In lakes and reservoirs worldwide, increased nutrient concentrations have led to an increase in harmful phytoplankton blooms. Phytoplankton blooms can increase hypoxia in the bottom waters of lakes and reservoirs, alter freshwater food webs, and impair drinking water with odors, tastes, and toxins that threaten human health. Thus, understanding the factors influencing their increase is critical for management. There is some evidence that phytoplankton may increase under future climate conditions, but this is largely based on predictions of warmer temperatures and stronger thermal stratification, which favor bloom-forming cyanobacteria. The responses of other phytoplankton taxa to future climate are less clear. For example, many regions are expected to experience future increases in storm intensity, which may alter phytoplankton community structure and bloom frequency through increased water column turbulence. To improve our understanding of phytoplankton community responses to storm events, we conducted a whole-ecosystem storm simulation experiment in a small drinking water reservoir. We used an engineered mixing system to simulate three storms with different intensities and durations during summer 2016 while monitoring phytoplankton community dynamics throughout the water column with high-frequency fluorescence profiles. We then compared these data to phytoplankton concentrations in an adjacent, non-manipulated reference reservoir.
Results/Conclusions
Our results suggest that the intensity and duration of storm events can substantially alter phytoplankton community response to mixing. Specifically, we observed that short (<6 hours), intense mixing events stimulated phytoplankton, including cyanobacteria, potentially by entraining nutrients from below the thermocline into the surface waters. This stimulatory effect may be most apparent early in the stratified summer period, when thermal stratification is weak. In contrast, less intense mixing events of longer duration (>20 hours) led to decreased light penetration into the water column by entraining sediments and thus had a negative effect on phytoplankton taxa with high light requirements, such as green algae, while cyanobacterial biomass exhibited a small, but not statistically significant decrease.
Consequently, our whole-ecosystem manipulation indicates that the effects of storm events on phytoplankton communities can be stimulatory and inhibitory, and that this may depend on storm duration and intensity. Moreover, our results demonstrate that mixing effects on cyanobacteria are highly context-dependent. As the intensity of storm events increases in some regions due to global change, our study indicates that it is essential to take whole-ecosystem dynamics – i.e., the interaction of light, nutrients, and thermal structure – into account when predicting storm effects on phytoplankton.