According to the Intergovernmental Panel on Climate Change report released in September 2014, unprecedented changes in temperature and precipitation patterns have been recorded globally in recent decades and further change is predicted to occur in the near future, mainly as the result of human activity. In particular, projections show that the Mediterranean climate zone will be markedly affected with significant implications for lake water levels and salinity. This may be exacerbated by increased demands for irrigation water. Based on long-term data from seven lakes and reservoirs covering a geographical gradient of 52°of latitudes and a literature review, we discuss how changes in water level and salinity related to climate change and water abstraction affect the ecosystem structure, function, biodiversity and ecological state of lakes and reservoirs. We discuss mitigation measures to counteract the negative effects on ecological status that are likely to result from changes in climate and water abstraction practices. Finally, we highlight research required to improve knowledge of the impacts of anthropogenically induced changes on lake water level and consequent changes in salinity.

Climate and land-use change drive a suite of stressors that shape ecosystems and interact to yield complex ecological responses, i.e. additive, antagonistic and synergistic effects.Currently we know little about the spatial scale relevant for the outcome of such interactions and about effect sizes. This knowledge gap needs to be filled to underpin future land management decisions or climate mitigation interventions, for protecting and restoring freshwater ecosystems. The study combines data across scales from 33 mesocosm experiments with those from 14 river basins and 22 cross-basin studies in Europe producing 174 combinations of paired-stressor effects on a biological response variable. Generalised linear models showed that only one of the two stressors had a significant effect in 39% of the analysed cases, 28% of the paired-stressor combinations resulted in additive and 33% in interactive (antagonistic, synergistic, opposing or reversal) effects. For lakes the frequency of additive and interactive effects was similar for all spatial scales addressed, while for rivers this frequency increased with scale. Nutrient enrichment was the overriding stressor for lakes, generally exceeding those of secondary stressors. For rivers, the effects of nutrient enrichment were dependent on the specific stressor combination and biological response variable. These results vindicate the traditional focus of lake restoration and management on nutrient stress, while highlighting that river management requires more bespoke management solutions.

The biota of European rivers are affected by a wide range of stressors impairing water quality and hydro‐morphology. Only about 40% of Europe's rivers reach ‘good ecological status’, a target set by the European Water Framework Directive (WFD) and indicated by the biota. It is yet unknown how the different stressors in concert impact ecological status and how the relationship between stressors and status differs between river types. We linked the intensity of seven stressors to recently measured ecological status data for more than 50,000 sub‐catchment units (covering almost 80% of Europe's surface area), which were distributed among 12 broad river types. Stressor data were either derived from remote sensing data (extent of urban and agricultural land use in the riparian zone) or modelled (alteration of mean annual flow and of base flow, total phosphorous load, total nitrogen load and mixture toxic pressure, a composite metric for toxic substances), while data on ecological status were taken from national statutory reporting of the second WFD River Basin Management Plans for the years 2010–2015. We used Boosted Regression Trees to link ecological status to stressor intensities. The stressors explained on average 61% of deviance in ecological status for the 12 individual river types, with all seven stressors contributing considerably to this explanation. On average, 39.4% of the deviance was explained by altered hydro‐morphology (morphology: 23.2%; hydrology: 16.2%), 34.4% by nutrient enrichment and 26.2% by toxic substances. More than half of the total deviance was explained by stressor interaction, with nutrient enrichment and toxic substances interacting most frequently and strongly. Our results underline that the biota of all European river types are determined by co‐occurring and interacting multiple stressors, lending support to the conclusion that fundamental management strategies at the catchment scale are required to reach the ambitious objective of good ecological status of surface waters.

Lake Pamvotis, NW Greece is a shallow Mediterranean eutrophic lake that has changed drastically over the past 50 years. Strong effects, resulted mainly from anthropogenic causes, in the hydrological regime are shown for this area using long term hydrological data and a GIS system for extracting land cover/use changes. A set of aerial imagery acquired in 1945 through 2002 were used to monitor and assess the spatial and temporal changes in land cover/use, focused mainly on the lake's surface area and its surrounding ecosystem (Natura 2000 area). The significance of the changes in land cover/use distribution within Pamvotis wetland is further discussed depicting the role of the anthropogenic influence on the fragile ecosystem that resulted in the shrinkage of lake's habitats extent. The purpose of this analysis was to examine the long-term changes on macrophyte community composition, species occurrence and relative abundance with water quality and water level changes over the past century, using historical data, aerial photos and GIS techniques. The results showed that for the last 25 years annual water level fluctuation ranged from 70 to 159 cm. Water level starts decreasing in mid June and increasing again gradually from November until March-April. Intra annual water level fluctuation seems to E. Papastergiadou et al. be affected by land use for agricultural purpose through intensive irrigation and the summer drought as well. A dramatic decline of the submerged vegetation is apparent mainly attributed to anthropogenic pressures. Regarding the land cover/use changes, the most notable and significant alterations are concerning the urban development around the lake, the disappearance of wet meadows and the extension of reed beds. Finally it seems that water budget data as well as the response of the key eutrophication parameters are affected from both hydrological alterations and point/non-point pollution sources.

