Nitrogenous nutrient uptake, primary production, and species composition of phytoplankton in the Carmans River estuary, Long Island, New York1

Uptake of NH4+, NO3−, and urea was measured at four stations in a small temperate‐zone estuary over a 10‐month period. Considering all four stations, NO3− accounted for 45% of all N taken up, followed by NH4+ (43%) and urea (12%). Nitrogen uptake was highly temperature‐dependent (r = 0.82), and an exponential relationship was apparent. Recycling was very important as a source of nitrogen, and mean NH4+ pool turnover times were rapid, ranging from 4.3 to 6.8 h in the lower estuary in summer. PN turnover times in the lower estuary were also rapid, averaging 28.8 h in summer and being as short as 3.1 h in July and August. An analysis of N uptake using the relative preference index (RPI) indicated that NH4+ was the preferred nutrient. However, high NO3− concentrations appeared to override the repressive effect of NH4+ concentrations on nitrate reductase, and NO3− uptake was significant even at relatively high NH4+ concentrations. Urea uptake did not appear to be affected by NH4+ concentration.