Temporal variability of groundwater seepage and brown tide blooms in a Long Island embayment

Blooms of Aureococcus anophagefferens, the alga responsible for brown tide in Long Island waters, have been hypothesized to occur during years in which groundwater discharge is low. The precise mechanism by which blooms are initiated, however, remains unknown. To better understand the influence of groundwater inputs on brown tide, a 2 yr sampling campaign was established at 'bloom-prone' embayment, West Neck Bay (WNB), Long Island, New York. During 1997 and 1998, changes in water-column chemistry and phytoplankton dynamics were observed, along with groundwater composition and flow rates. Groundwater entering WNB was enriched in nitrate (>250 μM). During 1997 and 1998, elevated levels of dissolved nitrate (1 to 25 μM) were measured in the water column of WNB during the annual peak in groundwater flow. Peak nitrate levels were followed by mixed-assemblage phytoplankton blooms that were succeeded by monospecific brown tide with densities >5 × 10⁵ cells ml^-1. Interannual differences in groundwater seepage were reflected in the magnitude of water-column nitrate concentrations and phytoplankton biomass. Fifty percent more groundwater recharge in spring of 1998 compared to spring of 1997 resulted in levels of nitrate and chlorophyll in 1998 (25 μM and 25 μg l^-1 respectively) exceeding those of 1997 (1 μM and 15 μg l^-1). Phytoplankton blooms preceding brown tide may supply A. anophagefferens with organic nutrients, as annual bloom densities seemed dependent on the magnitude of dissolved organic nitrogen inputs prior to brown tide events. A multivariate regression model is presented which accounts for 72 % of the variability in brown tide densities during the 2 yr study period at WNB. A highly significant correlation between groundwater seepage and A. anophagefferens densities in the model suggests that rather than repressing brown tide, groundwater inputs to WNB can stimulate A. anophagefferens growth by initiating phytoplankton blooms prior to the brown tide which supply remineralized organic nitrogen.