Effects of background concentrations of Aureococcus anophagefferens (brown tide) on growth and feeding in the bivalve Mercenaria mercenaria

Much research on the toxic alga Aureococcus anophagefferens (brown tide) has focused on its effects on bivalve suspension feeding, but less is known about how brown tide influences bivalve growth. This study examined the extent to which background levels, defined as concentrations too low for toxicity to inhibit bivalve feeding, of A. anophagefferens influenced the growth and feeding physiology of northern quahogs Mercenaria mercenaria compared to other phytoplankton common to Long Island, New York, waters. Juvenile quahogs were fed either unialgal cultures of A. anophagefferens, non brown tide species, or diets mixed with background levels of brown tide. Absorption efficiency (AE) was determined using the C-14:Cr-51 dual-tracer method, and growth was determined by overall biomass change. Results showed that unialgal diets resulting in the highest AE, specifically Isochrysis galbana and Thalassiosira pseudonana, were associated with rapid M. mercenaria growth. Conversely, Nitzschia closterium resulted in a comparatively low AE and a loss in quahog biomass. Diets mixed with brown tide resulted in a significantly lower AE than the corresponding unialgal diet for all phytoplankton species except N. closterium. Additionally, mixed diets exerted a small negative influence on quahog growth compared to unialgal diets. These observations suggest that quahogs may suffer subtle, chronic effects when A. anophagefferens is present in the field at background levels. Moreover, the different responses of M. mercenaria to each experimental diet have broad implications for understanding how phytoplankton community composition may influence bivalve growth patterns in the field.