Effects of the cyanobacteria Microcystis aeruginosa on eastern oyster feeding

The eastern oyster, Crassostrea virginica , is an estuarine consumer of considerable ecological and economic value, with the ability to selectively feed from a mixed phytoplankton community. Estuarine phytoplankton communities are experiencing an increased presence of small, nutritionally poor, salt‐tolerant, and potentially toxic cyanobacteria, such as Microcystis aeruginosa . To determine how the presence of the potentially harmful cyanobacteria M. aeruginosa affects oyster feeding, we quantified clearance rates, pseudofeces production, and pseudofeces composition across two feeding experiments conducted with a nontoxic strain. The first experiment consisted of bialgal feeding experiments in which single oysters were fed treatments consisting of (1) only M. aeruginosa , (2) only the diatom Thalassiosira pseudonana , or (3) a 50/50 mix of the two species. To quantify effects of M. aeruginosa on oyster feeding under more environmentally relevant conditions, which include numerous phytoplankton prey options and the presence of inorganic particles, a second feeding experiment was conducted using a natural phytoplankton community collected from a local estuary and oyster habitat. This natural community was then used to create two treatments: (1) an amended treatment in which nontoxic M. aeruginosa was added, and (2) a natural treatment in which only M. aeruginosa growth medium without cells was added. Results from both bialgal experiments (Exp. 1) and natural phytoplankton community experiments (Exp. 2) indicated that ecologically relevant concentrations of nontoxic M. aeruginosa did not significantly affect oyster clearance rates. In bialgal experiments, oysters showed no difference in pseudofeces production or the composition of that pseudofeces relative to the number of M. aeruginosa cells that were captured. However, oysters tested using the compositionally complex background of a natural community with inorganic particles produced significantly more pseudofeces when M. aeruginosa cells were added. These combined results indicate that prey community complexity and water quality metrics can drive the fate of oyster–cyanobacteria interactions in estuarine waters. Future research efforts should focus on implications to oyster fitness that a diet containing the nutrient‐poor species M. aeruginosa may have, specifically in relation to other climate‐ and human‐driven stressors.