The influence of water level on macrophyte growth and trophic interactions in eutrophic Mediterranean shallow lakes: a mesocosm experiment with and without fish

Abstract

Summary 1. Water‐level fluctuations are typical of lakes located in the semi‐arid Mediterranean region, which is characterised by warm rainy winters and hot dry summers. Ongoing climate change may exacerbate fluctuations and lead to more severe episodes of drought, so information on the effects of water level on the functioning of lake ecosystems in such regions is crucial. 2. In eutrophic Lake Eymir, Turkey, we conducted a 4‐month (summer) field experiment using cylindrical 0.8‐m‐ (low‐water‐level) and 1.6‐m‐deep (high‐water‐level) mesocosms (kept open to the sediment and atmosphere). Fish (tench, Tinca tinca, and bleak, Alburnus escherichii) were added to half of the mesocosms, while the rest were kept fishless. Ten shoots of Potamogeton pectinatus were transplanted to each mesocosm. 3. Sampling for physicochemical variables, chlorophyll a (chl‐a), zooplankton and per cent plant volume inhabited (PVI%) by macrophytes was conducted weekly during the first 5 weeks, and subsequently biweekly. Macrophytes were harvested on the last sampling date. During the course of the experiment, the water level decreased by 0.41 ± 0.06 m. 4. Throughout the experiment, fish affected zooplankton abundance (−), nutrient concentrations (+), chl‐a (+) and water clarity (−) most strongly in the low‐water‐level mesocosms and the zooplankton community shifted towards dominance of small‐sized forms. The fishless mesocosms had a higher zooplankton/phytoplankton ratio, suggesting higher grazing. 5. Greatest macrophyte growth was observed in the low‐water‐level fishless mesocosms. However, despite high nutrient concentrations and low water clarity, macrophytes were also abundant in the fish mesocosms and particularly increased following a water‐level decrease from midsummer onwards. Macrophyte growth was poor in the high‐water‐level mesocosms, even in the fishless ones with high water clarity. This was ascribed to extensive periphyton development reducing light availability for the macrophytes. 6. Our results indicate that a reduction in water level during summer may help maintain the growth of macrophytes in Mediterranean eutrophic shallow lakes, despite a strong negative effect of fish predation on water clarity. It is therefore probable that an expected negative effect of global climate change on water clarity because of eutrophication and enhanced top‐down control of fish may be, at least partly, counteracted by reduced water level, provided that physical disturbance is not severe.

Restoration of Eutrophic Lakes with Fluctuating Water Levels: A 20-Year Monitoring Study of Two Inter-Connected Lakes

“…Moreover, the simultaneous increases in both WL and Secchi depth in 2009 and 2013 may explain the lack of changes in macrophyte coverage. Our results corroborate those of previous monitoring and experimental studies in Turkish lakes, showing that the water level overrides the effects of the turbid water conditions [39][40][41]52]. The CIT analysis revealed that not only water depth but also water clarity are critical factors for determining macrophyte growth in Lake Eymir.…”

Ecosystem change in the large and shallow Lake Säkylän Pyhäjärvi, Finland, during the past ~400 years: implications for management

“…This is supported by an increase in macrophyte abundance as evidenced by the enhanced proportion of periphytic diatoms and of both macrophyte-and sedimentassociated cladocerans, with potential positive feedback on water clarity via reduced phytoplankton development, as seen in other studies (e.g. Coops et al, 2003;Bucak et al, 2012;Jeppesen et al, 2015). Accordingly, a decline in the proportion of planktonic diatom forms suggests a reduction in phytoplankton production and improved water clarity.…”

Ecological impacts of global warming and water abstraction on lakes and reservoirs due to changes in water level and related changes in salinity

“…Two recent mesocosm studies undertaken in warm Turkish shallow lakes located in a semi-arid dry climate with major water level fluctuations (Ö zkan et al, 2010;Bucak et al, 2012) have shown greater resistance by macrophytes to nutrient loading owing to major water level reduction than expected from studies of northern temperate lakes. A mesocosm experiment conducted in eutrophic Lake Eymir with two water depths, 0.8 m and 1.6 m, showed that the strongest macrophyte growth occurred in the low water level fishless mesocosms (Bucak et al, 2012).…”

“…A mesocosm experiment conducted in eutrophic Lake Eymir with two water depths, 0.8 m and 1.6 m, showed that the strongest macrophyte growth occurred in the low water level fishless mesocosms (Bucak et al, 2012). However, despite high nutrient concentrations and low water clarity, macrophytes were also abundant in the fish mesocosms and abundance increased further following a water level decrease from midsummer onwards.…”