Abstract
Cross-latitude studies on lakes have a potential to predict how global warming may cause major changes in phytoplankton biomass and composition, e.g., the development of favourable conditions for cyanobacteria dominance. However, results from these studies may be influenced by biogeographical factors, and the conclusions may, therefore, not hold when considering local response patterns. We used monthly monitoring data from 504 lakes in Denmark—a small and homogeneous geographical region—to establish empirical relationships between key phytoplankton groups and a set of explanatory variables including total phosphorus (TP), total nitrogen (TN), lake mean depth (DEP) and water temperature (TEMP). All variables had strong effects on phytoplankton biomass and composition, but their contributions varied over the seasons, with TEMP being particularly important in June–October. We found dominance of cyanobacteria in terms of biomass and also an increase in dinophytes biomass at higher TEMP, while diatoms and chlorophytes became less important. In May, however, the TEMP effect on total phytoplankton biomass was negative, likely reflecting intensified zooplankton grazing. Our results suggest that biogeographical effects are of minor importance for the response patterns of phytoplankton to temperature and that substantial concentration reductions of TN and TP are needed in eutrophic lakes to counteract the effect of the climate change-induced increase in TEMP.
Highlights
The global climate is changing [1,2], and this may affect the ecosystem structure and nutrient dynamics of lakes as well as their water quality
To assess the effects of changes in temperature, we analysed how the total phytoplankton biomass, chlorophyll a, the biomass of dominant phytoplankton groups and their contribution (%) of the total phytoplankton biomass were related to temperature, nutrient level and lake depth
Risk of cy-effect on the perature onthethe total phytoplankton biomass chlorophyll a, The a positive anobacteria dominance increased with increasing temperatures, andeffect the period withdiatom cyabiomasses of cyanobacteria and dinophytes and a negative on the biomass, nobacteria importance is predicted to last long (TEMP effect became positive already from while April), the effect on the biomass of chlorophytes was modest
Summary
The global climate is changing [1,2], and this may affect the ecosystem structure and nutrient dynamics of lakes as well as their water quality. Several studies suggest that the biomass of phytoplankton and the concentration of chlorophyll a will increase, and the relative contribution of cyanobacteria among the phytoplankton assemblage is expected to rise [3,4,5,6,7]. Warming intensifies stratification, and the mixing depth may become shallower, favouring cyanobacteria competition [9]; in addition, shallow lakes may become stratified [10]. This development creates optimal conditions for many cyanobacteria taxa due to their high optimum growth temperature and their ability to regulate buoyancy [11].
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