Phytoplankton Life History Events: Resting Stages and Physiological Cell Death

Abstract

Understanding and predicting changes in phytoplankton populations requires knowledge of key life history processes such as recruitment from benthic resting stages and losses due to sedimentation and cell death. Currently, these processes are poorly understood in freshwater systems. Phytoplankton resting stage and cell death life history events were separately examined in two freshwater systems in Wisconsin, four northern lakes and an urban pond. In the norther lakes, sedimentation and benthic recruitment were examined using sediment and recruitment traps that were sampled weekly over two summers. Sedimentation and benthic recruitment contributed little to changes in standing crop chl a, but rather vegetative growth, grazing pressure and mortality likely caused the observed changes in standing crop chl a. Food web structure affected sedimentation and recruitment for taxa that produce resting stages. The proportion of Dinobryon that encysted was greater in a planktivore-dominated lake compared to the lake that had greater grazing pressure due to dominance of the food web by piscivores. Benthic recruitment of dinoflagellates was greater in the planktivorous versus piscivorous system, suggesting grazer inhibition of benthic recruitment. In a separate study, the occurrence of cell death was examined in an urban pond. Florescence microscopy was used to detect necrotic cells, as indicated by positive staining with SYTOX Green®, and apoptotic cells, as indicated by positive staining with FITC Annexin-V, during the mid-July to mid-November 2010 sampling period. Cell death occurred in a number of phytoplankton taxa. Abiotic stress likely caused death near the end of the sampling period, during temperate autumn. Biotic factors, such as viral or chytrid fungal infection, and allelopathy, may have contributed to cell death when populations were increasing or at their peaks. Finally, though death is implicated as a potentially important process to nutrient cycling in aquatic systems, it could not fully explain observed changes in total dissolved phosphorus in the urban pond.