László G.-Tóth

Ecological effects of water-level fluctuations in lakes: an urgent issue

Water-level fluctuations (WLF) affect the ecological processes and patterns of lakes in several ways. Aquatic habitats and feeding or breeding grounds are gained or lost, as light, climate and wave impacts change to mention only a few phenomena. While the phenomenon of WLF has been studied in many rivers (better known as flood pulse, Junk & Wantzen, 2004) and environmental flows (Coops et al., 2003), so far it has not received due attention in lakes (Coops et al., 2003). A potential reason for why WLF in lakes have been overlooked for such a long time may be that central Europe and North America—the cradles of limnological research—are situated in landscapes which have been shaped by man for long time. Most central European lakes have been regulated since the middle ages; therefore, naturally fluctuating lakes such as Lake Constance (Fig. 1) are rare today.

Ecological threshold responses in European lakes and their applicability for the Water Framework Directive (WFD) implementation: synthesis of lakes results from the REBECCA project

The objective of this synthesis is to present the key messages and draw the main conclusions from the work on lakes in the REBECCA project, pointing out their links to theoretical ecology and their applicability for the WFD implementation. Type-specific results were obtained from analyses of large pan-European datasets for phytoplankton, macrophytes, macroinvertebrates and fish, and indicators and relationships showing the impact of eutrophication or acidification on these biological elements were constructed. The thresholds identified in many of the response curves are well suited for setting ecological status class boundaries and can be applied in the intercalibration of classification systems. Good indicators for phytoplankton (chrysophytes, cyanobacteria) and macrophytes (isoetids and charaphytes) responses to eutrophication were identified, and the level of eutrophication pressure needed to reach the thresholds for these indicators was quantified. Several existing metrics developed for macrophytes had low comparability and need further harmonisation to be useful for intercalibration of classification systems. For macroinvertebrates, a number of metrics developed for rivers turned out to be less useful to describe lake responses to eutrophication and acidification, whereas other species based indicators were more promising. All the biological elements showed different responses in different lake types according to alkalinity and humic substances, and also partly according to depth. Better harmonisation of monitoring methods is needed to achieve better precision in the dose–response curves. Future research should include impacts of hydromorphological pressures and climate change, as well as predictions of timelags involved in responses to reduction of pressures.

Updating water quality targets for shallow Lake Balaton (Hungary), recovering from eutrophication

The paper presents an overview about recovery of shallow Lake Balaton from eutrophication by assessing quantitative and qualitative changes in phytoplankton, zooplankton, and chironomids as a function of load reduction. The aim was to update the present water quality targets. The proposed targeting scheme supplements the existing one with a range of lake-specific ecological criteria. We conclude that simple targets (desired phytoplankton biomass and permissible load) are the best choice during the initial stage of eutrophication management, but more complex schemes including ecological criteria are needed to trace recovery when re-organization of the ecosystem takes place.

Stir-up effect of wind on a more-or-less stratified shallow lake phytoplankton community, Lake Balaton, Hungary

Microstratification of phytoplankton in the large shallow Lake Balaton (Hungary) was studied during a 24 h period. Dissolved O2 showed biological stratification; flagellates exhibited a definite circadian rhythm. In the middle of the investigation a heavy storm broke out which was followed by the disappearance of differences between different layers of water. Storm-induced destratification is described by cluster-analysis. Abundances of dominant species changed differently in connection with the storm. Numbers of Nitzschia sp. increased due to stirring up from the sediment surface. Numbers of single-celled or colony-forming species (Cyclotella comta, Crucigenia quadrata, Coelosphaerium kuetzingianum) practically did not change. Numbers of all the three dominant filamentous species (Aphanizomenon fos-aquae f. klebahnii, Lyngbya limnetica, Planctonema lauterbornii) significantly decreased, which might be attributed to an unknown loss process and was followed by a competitive displacement by algae of small cell size.

Alga consumption of four dominant planktonic crustaceans in lake Balaton (Hungary)

Between 1981–83 the gut contents of Daphnia galeata, D. cucullata, Eudiaptomus gracilis, and Cyclops vicinus were examined with light and scanning electron microscope to obtain information on the feeding of these species in Lake Balaton. The two Daphnia species feed mainly on abioseston, and it is assumed that their primary nutrient source was organic matter adsorbed onto the surfaces of the abioseston granules plus bacteria and detritus. E. gracilis feeds on algae, showing a preference for green algae and diatoms. C. vicinus is also a prodigious consumer of algae in Lake Balaton, utilizing the whole size spectrum of phytoplankton.

