Efficient removal of nitrogen and phosphorus in the Stockholm Archipelago, Baltic Sea

Increased anthropogenic input of nutrients from land have led to eutrophication of marine environments globally. Coastal systems reduce the flux of nutrients from land to the open sea, thereby acting as a coastal filter. The key processes that remove nitrogen and phosphorus in coastal areas are denitrification and phosphorus burial. Recent modeling of nutrient dynamics in the Stockholm Archipelago suggests that at least 72% of the nitrogen input and 62% of the phosphorus input from land is retained in the archipelago. In contrast, on average only 16% of nitrogen and 53% of phosphorus is retained in coastal environments in the Baltic Sea. Here, we assess the benthic processes controlling the efficient removal of nutrients in the Stockholm Archipelago. Based on data for four locations, we demonstrate that area-specific rates of nitrogen removal due to benthic denitrification are highest in the inner archipelago and decrease towards the open sea. The recycling of N through DNRA and production of N2 by anammox play only minor roles. Rates of phosphorus burial in the archipelago are high due to the combined effect of high concentrations of phosphorus in the sediment and high rates of sediment accumulation. Most of the phosphorus is buried in the form of organic matter. Inorganic forms act as both temporary and permanent sinks for phosphorus. We will discuss potential future trends in nitrogen and phosphorus removal in the archipelago, the link with changes in bottom water hypoxia, and the potential role of import of nutrients from the open Baltic Sea.