| I studied the responses of seston food quality and quantity to two large-scale environmental perturbations -- Dreissena mussel invasion and phosphorus abatement. A case study in Oneida Lake, New York examined seston carbon: phosphorus ratio (C:P) and edible C concentration changes after these two perturbations, and showed significantly increased seston C:P (lower seston P content) and decreased edible C concentrations after Dreissena invasion. Despite possible seasonal food quantity or P limitation, Daphnia growth was not limited by seston P content or edible food quantity on an annual scale, consistent with continued observations of high Daphnia abundances. I predicted the total P (TP) and water clarity thresholds at which Daphnia populations in Oneida Lake would face growth depression. Controlled mesocosm experiments were conducted with a full-factorial design to separate individual Dreissena, light, and P effects on pelagic primary productivity and seston stoichiometry. The experiments identified Dreissena rather than light or P as the primary factor driving seston C:N:P and algal biomass changes. Dreissena presence significantly increased seston P, but decreased seston N content, likely because Dreissena retained more N, but released more P. Primary production did not change after Dreissena invasion, perhaps due to algal compensatory growth under Dreissena grazing. The pronounced Dreissena polymorpha (zebra mussel) effect on increasing available P suggests adjustments in lake nutrient management are necessary to combat eutrophication. Last, algal growth experiments examined seston C:N:P changes with various temperature, light, and P conditions, and explored mechanisms by which major environmental factors drive seston C:N:P variations. The results showed significant temperature, light, and P effects on algal growth and C:N:P ratios. A tight regression between light:TP and seston C:P confirmed the "light:nutrient hypothesis" suggested by previous studies. Additionally, under P limiting conditions, seston C:P and N:P generally increased with higher light and temperature, but under sufficient P or low temperature, phytoplankton C:N:P were close to the Redfield ratios, and responded to light weakly. This work suggests that changes in light and nutrients, such as caused by Dreissena and P loading, along with changes in temperature, will combine to alter seston stoichiometry and quantity, with subsequent repercussions for pelagic consumers. |
