Organic N and P in eutrophic fjord sediments – rates of mineralization and consequences for internal nutrient loading

Abstract. Nutrient release from the sediments in shallow eutrophic estuaries may counteract reductions of the external nutrient load and prevent or prolong ecosystem recovery. The magnitude and temporal dynamics of this potential source, termed internal nutrient loading, is poorly under\-stood. We quantified the internal nutrient loading driven by microbial mineralization of accumulated organic N (ON) and P (OP) in sediments from a shallow eutrophic estuary (Odense Fjord, Denmark). Sediments were collected from eight stations within the system and nutrient production and effluxes were measured over a period of ~ 2 years. Dissolved inorganic nitrogen (DIN) effluxes were high initially but quickly faded to low and stable levels after 50–200 days, whereas PO₄³⁻ effluxes were highly variable in the different sediments. Mineralization patterns suggested that internal N loading would quickly (< 200 days) fade to insignificant levels, whereas internal PO₄³⁻ loading could be sustained for extended time (years). When results from all stations were combined, internal N loading and P loading from the fjord bottom was up to 121 × 10³ kg N yr⁻¹ (20 kg N ha⁻¹ yr⁻¹) and 22 × 10³ kg P yr⁻¹ (3.6 kg P ha⁻¹ yr⁻¹) corresponding to 6 (N) and 36% (P) of the external nutrient loading to the system. We conclude that the internal N loading resulting from degradation of accumulated ON is low in shallow eutrophic estuaries, whereas microbial mineralization of accumulated OP is a potential source of P. Overall it appears that, in N-limited eutrophic systems, internal nutrient resulting from mineralization of ON and OP in sediments is of minor importance.