The origin of Skagerrak coastal water off Arendal in relation to variations in nutrient concentrations

Evidence is provided that Skagerrak Coastal Water (0-30 m off Arendal) during winter and spring is a physical mixture of two main water masses entering the Skagerrak: Jutland Coastal Water (average 75%) and Kattegat Surface Water (average 25%). Jutland Coastal Water is itself composed of German Bight Water and Southern/Central North Sea Water. These water masses contribute on average 55% and 20%, respectively, to Skagerrak Coastal Water. Inorganic nutrient concentrations are largely determined by the concentrations in the parent water masses, with Kattegat Surface Water contributing 6-20% and German Bight Water 60-80% to the nitrate concentration, and 20-30% and 40-50% to the phosphate concentration, respectively. The rest originates from Southern/Central North Sea Water. Observed nitrate concentrations, and the N:P ratio, off Arendal during this time of the year have approximately doubled since the 1980s. Very similar results were obtained when calculating the expected concentration based on the relative contributions of the parent water masses, which suggests that the increase in Skagerrak Coastal Water is mainly caused by the increased nitrate concentration in the German Bight. Available data suggest that the large inter-annual variability in nitrate concentration in the Skagerrak area is partly explained by river run-off to the southern North Sea. Variability in phosphate and silicate was much smaller and apparently less dependent on run-off.