Phosphorus dynamics in sediments of the Stockholm Archipelago, Baltic Sea

Increased inputs of nutrients, mainly derived from anthropogenic sources, have led to eutrophication of coastal areas in the Baltic Sea. Bottom waters in many of these areas have become (seasonally) hypoxic or even euxinic. Insight in the processes responsible for nutrient (re)cycling, transformation and removal in coastal areas is critical for the design of management strategies to improve water quality. Recent studies suggest that a significant proportion of the phosphorus (P) inputs from land into the Baltic Sea are removed in coastal areas through burial. Particularly archipelagos appear to be very effective as a filter for P.

In this study, we identify and quantify the main P-burial phases at ten sites in the Stockholm Archipelago, capturing a range of bottom water redox conditions, from oxic to euxinic. Large differences are observed in the P contents in the surface sediments, which is attributed to the variable presence of iron(hydr)oxides, a temporary sink for P. Organic P is quantitatively the most important burial sink for P at all sites. At seven of our sites, there is no evidence for sink-switching of organic or Fe-oxide bound P to authigenic P minerals. At two sites in the inner archipelago, however, our data suggests formation of a vivianite-type Fe(II)-P mineral at depth. Furthermore, at the most seaward study site, we find evidence for a role of the manganese carbonate rhodochrosite as a quantitatively important P sink. We will discuss the implications of our findings for mitigation strategies of eutrophication in the Baltic Sea.