Nitrogen and silicon limitation of phytoplankton communities across an urban estuary: The East River-Long Island Sound system

Abstract

To understand how nutrient loading may impact phytoplankton community growth, structure, and photosynthetic efficiency in Long Island Sound (LIS), nutrient enrichment (N, P, or Si) experiments were conducted at stations along the longitudinal gradient of the East River-Long Island Sound system during high and low river flow conditions (spring and summer). During summer, East River (ER) phytoplankton showed no response to N, P, or Si additions. In contrast, N enrichment significantly increased growth rates of all algal size classes (pico- (<3 μm), nano- (3–20 μm) and microphytoplankton (>20 μm)) and most major groups (dinoflagellates, diatoms) beyond control treatments in all regions of LIS (western, WLIS, central, CLIS and eastern, ELIS). Nitrogen enrichment also increased particulate organic carbon production and the photosynthetic efficiency of the phytoplankton community. During spring, ER phytoplankton remained nutrient replete, while larger phytoplankton (nano- and microphytoplankton; diatoms, dinoflagellates) within WLIS and CLIS algal communities were stimulated by N and/or Si additions (1.5- to 7-fold increased growth rates). An absence of response in ELIS phytoplankton following nutrient enrichment during spring together with elevated dissolved inorganic nutrients concentrations and river discharge rates suggests that seasonal variation in river flow can impact the degree to which ELIS phytoplankton are nutrient-limited. A 14-year record of nutrient concentrations and ratios in LIS suggested that our experimental results were representative of long-term seasonal trends, with N-limitation most likely to occur during the spring, summer, and fall in CLIS, during summer and fall in ELIS, and during late spring and summer in WLIS. The existence of low silicate levels relative to inorganic nitrogen in WLIS during spring months suggested that anthropogenic nutrient loading in this region may promote the seasonal Si-limitation of diatoms at this time.