Nitrogen removal by phytoplankton uptake through a temperate non-turbid estuary

The impact of phytoplankton growth on the ambient levels of nitrate, ammonium and urea at three stations in the Southampton Water estuarine system was assessed over the productive springesummer period in 2001 using 15N tracer techniques. The relative contribution of the different nitrogen sources to total nitrogen uptake was also addressed. In general, ammonium uptake contributed >60% to total nitrogen uptake, but on some dates nitrate and urea uptake were dominant. Nitrogen depletion rates show that phytoplankton activity can, respectively, 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. Area-wide N-uptake estimations suggest that over time scales of hours to a few days, uptake by phytoplankton can potentially remove 100% of riverine nitrogen loads during the productive period. Mass balance calculations suggest that 13 and 36% of the nitrate and 80 and 68% of the ammonium annual inputs to the Test and Southampton Water (SW) estuaries are respectively removed. N-removal however, was less significant than N-export from the system. Estimates indicate that >100% and approximately 70% of nitrate and ammonium inputs are respectively exported from the Test and SW estuaries to the adjacent estuary and to the Solent. It appears that nitrification, ammonium regeneration and wastewater inputs can account for the extra nitrogen within the system.