Abstract
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.
Highlights
The nutrient loading of coastal ecosystems is often divided into internal and external sources, i.e., release from sediments resulting from organic N (ON) and P (OP) mineralization, and natural and anthropogenic supplies via the water shed and atmospheric deposition, respectively
The goals of the study were twofold: (1) to quantify the magnitude and temporal dynamics of internal nutrient loading resulting from mineralization of ON and OP accumulated in sediments and (2) to evaluate the role of internal nutrient loading for the recovery of eutrophic ecosystems
Our study shows the magnitude and temporal dynamics of the internal nutrient loading that can be expected in shallow estuaries recovering from eutrophication
Summary
The nutrient loading of coastal ecosystems is often divided into internal and external sources, i.e., release from sediments resulting from organic N (ON) and P (OP) mineralization, and natural and anthropogenic supplies via the water shed and atmospheric deposition, respectively. The external nutrient loading can be quantified by summing up the external sources (e.g., Petersen et al, 2009). It is difficult, to use a mass balance approach to obtain reliable estimates of internal nutrient loading, since release from sediments and export to adjacent water bodies are difficult to quantify with sufficient temporal and spatial precision in large and dynamic estuaries with extensive spatial variability and open boundaries.
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