Nutrient depletion studies in offshore North Sea areas

Nanomolar nitrate and nitrite concentrations are reported for offshore areas of the North Sea during a research cruise in July 1987. The detection system employed was a sea-going chemiluminescence analyser. Surface concentrations during a period of nutrient depletion are reported, not previously described for this sea area. Very low values for the N/P ratio are reported (<0.1), which suggested that phytoplankton primary production was potentially nitrogen limited during the study period for large areas of the North Sea. Some coastal and river influenced regions exhibited a N/P Redfield ratio of >16, suggesting potential phosphate limitation. As well as the horizontal structure for nitrate and nitrite, vertical profiles are reported showing some structure in the nitrate profile above the thermocline. Nitrate levels generally increased towards the surface, from a minimum at the thermocline, and this may represent an important source of nitrogen to the system during nutrient depletion. A nitrite maximum was observed at the base of the thermocline at the deep water sites, this is attributed to either nitrification or nitrate reduction, and indicates that the thermocline region was a site of active nitrogen regeneration. A significant correlation between primary production and the nitrate concentrations below 100 nmoles.dm^-3 was found, showing that the nitrate pool, as well as the recycling rate, remains an important influence during periods of nutrient depletion.