Deliverable 3.6

Abstract: Manuscript on current interactions between temperature, hydrology/water level in European lakes and consequences for lake restoration

This deliverable comprises four research papers and one PhD thesis on hydrology and temperature effects on lakes and consequences for restoration. Here we provide the abstracts for these. Copies of the full papers can be obtained by request from the Proejct Web site at http://www.refresh.ucl.ac.uk/contact.

  1. High water-level fluctuations are typical of lakes located in the semi-arid Mediterranean region, which is characterized by warm rainy winters and hot dry summers. Ongoing climate change with severe episodes of drought may lead to lower and more variable water levels, longer hydraulic retention time and eutrophication as nutrient concentrations increase in the remaining water. Higher hydraulic retention time also triggers salinization. In addition, the cascading effect of top-down control by fish as well as the nutrient cycling are sensitive to changes in temperature. Information on the effects of water level and temperature on in-lake trophic dynamics and their interactions is crucial in such regions and may have profound implications for restoration.
  2. It is hypothesized that in the Mediterranean region, major water level decline during summer due to higher evaporation, may potentially help maintain macrophyte growth even in eutrophic shallow lakes and therefore, at least partly, counteract the effect of reduced clarity from climate-driven enhanced eutrophication. An experiment was conducted at contrasting water levels in a Turkish shallow lake. This showed that lower water levels help maintain macrophyte growth  in a Mediterranean eutrophic shallow lake, despite a stronger (negative) cascading effect of fish predation on water clarity confirming the above hypothesis. Laboratory and field studies further showed large-bodied grazer Daphnia avoid submerged macrophytes and instead prefer to hide near the sediment when exposed to predation risk, because of high fish densities in the vegetation. These results confirm earlier findings that macrophytes are less efficient refugia for large bodied zooplankton in these lakes compared with northern temperate lakes making them lakes more vulnerable to a shift to a turbid state when the nutrient level increases, due to reduced grazing on phytoplankton.
  3. In semi-dry Mediterranean climatic regions, excessive water use especially for irrigated crop farming, and global warming also enhances salinization of inland freshwater lakes, whicyh has a major impact on the zooplankton community structure. A study of 31 Turkish lakes showed a shift towards smaller sized classes including rotifers, which are less efficient phytoplankton grazers. Salinity strongly control Daphnia populations through increased mortality, decreased reproduction, and reduced growth rate. Global warming-induced increase in salinity may therefore also cascade through the food web and lead to higher risk of having turbid waters in warm and dry region.
  4. Lake restoration may therefore be more difficult in warm regions. Nutrient loading reduction and reduced water abstractions are the first step needed to restore eutrophic lakes. In Mediterranean climatic regions, restrictions on human use of water especially irrigation purposes as well as on nutrient loading to lakes are urgently needed to achieve good ecological status (WFD). To fulfil this objective, adaptation measures are required. In Mediterranean climatic zones the obvious methods are to change agricultural practices for reducing demand for water and the loss of nutrients to surface waters, to improve sewage treatment and to reduce the storm-water nutrient runoff. In warm Mediterranean zones adaptations may also include re-establishment of artificial and natural wetlands, introduction of riparian buffer zones and re-meandering of channelised streams, which may all have a large impact on the N loading in lakes. In-lake measures may increase the rate of recovery after measures to reduce the nutrient loading have been taken. One such method is biomanipulation. The review of biomanipulation methods and results included here indicates that it might be possible to enhance water clarity even in warm freshwater lakes by removing the dominant benthic fish, such as carp, though the long term effects of such measures are unknown. It is suggested that dual treatment including both biomanipulation and chemical treatment of the sediment to immobilise phosphorus might be a way forward to enhance the chance of long-term recovery of lakes in all climate zones.

Link - http://www.refresh.ucl.ac.uk/webfm_send/1959

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