Transformations of riverine nutrients in the Daugava river plume (Gulf of Riga)

Abstract The river Daugava is the main source of terrestrial nutrients to the Gulf of Riga. In 1997, two cruises were conducted in the Daugava plume zone to study nutrient transformations in the freshwater mixing zone. Phosphate and ammonium were rapidly removed from the freshwater plume in April (approx. 70% removal for phosphate), while nitrate and silicate behaved conservatively. Excess of particulate P and N, N:P ratios of the estuarine particles, and correlation of particulate N and P with Chl a suggest phytoplankton uptake as a mechanism for the estuarine nutrient removal. In the middle of October, conservative behaviour was also observed for ammonium and phosphate. This indicates that biological activity in late autumn is not sufficient to transform a significant part of the riverine nutrient load.