Feeding of Cyclops vicinus (Uljanin) (Copepoda: Cyclopoida) in Lake Balaton on the basis of gut content analyses

Consumption of planktonic algae by Cyclops vicinus living in Lake Balaton were studied in eighteen feeding experiments during 1980–1981. Gut contents of 284 adult C. vicinus were analysed in comparison with the natural phytoplankton assemblage. Rates of algal cells elimination were also studied and compared to controles which did not contain animals. C. vicinus in Lake Balaton was found to be an omnivore.

Biomass of planktonic crustaceans and the food of young cyprinids in the littoral zone of Lake Balaton

The littoral zone of Lake Balaton and its periphyton-zooplankton-fish communities have been investigated intensively in recent years. Total average number of crustacean plankton varied from 36 to 126 ind l−1, their biomass from 0.49 to 1.86 mg ww l−1 month−1 at different areas of the littoral zone. In general, these values for the above parameters were higher in hypertrophic areas. 23 fish species occurred in the littoral zone with cyprinids dominating. The seasonal food spectra of Y-O-Y roach (Rutilus rutilus), white bream (Blicca bjoerkna) and bream (Abramis brema) were based mainly on planktonic crustaceans and benthic/periphytic invertebrates. According to the frequency of occurrence of crustaceans and other invertebrates, the food composition of young cyprinids differed significantly in the NE and SW-basins of the lake.

Ecological effects of water-level fluctuations in lakes.

Ecological effects of water-level fluctuations in lakes: an urgent issue.- Potential effects of water-level fluctuations on littoral invertebrates in lowland lakes.- Effects of water-level regulation on the nearshore fish community in boreal lakes.- Natural lake level fluctuation and associated concordance with water quality and aquatic communities within small lakes of the laurentian great lakes region.- Climate change and lakeshore conservation: a model and review of management techniques.- The impact of water-level regulation on littoral macroinvertebrate assemblages in boreal lakes.- Quantitative trends of zebra mussels in Lake Balaton (Hungary) in 2003-2005 at different water levels.- Patterns, origin and possible effects of sediment pollution in a mediterranean lake.- Temporal scales of water-level fluctuations in lakes and their ecological implications.- Effects of water-depth and water-level fluctuations on the macroinvertebrate community structure in the littoral zone of lake constance.- Flooding events and rising water temperatures increase the significance of the reed pathogen pythium phragmitis das a contributing factor in the decline of phragmites australis.- Leaf litter degradation in the wave impact zone of a pre-alpine lake.- Use of the water-level fluctuation analysis tool (Regcel) in hydrological status assessment of finnish lakes.- Mass mortality of the invasive bivalve Corbicula fluminea induced by a severe low-water event and associated low water temperatures.- An extension of the floodpulse concept (FPC) for lakes.- Effects of water-level fluctuations on lakes: an annotated bibliography.

First steps in the Central-Baltic intercalibration exercise on lake macrophytes: where do we start?

The Water Framework Directive (WFD 2000) defines macrophytes as one of the biological groups required for the ecological assessment of European surface waters. Several indices for macrophyte assessment have been proposed or are currently in use by different European states. As a first step towards performing an intercalibration of these indices a common dataset was developed. This dataset contains abundance data on 789 macrophyte species from 316 different lake sites in ten European countries. Various common species and genera were identified as indicators of reference and impacted conditions within the dataset. Cluster analysis of macrophyte data, supported by non-metric multidimensional scaling, indicated that clusters formed were more reflective of their source country rather than lake type. This might be caused by differences in regional climate, biogeography, monitoring techniques, or a combination of these factors. A total of six national indices were applied to assign quality classes to the lakes. However, this produced results that often differed by one or two quality classes for the same site. We foresee that a more precise intercalibration exercise is necessary, and it should be based on more detailed data considering both seasonality and the latitudinal differences within the area covered.

Management measures and long-term water quality changes in Lake Balaton (Hungary)

A reduction in the external and internal P loadings along with the removal of benthivorous cyprinids resulted in a significant improvement of the water quality in the hypertrophic western part of Lake Balaton by 1996, some ten years following first management measure. Enhanced internal P loading after the reduction in external nutrient load is clearly an important factor explaining the duration of recovery in shallow lakes. Phytoplankton responded quite rapidly to reductions in nutrient loading and fish stock, whereas the effect on both the structure and abundance of the crustacean plankton was less pronounced. The role of top-down control of phytoplankton by crustaceans following nutrient reduction is not clearly understood in shallow lakes. Grazing did not play a significant role in the overall reduction of algal biomass in Lake Balaton. Since high concentration of suspended mineral particles from both sediment resuspension and lime precipitation causes permanent food limitation of daphnids, the top-down effects were negligible in Lake Balaton following fish stock reduction. Fish community structure did not respond to the lower external and internal TP loads cyprinids being dominant in both the pre- and post-management periods.