Spatial and temporal trends in nutrient concentrations in the Belgian Continental area of the North Sea during the period 1993–2000

Statistical analysis of nutrient data obtained during 8 years of monitoring indicated strong seasonal and spatial variability, with highest concentrations in winter and significantly higher concentrations (on average O2 times higher) in the salinity zone S!33 than in the salinity zone SR33. In the North Sea ammonium concentrations significantly decreased from 1995 on (47% in zone S!33 and 64% in zone SR33), while for nitrate no significant decrease was observed in the zone SR33. Despite a drastic reduction (50%) in the riverine inputs of phosphorus to the North Sea during the last decade, phosphate concentrations decreased only slightly in the zone S!33 (!20%). This observation was somewhat similar to what happened with nitrate. Phosphate concentrations were generally low (mean: 0.75 mM; maximum: 2.4 mM) compared to the other dissolved nutrients. Mean nitrate concentrations were about 18 mM but some extreme concentrations, up to 140 mM were observed. Finally, the dissolved organic nitrogen (DON) content represented a major fraction of the total dissolved nitrogen (TDN) (median of 72% for all data), especially in the zone SR33, but no significant spatial differences in the DON concentrations were found in the Belgian area of the North Sea.