Seasonal Nutrient Supply to Coastal Waters

Abstract

Abstract Monthly nutrient and salinity observations have been undertaken during the years 1982 - 1985 in the southeastern part of the Kattegat on the Swedish west coast. Two minor embayments in that area, the Laholm bay and the Skalderviken receive large amounts of inorganic nitrogen from a couple of small rivers. Oxygen deficit occurs as a frequent feature in the deep water outside the bays. The observations have been used to calculate bimonthly mean concentrations of total nitrogen, total phophorus, inorganic nitrogen and phosphate within three different water masses; one defined as local surface water within the Laholm bay, one as Kattegat surface water, outside the bay and the third as Kattegat deep water. Using the observed landbased supply of nutrients, the deep water supply, calculated from entrainment theory and the measured nutrient gradients, we have determined the exchange of water and nutrients between the Laholm bay surface water and the Kattegat surface water. We have also calculated the “net assimilation” of inorganic nitrogen and phosphate within the Laholm bay. In an earlier report, we did similar calculations on an annual mean basis. Here, we have done a separation between a winter period from November to February when the assimilation is low, and the rest of the year, when most of the primary production occurs. From March to October, approximately 3/4 of the inorganic nitrogen supply to the local water is of land based origin while the phosphate supply is dominated by entrainment from the deep water. 2÷3 of the total inorganic nitrogen supply is assimilated by the primary production within the local water. The rest may be found as a loss from the local water to the Kattegat, which occurs mainly during the winter, but also during the spring, when the supply of phosphate is frequently too small for the primary production. The supply of phosphate to the bay seems to be low for the rest of the season as well, and the IN/IP ratio based on the external nutrient supply is well above the Redfield ratio. Still, however, the surface water concentrations of inorganic nutrients points towards nitrogen as the limiting nutrient for the main part of the productive season, indicating a more effective internal regeneration of phophorus and/or denitrification at the bottom.