Hydrographic and hydrochemical conditions in the Gotland Deep area between 1992 and 2003

The paper describes the hydrographic-hydrochemical development in the eastern Gotland Basin between the major saltwater inflows into the Baltic Sea in 1993 and 2003. This period is characterised by only low inflow activity. The most important hydrographic events were the effects of the very strong inflow in 1993 and the weak inflows in 1993/1994 and 1997. The 1993/1994 inflows led to deep-water renewal, a steep fall in deep-water temperatures, and increasing salinity. The effects of the inflow of very warm, saline and oxygen-rich water in autumn 1997 were observed in the deep water in 1998, resulting in temperatures rising to 7◦C. The recent renewal in spring 2003 is reflected in the decreasing temperature, higher salinity and improved ventilation of the bottom water. Changes in the redox conditions exert a considerable influence on the nutrient distribution. During stagnation periods, there is enrichment of phosphate and ammonium, while nitrate is absent. Thus, around 31 μmol l−1 ammonium and 7 μmol l−1 phosphate were measured prior to the water renewal in 2003. Deepwater ventilation results in lower phosphate concentrations of around 2 μmol l−1, the nitrification of ammonium and the occurrence of nitrate. For the observation period, an estimate of nutrients stored in the deep water was done for the eastern Gotland Basin. During the recent stagnation period, there was an increase of up to 150% in the phosphate pool below the halocline, whereas the pool of inorganic nitrogen compounds decreased to 80% compared with 1992 when the previous stagnation period had ended. Under specific circumstances, these unbalanced nutrients can be made available to the upper water layers and can induce large-scale blooms of algae, especially of cyanobacteria.