Phytoplankton ecology of arctic lakes.

Abstract

The amount of light and nutrients available to a phytoplankton cell ultimately control photosynthesis. Light and nutrients, in turn, are controlled by sporadic physical events that bring new nutrients into lakes and redistribute nutrients and phytoplankton to areas with more or less light. Much of the enhancement from these events is unaccounted for in current estimates of pelagic photosynthesis; this dissertation contributes to understanding of these controls. This work also contributes to the ecological understanding of sporadic events and ecosystem response to climate change. Internal waves have been predicted to enhance photosynthesis by moving phytoplankton through non-linear light fields. Field experiments and mathematical models show that, in situations where photosynthesis is light limited, phytoplankton circulated at depths mimicking internal waves had photosynthetic rates up to two-fold higher than in static incubations and the effects of surface light variation due to cloud cover interact strongly with those of internal waves. Internal waves in a wide variety of aquatic systems show a strong potential for wave-induced enhancement of photosynthesis. The effects of storm-induced inflow and mixing events were investigated. Inflow events are shown to cause a sustained enhancement of photosynthesis when mixed into the upper water-column, but little impact if isolated from the overlying water. The exploration of storm impacts on photosynthesis was expanded to include the direct and interactive effects of storm events, lake fertilization, lake depth, and seasonality. Results indicate that even low-level fertilization increases lake photosynthesis, causes a change in seasonal distribution of photosynthesis, and causes a change in the depth distribution of phytoplankton response to mixing events. Lake depth, but not fertilization level, was found to affect event-driven changes in water column photosynthesis. These results have implications for lake dynamics during the long ice-covered season. A combined analysis of these various factors (inflow, mixing, internal waves, fertilization, and temperature) and the stochastic timing of storm events showed that the frequency of storm events has a saturating positive effect on seasonal photosynthesis, and increasing temperature decreases the frequency of lake mixing but increases seasonal photosynthesis. Moderate climate changes could result in large impacts on seasonal photosynthesis in arctic lakes.