Contributions of phytoplankton and nitrifying bacteria to ammonium and nitrite dynamics in coastal waters

Ammonium and nitrite dynamics in coastal waters off Washington were examined using stable nitrogen isotope methods. Assimilation rates of ammonium into particulate nitrogen exhibited maxima (up to 500 nmol l⁻¹d⁻¹ at shallow depths and were negligible below the photic zone. Rates of ammonium oxidation by nitrifying bacteria showed surface minima and increased with depth (up to 35 nmol l⁻¹ d⁻¹). Both processes showed evidence of control by light intensity: light stimulated assimilation and inhibited oxidation. Ammonium turnover was dominated by phytoplankton assimilation at shallow depths and at inshore stations, while the relative contribution of bacterial oxidation of ammonium to turnover increased with increasing depth and increasing distance from shore. Nitrite turnover due to ammonium oxidation approached 60% per day in the primary nitrite maximum at the outer stations.