Vertical location of seasonal nutriclines in the western Gulf of Finland

Abstract About 350 profiles of phosphate, nitrate and silicate concentration were collected on board the R/V Aranda in the western Gulf of Finland in summers 1992–1999. The study sites were located close to the coastal slope where the effect of wind-forced physical processes is well expressed. To determine the mesoscale background, hydrographical fields were mapped with a towed undulating CTD. The seasonal nutricline is a layer where the nutrient concentration increases just below the nutrient-depleted upper layer and is characterised by its depth and gradient. A clear ordering of seasonal nutriclines in the pycnocline/thermocline was observed with increasing depth: silicocline, phosphacline and nitracline. The silicocline and phosphacline were found close to each other. The spacing between the phosphacline and the nitracline was usually 3–5 m and about 14 m in the cases of a deep nitracline. The latter can be explained by the utilisation of nitrate reserves by a vertically migrating dinoflagellate Heterocapsa triquetra in cold summers when the surface layer temperature remained below 16 °C. Almost all nutricline depths (except the deep nitraclines) were well correlated with the pycnocline depth. Cold nutrient-rich intrusions in the nutricline layer, mostly 4–9 m thick, were frequently found in the vicinity of internal thermohaline/nutrient fronts in the Finnish coastal sea area. It is shown that the vertical transport of phosphate and nitrate by upwelling and turbulent mixing essentially depends on the separation of the phosphacline and nitracline.