Concentration-dependent effects of toxic and non-toxic isolates of the brown tide alga Aureococcus anophagefferens on growth of juvenile bivalves

Suspension-feeding bivalve mollusks are highly susceptible to the deleterious effects of blooms of the picoplankter Aureococcus anophagefferens (brown tide) in coastal bays of the mid-Atlantic USA. Although short-term exposure to A. anophagefferens is known to cause feeding inhibition of bivalves, longer-term effects on growth and survival are poorly documented. This laboratory study examines the concentration-dependent effects of 2 Long Island, New York, isolates of A. anophagefferens on the juvenile hard clam Mercenaria mercenaria and the mussel Mytilus edulis. Concentrations greater than or equal to400 x 10(3) cells ml(-1) of a toxic A. anophagefferens isolate (CCMP 1708) arrested shell growth and caused significant soft-tissue weight loss and reduced condition in both species, effects comparable to those of starvation, but did not induce mortalities over the 3 wk study period. No histopathology was detected at the cellular level; brown tide caused reduction in digestive epithelium height and overall appearance of absorptive cells similar to that observed under starvation. Optimum concentrations of non-toxic, nutritious algae in a mixed assemblage with A. anophagefferens CCMP 1708 did not mitigate effects of brown tide at 400 x 10(3) cells ml(-1), but at 80 x 10(3) A. anophagefferens cells ml(-1) the mixture resulted in positive growth and progressive acclimation to the diet. In contrast, exposure of hard clams to 400 x 10(3) and 1 x 10(6) Cells ml(-1) of a non-toxic A. anophagefferens strain (CCMP 1784) supported growth rates only 18 and 29% below a volume-equivalent control diet of Isochrysis galbana respectively. We thus conclusively demonstrate that the detrimental effects of brown tide on bivalve growth are mainly attributable to cell toxicity, rather than high cell density, nutritional deficiency or poor retention of small (2 mum) cells. The implications of these results to recruitment success and stock-enhancement efforts of bivalve populations in brown tide-affected estuaries are discussed.