The seasonal cycles of temperature, salinity, nutrients and suspended sediment in the southern North Sea in 1988 and 1989

Simple statistical analyses are used to summarize the large data set available from the 15 consecutive monthly surveys of the U.K. North Sea Project (NSP). The seasonal cycles of temperature, salinity, phosphate, nitrate, nitrite, silicate, ammonium and suspended particulate matter (SPM) are approximated by a mean value plus a year-long cosine wave. The mean concentrations, with standard deviation(c) given in parentheses, for each of these water quality parameters covering the whole area throughout the 15-month period are: salinity 34.26 (+/-0.74), ammonia 1.3 (+/-1.0) mu M, nitrate 4.9 (+/-6.0) mu M, nitrite 0.4 (+/-0.5) mu M, phosphate 0.5 (0.3) mu M, silicate 2.5 (+/-2.5) mu M and suspended sediment 2.6 (+/-3.5) mg l(-1). This approximate seasonal cycle accounts for most of the variance in temperature and nutrients. The spatially-averaged seasonal amplitudes for both nitrate and silicate are approximately equal to their mean values-this is consistent with these being limiting nutrients. Salinity shows little seasonality. Spatial distributions are shown of the mean values, the seasonal amplitudes and the percentage variances accounted for by a combination of these mean values and seasonal amplitudes. Correlations between the determinands are calculated; these confirm the similarity in the spatial distributions for the nutrients, especially between nitrate, phosphate and silicate. Maximum concentrations are confined to the coastal regions, except for ammonium and nitrite for which they occur offshore. Spatial distributions of the anomalous (non-seasonal) components can be interpreted to indicate the effect of specific riverine and oceanic exchanges. Correlations between nitrate, nitrite and ammonium correspond to the interconversion of these compounds. The oceanic/riverine inflow rates of phosphate, nitrate and silicate are shown to be insufficient to support their seasonal variability, suggesting that internal recycling is required to maintain the seasonal cycle. (C) 1997 Academic Press Limited.