Storms and nutrient competition drive high-frequency phytoplankton size dynamics
Cell sizes determine nutrient competition traits, implying that size dynamics should be strongly influenced by variability in environmental nutrients. Lab experiments have borne out theoretical predictions that small cells are more efficient at nutrient use and should dominate under low-nutrient conditions, while large cells benefit from nutrient pulses. However, the sporadic nature of nutrient supply in natural systems has limited our ability to connect environmental changes with changes in microbial traits and ecosystem processes. To address this, we used a scanning flow cytometer to monitor high-frequency individual-level trait dynamics in a eutrophic lake in northern Switzerland (Greifensee), and coupled this with weekly sampling of nutrient concentrations and grazer abundances. We measured approximately 50 million individual cells over 3 months in summer, capturing changes in cell size and pigment fluorescence every four hours. Over the entire sampling period, the lake experienced 3 large storms, which are expected to supply limiting nutrients via deep mixing and stream runoff.
Storms supplied pulses of phosphorus that were rapidly taken up by the phytoplankton community, with concentrations returning to pre-storm levels within days. These pulses disproportionately benefited large cells, with the median community cell size increasing by up to 40% in the 3-4 days after a storm. In between storms, available phophorus was depleted and cell sizes decreased steadily, consistent with predictions based on competition for limiting nutrients. Our results highlight the predictive capacity of resource competition theory in natural communities and connect important patterns in environmental variation with community dynamics. Furthermore, they illustrate the importance of high-frequency monitoring of microbial communities, where large changes in communities with potentially important consequences for biogeochemical processes can occur over the course of hours or days.