A Synoptic Study of Nutrients in the North-west Irish Sea

Concentrations of nitrate and nitrite (N), phosphorus (P) and silica (Si) were measured at a grid of stations in the north-west Irish Sea throughout 1992. The data were treated synoptically by mapping and graphical analysis to integrate data from 28 different stations. In spring, there was concurrent depletion of N, Si and P in the molar ratio 9•5:6•4:1, indicating that N was depleted before P and Si. Areal integrals were calculated for N and P at each station, which ranged in depth between 15 and 150 m. Over the summer, P integrals reducedpro ratawith depth up to 60 m, but in stations deeper than 60 m, there was no further reduction in P. In contrast, the areal N integral decreased in proportion to depth even in the deepest stations. It is suggested that loss of N in the deepwater stations may be due to denitrification in the bottom water. Over the winter, N, P and Si concentrations returned to the winter maximum in more or less constant proportions. Mechanisms for the control of the nitrate concentration are discussed. A desk budget suggests thatin situregeneration of nitrate is the dominant process.