Beklioglu, M; Levi, E; Cakiroglu, I; Tavsanoglu, N; Bezirci, G; Ozen, A; Turner, S; + view all Beklioglu, M; Levi, E; Cakiroglu, I; Tavsanoglu, N; Bezirci, G; Ozen, A; Turner, S; Kernan, M; Bennion, H; Jeppesen, E; - view fewer (2014) Manuscript on management options to deal with rising temperature, reduced water input and enhanced nutrients in European lakes. (EU REFRESH Deliverables Delivera ). UCL Faculty of S&HS: London, UK.

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Abstract

Shallow lakes, besides being the most widespread inland water bodies in the world and are very sensitive to external perturbations. Unprecedented rates of warming threaten the functioning and biodiversity of shallow lakes, not least lakes located in Mediterranean climatic zones that subjected to additional multiple stressors such as intensive land use. They are highly sensitive to the balance between evaporation and precipitation due to their large surface:volume ratio which is particularly marked in the Mediterranean climate owing to the strong linkages between climate and the hydrological cycle. In Turkey, climate models predict dramatic summer drought episodes across western and southern parts due to a >30% decrease in winter precipitation and enhanced summer evaporation. Such drought conditions will lead to an even stronger reduction in runoff waters, projected to be as large as 30-­‐40% in Turkey. This may have serious consequences for the ecology of shallow lakes in this region and may even ultimately lead to complete dry out. To further our understanding of the responses of shallow lakes located in the Mediterranean to a changing climate along with other stressors and to contribute to their conservation with adequate mitigation measures, we carried out research using space for time substitute with large number of lakes, paleoecology, mesocosms experiments approaches. Additionally, detailed paleoecological analysis of three lakes with instrumental measurements of water level changes were undertaken to explore the impact of hydrological alterations on shallow lakes. Lakes located in the northern highlands, with the lowest agricultural activity and temperatures, had low concentrations of nutrients and chlorophyll a (Chl a), low proportions of small fish, large proportions of piscivores, dominance of large-­‐bodied cladocerans and calanoid copepods, all characteristics indicative of low top-­‐down control of fish. In contrast, lowland lakes with higher temperatures and higher agricultural impact had a higher proportion of small fish, higher nutrient and Chl a concentrations with higher phytoplankton biomass (with cyanobacteria dominance). Higher Chl a:TP ratio and a high top-­‐down control of fish on zooplankton was evident with higher omnivorous fish biomass, and higher fish:zooplankton biomass ratio lower zooplankton:phytoplankton biomass ratio and lower macrophyte coverage. The warmest lakes located in the south further suffer from salinization in addition to eutrophication with enhanced top-­‐down control and species diversity of most organism groups was lower. Nutrients (TP and TN) and salinity were significant explanatory variables for the contemporary and surface sediment assemblage of the subfossils cladocera and macrofossils of aquatic plants. In warmer lakes, fish were strongly associated with the submerged plants, thereby impoverishing the usefulness of plants as a daytime refuge for zooplankton, particularly for large-­‐bodied species. Diel vertical migration was the most frequent anti-­‐predator avoidance behavior performed by the zooplankton whose size structure and diel movement are notably affected by predators, the response being size and water clarity dependent. Strong cascading effects of fish on the microbial community especially at shallow waters was also evident. Thus a lowering of the water level in warm shallow lakes will enhance the importance of the microbial community at the expense of phytoplankton, likely due to increased density of submerged macrophytes especially in the absence of fish. Long term paleoeoclogical records with instrumental water level data revealed that during high water levels there are shifts to more pleagic dominated system from more benthic-­‐ dominated through increases higher planktonic diatom abundance, pleagic cladocerans and tall growing submerged plants. High water levels accompained with enhanced nutrient levels may completely eliminate submerged plants whereas macrophytes apperared to tolerate rlatively low water clarity in during low water level periods. Enhanced salinity and temperature along with eutrophication are apparently the key factors determining the trophic structure and community composition and diversity of organisms in Turkish shallow lakes. The southern lakes were more saline and eutrophic than the northern lakes as a result of hydrological constraints at similar animal density and fertilization levels, particularly in the lowlands, and they overall had lower species diversity of most organism groups. Climate warming, temperature rise and drought events are anticipated to occur more frequently in the future and with higher intensity in semi-­‐arid Mediterranean climatic regions with a resulting risk of obtaining much higher water temperatures, salinisation, high proportions of small and omnivorous fish, eutrophication and, thus, potentially extensive and frequent cyanobacteria blooms in shallow lakes or, perhaps, desiccation. To counteract the effect of warming, strict control of nutrients to maintain a clear water state in shallow lakes is a prerequisite for avoiding cyanobacteria blooms and turbid water. In addition to nutrient control, restrictions on the human use of water are needed to mitigate the adverse impacts as >80% of the freshwater abstraction in most Mediterranean countries is used for irrigation. The need for water is predicted to increase significantly following climate warming. Stringent rules should be introduced regarding alteration of natural hydrological regimes through damming and irrigation to counteract the effect of future warmer and drier conditions. Furthermore, improved recycling of water, including wastewater and grey water, would be a valuable contribution. This is likely to entail changes in water rights and water pricing. In critical areas, intensive irrigated agriculture will have to be substituted by less water demanding rain-­‐fed agriculture or by grazing livestock systems.

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