Since 1985, Brown Tides of the pelagophyte Aureococcus anophagefferens have plagued multiple mid-Atlantic estuaries and have decimated commercial shellfish industries in New York. Although nutrient inputs are the most frequently cited cause of Brown Tides, there is no consensus as to which nutrient(s) controls Aureococcus growth in the field. Moreover, a comprehensive understanding of Brown Tide bloom dynamics is lacking due, in part, to the paucity of biogeochemical studies in the New York embayments which host blooms. This study used a combined approach of laboratory experiments, field experiments and field measurements to evaluate the role of organic, inorganic and trace metal nutrients in the initiation, persistence and demise of Aureococcus anophagefferens blooms within Long Island embayments.

During 1997 & 1998 Brown Tides in West Neck Bay (WNB), Shelter Island, groundwater was found to be the primary source of dissolved inorganic nitrogen to WNB, as elevated levels of nitrate were measured during the annual peaks in groundwater flow. Peak nitrate inputs were followed by mixed assemblage phytoplankton blooms that were succeeded by monospecific Brown Tides with densities > 500,000 cells / ml. Phytoplankton blooms preceding Brown Tides are hypothesized to supply Aureoccocus with organic nutrients, as annual bloom densities seemed dependent on the magnitude of DON inputs prior to Brown Tides. To evaluate the ability of nutrients to enhance Aureococcus growth during blooms, a series of nutrient enrichment experiments were conducted during a WNB bloom. The nutrient enhancing Brown Tide growth was a function of ambient nutrient levels, as the bloom shifted from organic carbon to N-limitation when nitrate levels in WNB decreased from elevated (2 to 20 µM) to low (< 0.5 µM) levels. Despite occasional growth enhancement by additions of nitrate or urea, these additions either had no effect or significantly decreased the relative abundance of Brown Tide among the algal community. In contrast, augmentation of Aureococcus growth and decreases in non-Brown Tide phytoplankton growth during glucose (DOC) additions resulted in significant increases in the relative abundance of Brown Tide among phytoplankton. Therefore, it is hypothesized that processes introducing copious amounts of labile DOC during Aureococcus blooms could promote monospecific Brown Tides.

To evaluate the role of Fe in Brown Tides, its physicochemical speciation in the bloom-prone Peconic Estuary was investigated. Across the estuary, dissolved Fe varied (9 to 240 nM) around the level reported to cause physiological Fe stress in this species (100 nM). High levels of inorganic and low molecular weight Fe measured during a Brown Tide suggested efficient Fe reduction by Aureococcus, potentially accounting for its Fe-replete growth during blooms. Finally, dissolved Fe inputs preceding Brown Tides are hypothesized to be a diagenetic indicator of organic matter input (e.g. DOC, DON) from sediments, which could fuel heterotrophic growth by blooms.

Since bloom densities of Aureococcus anophagefferens can repress benthic and pelagic grazing, viral lysis could be the primary removal mechanism for NY Brown Tide blooms. A laboratory investigation was conducted to establish the biogeochemical cycling of C, N, P, Fe and Se following the viral demise of Aureococcus blooms. Results demonstrate that viral lysis of Aureococcus blooms creates a direct trophic transfer of C, N, P, Fe and Se to bacteria and other phytoplankton, and a significant net release of DOC.