Carbon and nitrogen primary productivity in three North Carolina estuaries

Uptake of inorganic carbon and ammonium by the plankton community of three North Carolina estuaries was measured using 14C and 15N isotope methods. At 0% light, C appeared to be lost via respiration, and at increasing light levels uptake of inorganic carbon increased linearly, saturated (mean I k = 358±30 μEin m −2 s −1), and frequently showed inhibition at the highest light intensities. At 0% light NH 4 + uptake was significantly greater than zero and was frequently equivalent to uptake in the light (light independent); at increasing light levels NH 4 + uptake saturated (mean I k = 172±44 μEin m −2 s −1) and frequently indicated strong inhibition. Light-saturated uptake rates of inorganic carbon and NH 4 + were a function of chlorophyll a ( r 2 = 0·7−0·9); average assimilation numbers were 625 nmol CO 2 (μg chl. a) −1 h −1 and 12·9 nmol NH 4 + (μg chl. a) −1 h −1 and were positively correlated with temperature ( r 2 = 0·3−0·7). The ratio of dark to light-saturated NH 4 + uptake tended to be near 1·0 for large algal populations at low NH 4 + concentrations, indicating near light independence of uptake; whereas the ratio was lower for the opposite conditions. These data are interpreted as indicative of nitrogen stress, and it is suggested that uptake of NH 4 + deep in the euphotic zone and at night are mechanisms for balancing the C:N of cellular pools. A 24-h study using summed short-term incubations confirmed this; the cumulative C:N of CO 2 and NH 4 + uptake during the daylight period was 10–20, whereas over the 24-h period the ratio was 6 due to dark NH 4 + uptake. Annual carbon and nitrogen primary productivity were respectively estimated as 24 and 4·0 mol m −2 year −1 for the South River estuary, 42 and 7·3 mol m −2 year −1 for the Neuse River estuary, and 9·6 and 1·6 mol m −2 year −1 for the Newport River estuary.