Climate change is expected to profoundly affect both temperature and net precipitation, with implications for lake water level. We describe the design of a harmonized, simultaneous, cross-European mesocosm experiment to elucidate the effects of climate change on community structure, functioning, and metabolism in shallow lakes at low and high nutrient levels with contrasting depths (1 and 2 m). We used cylindrical (D = 1.2 m) tanks that were either 1.2 or 2.2 m high, each having a 10-cm sediment layer. We inoculated the mesocosms with a mixed sample of sediment and plankton from lakes with contrasting nutrient concentrations and added macrophytes and planktivorous fish. Sediment was pre-equilibrated to the required experimental nutrient concentration. During the experiment the water level decreased with increasing temperature (up to 90 cm in the Mediterranean mesocosms) while conductivity increased. The average chlorophyll a concentration increased with temperature in the deep mesocosms but was more variable in the shallow mesocosms. Macrophyte F. Landkildehus et al. 72abundance increased with temperature, while the oxygen data suggest that net primary production peaked at intermediate temperatures. We conclude that our experimental design has the potential for tracking the interacting effects of global warming and eutrophication in shallow lakes.

Key-words: fish, macrophytes, water level fluctuation, water quality, zooplanktonWater level fluctuations play a significant role in the lake nutrient dynamics, and consequently may have a strong influence on the biological communities and productivity. In this article we investigated the effects of a long term water level reduction on key chemistry parameters and major biological communities in an eastern Mediterranean lake. Our approach is based on temporal data regarding water quality, fish, zooplankton and aquatic vegetation that are representative of different water level periods. The results revealed significant correlations between water level, conductivity and chloride concentration suggesting a clear effect of the water level reduction on the water quality. Among the key findings of this study is the significant increase of zoobenthivorous fish (roach and carp) from 1973 to 1999 that correlates with the water level reduction. A decline of charophytes is also noted whereas the reed beds appear to have expanded at the shallower parts of the lake. The zooplankton composition of the lake is mostly dominated by nauplii, rotifer and small-sized crustaceans indicating a possible effect of fish predation. Overall, this article has ascertained an alarming shift of water quality and composition of biological communities that can be attributed to the combined effects of eutrophication and the extreme water level decrease. charophytes est également notée alors que les roselières se sont élargies dans les parties peu profondes du lac. La composition du zooplancton du lac est principalement dominée par les nauplii, rotifères et crustacés de petite taille indiquant un possible effet de la prédation des poissons. Dans l'ensemble, cette étude a constaté une évolution alarmante de la qualité de l'eau et de la composition des communautés biologiques qui peuvent être attribuées aux effets combinés de l'eutrophisation et de la baisse importante du niveau d'eau.

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Submerged hydrophyte vegetation consists of a highly important biotic component of maintaining lake ecosystems towards a ''clear water'' ecological status. Aquatic macrophytes are well known to play a significant multidimensional role in lakes by competing with phytoplankton growth, stabilising sediment and offering refuge to fishes, macro-invertebrates and littoral zooplankton, amongst others. Zooplanktons that are associated with macrophyte beds, in particular, may act as a positive feedback mechanism that contributes to maintaining a clear-water state. Although there are several studies investigating the relationships between macrophytes and zooplankton in European lakes, few have yet been carried out in Greek lakes. Seasonal field sampling was conducted from spring 2006 to autumn 2008 in four lakes of northwestern Greece. Zooplankton samples were collected from within hydrophyte beds in each lake to estimate their relative abundance and species density. Hydrophyte abundance and composition was recorded on a five-point scale. Moreover, water samples were analysed to determine nutrient and chlorophyll-a concentration. Pearson correlations between zooplankton density and key physicochemical variables were conducted to distinguish significant abiotic variables related with major zooplankton groups. Kruskal-Wallis non-parametric analysis was used to test for significant differences in zooplankton composition and environmental variables amongst the five hydrophyte abundance classes. In addition, Canonical correspondence analysis was used to distinguish possible correlations amongst the macrophyte and zooplankton species. Zooplankton density was significantly higher in dense macrophyte vegetation. Small-sized species (e.g. Rotifera) dominated the zooplankton community, indicating the eutrophic nature of the lakes. Large Cladocera were present in low abundance and were mostly littoral. The current research contributes to a better understanding of relationships between biotic groups in selected Greek lakes.