Nutrients in the interstitial waters of the Southern Bight of the North Sea

Interstitial water samples from the Southern Bight of the North Sea were analysed for the nutrients NH4+, NO2-, NO3-, PO43- and Si. Ammonia, phosphate and silica concentrations in the upper 15 cm were on the average 2 to 3 times higher in a coastal zone of about 2 nautical miles wide than they were offshore. In this coastal zone the average organic carbon content was higher and the aerobic layer thinner. The major part of the high inter-core variance was also present between cores taken only 20 m apart. Nutrient concentrations were significantly lower in a station in a region with high sand waves. Average silica concentrations were in summer twice as high as they were in winter. Ambiguous results were obtained regarding a possible seasonal variation in NH4+ and PO43- concentrations. The general shape of depth profiles of nutrient concentrations was determined from 26 coastal and 21 offshore 35cm cores. Below 2,5cm, nitrate reduction was always found, indicating anoxic conditions. The increase with depth of ammonia, phosphate and silica was highest in the upper 2,5cm of the sediment. Fluxes of nutrients to the water column were estimated from concentration gradients in this upper layer and were compared with other nutrient inputs to the area. It was concluded that the relative contribution from regeneration in the sediments to the nutrient budgets was largest for silica. In summer, the sediment yields an estimated 30 to 50% of all silica inputs to the area