Meteorological and climatic impacts on the phytoplankton community of a small meso-eutrophic lake

Gray, Emma and Jones, Ian D. and Mackay, Eleanor B. and Elliott, J. Alex and Folkard, Andrew (2019) Meteorological and climatic impacts on the phytoplankton community of a small meso-eutrophic lake. PhD thesis, Lancaster University.

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Abstract

Climate change is having long term impacts on the physical structure of lakes including potential shallower surface mixing, increases in stratification strength and increases in surface water temperature. It is therefore vital that lake thermal dynamic properties, like the depth of surface mixing, are quantified in a robust way. Comparing methods of mixed depth estimation in this thesis, however, found that values were highly inconsistent leading to disparate interpretations of ecological and chemical water quality parameters. Future limnological studies using the mixed depth should be vigilant to this issue and test multiple methods to ensure that findings are not dependant on the choice of method. Long term changes in lake water temperature and surface mixing as a result of climate change occur concurrently, but they have separate consequences for phytoplankton growth. Using a phytoplankton model, water temperature and mixed depth were changed independently. Increases in water temperature led to an increase in phytoplankton growth at most mixed depths. Contrary to previous research, mixed depth shallowing did not always lead to increases in cyanobacteria biomass, with shallowing from deep to intermediate depths resulting in a reduction in cyanobacteria. The magnitude and frequency of episodic weather events such as storms are also expected to increase as a result of climate change. It is therefore becoming increasingly likely that extreme events will occur at a time of heightened stratification strength, but the interaction of these two extremes have not yet been explored in detail. Despite concerns that episodic weather events will push lakes into more extreme states, evidence here suggests that in some cases storms can return the thermal stability and seasonal phytoplankton community succession to conditions more typical for the time of year. This highlights the importance of setting the responses of episodic weather events in the context of long term averages.