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| Benthic filter feeders (BFF) can reduce phytoplankton concentration (abundance) thereby controlling eutrophication in several ecosystems including rivers. However experiments suggest warming can alter the relationship between BFF grazing rate and the growth rate of (heterotrophic) planktonic prey. To investigate how eutrophication control by grazers is altered with temperature under the influence of other important abiotic (water depth and speed light and turbidity) and biotic factors (initial phytoplankton concentration [hereafter: Pin value] BFF density and spatial BFF distribution) we developed a spatially-explicit computer simulation model. This model simulates the dynamics of a phytoplankton population traveling through a simplified river channel while being grazed by BFF. Our model includes the thermal responses of BFF grazing and phytoplankton growth. The results show that BFF grazing can qualitatively alter and in some circumstances even reverse the response of phytoplankton to warming. Moreover the response of grazer-controlled phytoplankton to warming water depth and Pin value is non-linear and phytoplankton can increase steeply with slight changes within some ranges of these variables. In addition these variables can interact causing their combined effects on eutrophication to differ from what is expected considering their isolated effects. Generally the effect of most variables including temperature Pin value and BFF density and spatial distribution is larger at shallow waters. Moreover our study shows that phytoplankton control can be substantially improved by heterogeneous BFF distributions where the BFF are located at the extremes of the river either upstream or downstream instead of homogenously distributed along the whole river. However warming can cause a switch between these two optimal distributions or even can cause differences among the spatial distributions to disappear. In general the homogeneous BFF distribution can be used as conservative estimate of eutrophication control. In conclusion this work shows that trophic control can qualitatively alter the response of eutrophication to warming supporting previous studies suggesting that the prediction of global warming effects requires considering not only the thermal responses of organisms but also their trophic interactions. In addition to these biotic variables this thesis reveals that considering the interactions between abiotic and biotic variables and including their spatial distribution are important for eutrophication control. Especially the detection of thresholds in the response of grazer-controlled phytoplankton to temperature water depth Pin value and spatial BFF distribution indicates that one should be careful with predictions because of potential abrupt changes. Although further studies are needed to make specific recommendations for water quality management our work provides preliminary suggestions on the conditions where grazers or Pin reductions can be more efficient to control eutrophication. |
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