Contrasting microplanktonic composition and food web structure in two coastal embayments (Long Island, NY, USA)

The neighboring Great South Bay (GSB) and Peconic Bay (PB) of Long Island, NY, USA, were observed to support distinctive microplanktonic communities and trophic structure over most of an annual cycle (1998-99). Trophic structure analyses were based on 15 months of sampling for inorganic and organic nutrients, size-fractionated chlorophyll a (Chl a) and nanoplankton and microplankton abundances. While dissolved inorganic nitrogen (DIN) inventories were not demonstrably different between bays, dissolved organic carbon (DOC) and nitrogen were significantly higher in GSB than in PB and covaried with Chl a concentrations. Likewise, total biomasses (mu g C L-1) and mean seasonal biomass ratios of heterotrophic nanoplankton (HNAN) to autotrophic nanoplankton (ANAN) were substantially higher in GSB (> 0.30) than in PB (< 0.15) from spring to autumn 1998. The higher nanoflagellate biomass in GSB appears to have been indirectly supported by elevated concentrations of dissolved organic matter (DOM). During winter and spring 1999, biomass ratios in GSB dropped to levels similar to those in PB and coincided with a clear water event in GSB that may have been caused by increased bivalve suspension feeding. Even though these bays share similar broad-scale oceanographic/hydrogeologic settings and a common assortment of planktonic taxa, the structure and function of their planktonic communities were fundamentally distinct.