Are legacy effects important for the response of phytoplankton communities to nutrient and dissolved organic matter pulses?

Climate change predictions include increasing precipitation and runoff events that expose phytoplankton communities to colored dissolved organic matter (cDOM) and nutrient pulses of varying intensity and frequency. The consequence of different nutrient/cDOM pulse regimes on phytoplankton communities and the role of legacy effects related to the characteristics of previous exposure regimes remain largely unresolved. To investigate this, we implemented add-on bottle experiments with water collected from a mesocosm experiment conducted in Lake Erken (Sweden) with four nutrient regime treatments with varying intensity and frequency. The nutrient additions of the mesocosm experiment lasted for three weeks and were followed by a two-week recovery period. At the end of both periods, water from three mesocosm treatments was filled into microcosms and either exposed to a second single nutrient pulse or left as control. The microcosms were incubated for 8 days in the lab and changes in phytoplankton biomass and composition were analyzed to determine the legacy effect of antecedent nutrient regimes on the community response to a second nutrient addition. The results showed that past nutrient regimes created legacy effects on the community, dampening further changes in response to a second nutrient addition. However, the effects were transient, since they were not detected in the second microcosm experiment. Further, the lack of legacy effects in the second microcosm experiment suggests that the communities from the mesocosm treatments in fact recovered from the nutrient regimes they were exposed to in the two-week period. Consequently, the results of the thesis suggest that legacy effects may be important in predicting phytoplankton community response to extreme nutrient and cDOM pulses but they might be temporary