient littoral zone was possible, abundance and biomass of G. roeseli were unexpectedly high in the mesocosms stocked with burbot, indicating behavioural responses to fish presence. During the experiment, the burbot gained ca. 15% in length and about 60% in body mass. According to stomach content analyses at the end of the experiment, the ingested prey consisted mainly of relatively large and abundant invertebrates. Our study indicates that predation by juvenile burbot should be an important factor in structuring the benthic invertebrate community in terms of qualitative and quantitative effects.

No Evidence for Adaptive Micro-Evolution to a Decrease in Phosphorus-Loading of a Daphnia Population Inhabiting A Pre-Alpine Lake

Many polluted lakes in Europe are being restored and phosphorus concentrations have dropped dramatically in these lakes. We studied the genetic structure of Daphnia galeata over the past 30 years in Lake Greifensee, Switzerland, a period during which the phosphorus concentration in the lake reduced dramatically. Distinct genotypes of D. galeata were hatched from diapausing eggs extracted from six different time horizons in dated sediments. We compared juvenile growth, size and egg production of D. galeata reared on high-P and low-P algae to investigate whether Daphnia have evolved to grow better on phosphorus-limited algae. Our results indicate that life histories of D. galeata differed significantly between both food types. We also found significant clone effects for size and egg number. But we found no significant interaction between the depth from which the clones were selected and food quality. This means that we found no evidence for adaptive micro-evolution in response to P reduction in the lake. We discuss our results in relation to other studies that found evidence for adaptive micro-evolution in comparable time frames.

Die Auswirkungen der Wasserstände am Bodensee auf das Deschampsietum rhenanae zwischen 1989 und 2003

The Deschampsietum rhenanae is an endemic and endangered lakeshore community growing in the upper littoral of Lake Constance, the occurrence of which has been reduced dramatically during the last 100 years. The existence of this community depends on the seasonal water level changes of the unregulated, prealpine Lake Constance. Long-term monitoring indicates that the typical species of the Deschampsietum rhenanae and their competitors may be strongly affected by extreme water levels. An earlier onset of the water level increase in spring could potentially pose a threat to the Deschampsietum rhenanae, especially if the water level rises before seed ripening in the Deschampsietum rhenanae.

Comparison of production and degradation of organic matter at a littoral site of the prealpine Lake Constance

Biotic structures and processes of the comparatively large and deep Lake Constance are dominated by pelagic water bodies. Therefore, much more attention has thus far been paid to the carbon cycle of pelagic habitats as compared to littoral areas. Nevertheless, there is also an ongoing debate on the importance of littoral areas for the carbon cycle in this lake. Although actually existing data are certainly insufficient for a realistic carbon budget, investigations made during the past decade provide at least a rough basis for a first evaluation. Accordingly, littoral areas appear to contribute overproportionally (related to surface) both to production as well as to degradation of organic carbon in the lake. However, the relative effect seems to be more pronounced for heterotrophic activities, mostly due to intense degradation processes in littoral sediments. Related to volume, littoral water bodies exhibited similar or slightly higher microbial activities as compared to epilimnetic pelagic water bodies, while microbial abundances and activities observed for littoral sediments exceeded the corresponding overlying water layers by one to three orders of magnitude. As was confirmed by in situ measurements microbial oxygen consumption frequently exceeded photosynthetic oxygen production in the investigated littoral sites. Moreover, significant qualitative differences between water and sediment were found for the degradation potentials of xenobiotics as was exemplified for phenol and nitrilo-triacetic acid (NTA). Overall, a buffer function at the landwater interface may be ascribed to littoral areas due to which the large pelagic water bodies of the lake may become additionally protected against loads of undesired organic substances due to high littoral degradation capacities.

Ecological Integrity Assessment of Lakes Using Fish Communities – Suggestions of New Metrics Developed in Two Austrian Prealpine Lakes

AbstractAn ecological assessment method for lakes based on fish communities similar to the IBI‐system was developed and further tested on two Austrian prealpine lakes. Important quality criteria used were species composition, reproductive success, size structure and total fish biomass. The reference status of each lake was established by the reconstruction of the native fish community and the native total fish biomass.In Hallstättersee only minor deviations from the reference status could be observed. Additionally to the 12 native (reconstructed) fish species two exotic species were found inhabiting this lake. There was no change in the number of intolerant fish species and for 10 native fish species reproduction success was confirmed.The fish community of Irrsee differed to a higher extent from the reference status. Three native fish species were missing. Seven exotic species were confirmed by catches and four of these are reproducing in Irrsee. No reproductive success could be verified for five native species. The sum of scores in our proposed scoring system revealed a value of 92 points for Hallstättersee, which corresponds to a “high ecological status” and of 60 points for Irrsee, which corresponds to a “moderate ecological status” of the fish community.

Reconciling the dichotomy between single species and ecosystem conservation: black kites ( Milvus migrans) and eutrophication in pre-Alpine lakes

In recent times there has been a growing dichotomy between preservation of single species and broader, ecosystem-based approaches to conservation. Freshwater habitats are among the most highly human-impacted ecosystems. We used a long-term data set on black kites ( Milvus migrans), a threatened raptor dependent on aquatic habitats, to explore ways to reconcile single species and biodiversity-driven approaches to conservation. In the pre-Alpine lakes of northern Italy, black kite populations showed medium-low density, extremely low breeding success and widespread declines. Spatio-temporal variations showed density and breeding success to be positively related to ecosystem productivity (as estimated by phosphorus concentrations in lakes), availability of aquatic habitats and grassland, and to be negatively related to extent of farmland and fish harvest by professional fishermen. Fish species richness, used as a surrogate of biodiversity, was highest in one oligotrophic lake, but on average increased with increasing ecosystem productivity (i.e. lake eutrophication). Given expected future declines in ecosystem productivity, kite conservation will be helped by enhancing populations of alternative prey in terrestrial habitats (e.g. through incentives for grassland), and higher regulation of fish harvest. On the other hand, ecosystem management may profit from the use of black kites or other aerial piscivores as indicators of biodiversity and of diffuse ecosystem stress, such as sustainability of fishing practices.

Investigation on competitors and predators of herbivorous aquatic Lepidoptera (Acentria ephemerella) on submersed macrophytes in a large prealpine lake

Submersed macrophytes offer a large colonization area for many macroinvertebrates. Numerous studies investigated the interaction between plant species, growth and leaf form and the abundance and species composition of invertebrates (e.g. SOSZKA 1975, KORNIJOW & KAIRESALO 1992). Grazers and detrivores assist the plants by removing epiphyton and allowing more light to reach the photosynthetic tissue. Only recently has the paradigm that aquatic angiosperms are not subjected to a high degree of herbivory been challenged (LODGE 1991, NEWMAN 1991, KORNIJOW 1996, LODGE et al. 1998). A detailed study of the literature reveals that aquatic insects were frequently observed feeding on submersed macrophytes. For example, Acentria ephemerella, a true shredder–herbivore, was found associated with pondweeds and other macrophytes in several studies (BERG 1942, MULLER-LIEBENAU 1956, SOSZKA 1975). Acentria seems to prefer large lakes to small ponds as habitat (JOHNSON et al. 1998). Data on its abundance in different lakes range from a few dozen to many hundred species per square meter (SOSZKA 1975, HEDAL & SCHMIDT 1992, BANZIGER 2000, JOHNSON et al. 2001, GROSS et al. in prep.).