Sinks of macronutrients

In the short term, the most significant temporary sink for dissolved nutrients is uptake by phytoplankton. This is followed by remineralisation, which returns nutrients to solution or loss from the system by burial in sediments or export to ocean waters. These processes effectively separate N, P and Si because reminer-alisation returns them to solution at different rates (Officer Ryther, 1980). In particular, Si dissolves slowly from shell material, and denitrification leads to the loss of biologically available nitrogen from the system as nitrogen gas. 

In temperate waters, nutrients undergo seasonal cycles with algal uptake dominating during spring and summer. Joint and Pomroy (1993) estimated annual levels of production in the different North Sea Task Force (NSTF) areas of the southern North Sea. Hydes etal. (1999) compared estimates of production with estimates of available nitrogen based on observed loads before 

the estimate was 0.3molNirf2y-1 (Simpson Rippeth, 1998). These estimates are in line with newer field measurements of denitrification (Lohse etal., 1996 Trimmer etal., 1999). Hydes etal. (1999) used the NERC-NSP data from January 1989 to show that the concentration of P predicted, from a concentration-salinity relationship, at the salinity of source waters at the shelf break, matched the observed concentration. However, the predicted concentration of nitrate was significantly lower than at the shelf break. The nitrate deficit in the southern North Sea was equivalent to a denitrification rate of 0.25 mol N m-2 y-1, assuming a flushing time of one year for the North Sea. 

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