An estimate of oxygen consumption and denitrification in coastal waters of the Swedish west coast, using indirect methods

Abstract

Intensive hydrographic observations during 48 h, covering an area of ca. 100 km 2 on the Swedish west coast, have been used to determine oxygen consumption at the bottom and its relation to regeneration of inorganic nitrogen and phosphorus. The results are based on measurements of salinity, O 2, PO 4 3−, NO 2 −, NO 3 −, NH 4 +, and currents in a very thin deep water layer. Oxygen concentrations decreased, corresponding to a bottom oxygen consumption of 0·27 l m −2 day −1, while the mean concentrations were about 5·5 ml l −1. Phosphate concentrations increased, corresponding to a regeneration of 0·094 mmol m −2 day −1. Inorganic nitrogen concentrations, on the other hand, decreased which was interpreted as a net effect of simultaneous remineralization, nitrification and denitrification. Denitrification was estimated to be 1·87 mmol m −2 day −1. The results indicate that high oxygen concentrations within the water (favouring rapid nitrification), in combination with a large supply of organic matter to the bottoms (resulting in oxygen depleted ‘bottoms’) are effective triggers for denitrification. The results are compared with small-scale laboratory and field experiments from similar areas, and with seasonal changes in oxygen and nutrient conditions within these waters.