The effect of phytoplankton growth on nitrogen cycling in a non-turbid estuary, Southampton Water, UK

Stable isotope tracer incubation techniques were used to estimate the uptake of nitrogen by phytoplankton growth in the Southampton Water estuarine system over the productive spring-summer period in 2001 and 2002. Nutrient uptake experiments were carried out at approximately 2 week intervals with water samples collected from three stations in the estuary, representative of coastal waters of the Solent, the mid-estuary and brackish waters of the system. In 2001, separate subsamples were incubated with 13C-HCO3 and 15N-NO3-, 15N-NH+4, and 15N-urea, and were deployed in situ at 4 photic depths plus a dark incubation. In 2002, separated subsamples were incubated with 15N-NO-3 and 15N-NH4+, and were deployed at 2 photic depths. In addition, in 2002 the release of DON and the regeneration of NH4+ were also quantified. Vertical profiles of temperature, salinity and irradiance were made at each station, and water samples from 5 depths were collected for later analysis of nutrients (nitrate, ammonium, urea, and silicate) and chlorophyll a. Chlorophyll a concentrations of up to 64 μg L-1 were measured, and generally increased from the coastal waters towards the upper estuary. Nutrient concentration were also higher in the inner estuary and appear to be affected by the seasonal cycle of phytoplankton growth, showing lower concentrations during the summer months. Uptake rates of nitrogen within the Southampton Water estuary were comparable with value reported in the literature for other similar estuaries and coastal systems, and showed that during extensive phytoplankton blooms within these systems comparatively elevated uptake rates can be reached. In general, the temporal variation in the uptake of N-nutrients was consistent with the seasonal variations in the Chl-a levels, showing higher rates between April/May to August during the two years investigated. Uptake rates during both years were higher in the inner estuary relative to the coastal waters of the system. Results showed that ammonium was the dominant source of nitrogen, contributing on average >60% to the total nitrogen uptake. Nitrate and urea however, showed dominant contributions on some occasions. The 15N-tracer experiment carried out during this investigation demonstrated that phytoplankton activity can remove nitrate, ammonium and urea from the euphotic water column at rates of up to ˜9, 28 and 51% h-1 of the ambient levels, and can potentially increase by threefold during periods of high water slack. 15N-incubation experiments showed that an average of 74±32% of the nitrate taken up by phytoplankton was released as DON, and that ammonium uptake rates within correction for isotope dilution represented an average of 31±9% of those corrected. Results thus demonstrated that the impact of phytoplankton growth on the nitrogen levels can be underestimated if the gross uptake of nitrate and ammonium are not taken into account.