<i>Aureococcus anophagefferens</i>: Causes and ecological consequences of brown tides in U.S. mid‐Atlantic coastal waters

<jats:p><jats:italic>Aureococcus anophagefferens</jats:italic> is a picoplankionic alga that since 1985 has bloomed in coastal embayments of the western mid‐Atlantic, ranging from Narragansett Bay, Rhode Island, to Barnegat Bay, New Jersey, with greatest incidence of recurrence in Long Island bays, New York. Blooms of this small alga, referred to as “brown tide,” can persist for several months during late spring and summer at densities in excess of 1.0×10<jats:sup>6</jats:sup> cells ml<jats:sup>−1</jats:sup>. They are not associated with anomalous chlorophyll <jats:italic>a,</jats:italic> dissolved oxygen, or inorganic macronutrient (N, P) levels. Meterologically induced reduced flushing rates, elevated salinities, and delivery of micronutrients (e.g. iron) from the watershed have been implicated in bloom initiation. Brown tides have had severe detrimental effects on the benthos, especially eelgrass (<jats:italic>Zostera marina</jats:italic>) and suspension‐feeding bivalves, including bay scallops (<jats:italic>Argopecten irradians</jats:italic>) and blue mussels (<jats:italic>Mytilus edulis</jats:italic>). Adult bivalves experience sublethal effects (e.g. inhibition of clearance rates) at <jats:italic>Aureococcus</jats:italic> concentrations as low as ∼2×10<jats:sup>5</jats:sup> cells ml<jats:sup>−1</jats:sup> and mortalities at ∼10<jats:sup>6</jats:sup> cells ml<jats:sup>−1</jats:sup>, attributed to toxicity of this microalga. Impacts of brown tide on zooplankton are less clear, but reduced egg production rates of copepods and reduced population growth rates of ciliates are documented at higher brown tide concentrations (≥1.0×10<jats:sup>6</jats:sup> cells ml<jats:sup>−1</jats:sup>). We summarize the state of knowledge about the physical, chemical, and biological factors that may contribute to brown tide initiation, maintenance, and decline and assess its ecological effects.</jats:p>