Variability in fluxes of nutrients (N, P, Si) into the North Sea from the Atlantic Ocean and skagerrak caused by variability in water flow

Abstract

Fluxes of nutrients (dissolved phosphate, nitrate and silicate) from the Atlantic Ocean (Northern Atlantic and Channel) and Skagerrak to the North Sea were calculated for the upper 30 meters and for the total water depths. Monthly average water flows and their standard deviations were taken from the model calculations of the If M (HAMSOM model) and IMR (NORWECOM model) for the period 1955–1993 and 1976–1995, respectively. Climatological monthly mean nutrient data were collected for the same transects from the NOWESP Research Data Base, and extracted from additional literature. With the water flow from the models and the available nutrient data rather precise fluxes can be calculated. The aim is to compare these fluxes with those published in the literature which are often based on yearly averaged water flow and some nutrient concentrations. The northern Atlantic inflow (total depth) of the mean fluxes of nitrate, phosphate and silicate is 3,972 ±1,604, 713 ±310 and 3,847 ±1,527 ktonnes/year. About 20% of these fluxes transported in the top 30 m water layer (IMR model). For the upper 30 m the IfM model calculated higher fluxes for these nutrients: 1,654 ±621, 301 ±111 and 1,549 ±565 ktonnes/year, respectively. For the Skagerrak the IMR model estimated a net mean outflow to the Baltic, while the IfM model estimated a net mean inflow of nutrients to the North Sea. Both models agree rather well in the mean inflow of nutrients through the Channel, but with rather high standard deviations as compared to the other sections. For example, for the IMR total depth estimation: 162 ±236, 27 ±39 and 161 ±237 ktonnes/year for N, P, Si respectively. In the northern sections, literature data on nutrient fluxes are at the lower end of the range estimated in this study, while in the Channel the literature data are in the upper range. It will be discussed that the largest improvements in nutrient flux estimations are to be expected when the precision and accuracy of water flow data become better.