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 whic h groundwater discharge is low. The precise mechanism by which blooms are i nitiated, however, remains unknown. To better understand the influence of g roundwater inputs on brown tide, a 2 yr sampling campaign was established a t 'bloomprone' embayment, West Neck Bay (WNB), Long Island, New York. Durin g 1997 and 1998, changes in water-column chemistry and phytoplankton dynami cs were observed, along with groundwater composition and flow rates. Ground water entering WNB was enriched in nitrate (> 250 muM), During 1997 and 199 8, elevated levels of dissolved nitrate (1 to 25 muM) were measured in the water column of WNB during the annual peak in groundwater flow, Peak nitrat e levels were followed by mixed-assemblage phytoplankton blooms that were s ucceeded by monospecific brown tide with densities >5 x 10(-5) cells ml(-1) , Interannual differences in groundwater seepage were reflected in the magn itude of water-column nitrate concentrations and phytoplankton biomass. Fif ty percent more groundwater recharge in spring of 1998 compared to spring o f 1997 resulted in levels of nitrate and chlorophyll in 1998 (25 muM and 25 pg 1(-1), respectively) exceeding those of 1997 (1 muM and 15 mug 1(-1)). Phytoplankton blooms preceding brown tide may supply A. anophagefferens wit h organic nutrients, as annual bloom densities seemed dependent on the magn itude of dissolved organic nitrogen inputs prior to brown tide events. A mu ltivariate regression model is presented which accounts for 72% of the vari ability in brown tide densities during the 2 yr study period at WNB. A high ly significant correlation between groundwater seepage and A. anophageffere ns densities in the model suggests that rather than repressing brown tide, groundwater inputs to WNB can stimulate A. anophagefferens growth by initia ting phytoplankton blooms prior to the brown tide which supply remineralize d organic nitrogen.