Observed distributions of macronutrient concentrations and ratios

The core work of the EU-MAST project NORWESP was the collection of all the available nutrient and productivity data from the region of the northwest European shelf (Radach Gekeler, 1996). Available time-series data have been analysed by Visser etal. (1996). On the basis of the NERC-NSP data, Prandle etal. (1997) demonstrated that the annual cycle in concentrations can be effectively represented by a cosine wave. The spatially averaged seasonal amplitudes of both nitrate and silicate are approximately equal to their mean values, which is consistent with these being the limiting nutrients. 

The starting point for production on the shelf is the build up of concentration over winter. The concentration at a particular point is determined by recycling from organic matter in water column and seabed, input from the atmosphere, and exchange with surrounding waters, themselves influenced by oceanic and riverine source waters. In the case of a well-mixed shelf sea in winter, the 

This rapid increase with width gives broader shelf seas an interior, across which transports should be considered in distinction to the effectively separate oceanic and coastal boundaries (Huthnance, 1995). 

A consequence of the relatively long transport times in northwest European shelf seas is that, in the interior, concentrations cannot be predicted from end member concentrations. As we have seen above, this is particularly true for concentrations of nitrate due to denitrification. In winter, concentrations of nutrients can be lower in shelf seas than in adjacent ocean surface water masses (Hydes etal., 2001 Gowen etal., 2002). Maps of winter distributions of the ratio of N P show regions of minimum values in those waters furthest from river and ocean source waters (Hydes Edmunds, 1992 Brockmann Wegner, 1995 Brockmann Kattner, 1997). 

For the southern North Sea, Hydes etal. (1999) report an average N P ratio at the end of winter (March) of 10 with a corresponding nitrate concentration of 12 pM. During the spring bloom, the N P ratio rose to 25 and then fell back to below 3 in the summer. This represents an initial limitation of growth by phosphate due to the high ratio of N P in waters where the dominant source of nutrients is river water. The ratio then fell as production continued using phosphate which was recycled more quickly than nitrate, and nitrate was lost due to denitrification. 

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