The silicon (Si) cycle in coastal seas have been strongly influenced by human activities in the past decades, which in turn alters primary production and nutrient export efficiency to the open ocean. It is crucial to understand how and to what degree the coastal Si cycle responds to environmental change caused by human activities. In this study we present a field investigation conducted in March 2016 on the distribution of stable Si isotope of dissolved silica (δ30SiDSi) in a highly eutrophic coastal system, the Baltic Sea. A box model was used to simulate variations in DSi concentration and δ30SiDSi values and examine the impact of different human disturbances including damming, eutrophication and stratification, on the coastal Si cycle for the first time.

Under the influence of both physical mixing and different stages in the spring diatom growth, δ30SiDSi values in the mixed layer decreased gradually from the shallow straits in the west (~ +2.2 ‰) to the deep central basin in the east (~ +1.4 ‰). An uncommon vertical distribution pattern of DSi was observed with heavier δ30SiDSi in the deep waters (+1.57 to +1.95 ‰) than in the surface waters (+1.24 to +1.68 ‰) in the central basin. Based on our box model, we show that although damming had an impact on the δ30SiDSi values, such a pattern was mainly attributed to the strong internal recycling of Si driven by eutrophication. Intensive diatom production exported highly fractionated Si from the surface to deep waters, and subsequently, strong benthic recycling brought heavier δ30SiDSi back into the deep water due to the fractionation during authigenic clay formation. Our findings provide solid evidence on the response of Si cycling to environmental changes and important implications for future development of the Si cycle in coastal systems and changes in nutrient export to the ocean under high human pressures and global warming.