Differential effects of warming and nutrient loading on the timing and size of the spring zooplankton peak: an experimental approach with hypertrophic freshwater mesocosms

In shallow lakes, environmental warming and nutrient loading are important influences on the likelihood of a shift between clear and turbid ecosystem states. With temperatures and nutrient runoff predicted to increase within the next decades, climate change poses a threat to lake communities. However, current predictions on the effect of these environmental factors on the abundance and timing of peak zooplankton numbers are based on correlations rather than on experimental isolation of thermal from other confounding effects. We present results of warming (4 degrees C above ambient) and increased nutrient loading on plankton communities in 48 outdoor mesocosms, simulating fishless and hypertrophic ponds. The timing of the chlorophyll a peak and crustacean zooplankton peak abundance, dominated by Daphnia pulex, responded strongly to temperature and nutrient addition. Daphnia numbers reached peaks 22-24 days earlier in heated than in unheated mesocosms. The chlorophyll a peak abundance advanced by 15-19 days with heating. Phytoplankton, total zooplankton and D. pulex reached peak abundance 12-19 days later when doses of nitrogen and phosphorus were added; this finding contradicts predicted earlier phytoplankton and zooplankton spring peak abundances with nutrient enrichment. Peak zooplankton and D. pulex abundances did not differ with temperature treatment, contrary to our expectations, but peak abundances occurred at similar actual temperatures. Nutrient additions had no effect on the peak zooplankton and D. pulex abundances in our mesocosms. Overall, climate warming is likely to advance plankton phenology in fishless ponds; however, this advance could be dampened in systems with high nutrient concentration. We found very high zooplankton abundances with warming and high nutrient loadings inducing a clear water state in all our tanks owing to heavy zooplankton grazing despite high nutrient concentrations.