Short-term changes in the hydrochemical constituents in the water column of the Gdańsk Deep (Baltic Sea) in spring. Part 1. Nutrient and oxygen concentrations in relation to the density stratification

Both the seasonal thermocline in the uppermost layer of the sea and the halocline in the near-bottom layer were found to constrain transport of chemical compounds to the productive layer of the Gdansk Deep. The occasional disappearance of the halocline resulted on the one hand in the flow of nitrogen and phosphorus compounds through the intermediate layer towards the surface, and on the other in a considerable improvement in oxygen conditions in the near-bottom layer, this usually being either poorly oxygenated or anoxic. A statistically significant negative correlation between nitrate and oxygen concentrations was found in the isohaline layer. The N:P ratio was usually low during the day but increased at night. During spring this ratio increased in the euphotic layer. The large number of high N:P ratios may attest to the shortage of phosphates and to the change in the limiting factor - from nitrogen compounds in early spring to phosphates in late spring.