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Nitrogen Fixation and Dentrification in Sediments of Eutrophic Mediterranean-Type Estuaries: Seasonal Patterns and Responses to Anthropogenic Nitrogen Inputs


Estuaries are dynamic ecosystems at the interface of terrestrial and oceanic nutrient cycles. Nitrogen (N) fixation and denitrification are key N-cycle components that add or remove N, respectively. In mediterranean-type estuaries, wet and dry seasons affect freshwater inflows and nutrient inputs, and greater coastal development and agriculture increases anthropogenic nutrient inputs, often leading to eutrophication. My research expands understanding of N-cycling in modified mediterranean-type estuaries subject to increasing nutrient influences. Using field surveys and manipulations, I investigated spatial and temporal patterns of N-fixation and relationships with abiotic characteristics and impacts of nutrient inputs on N-fixation and denitrification in unvegetated sediments of intertidal mudflats in southern California estuaries.

A 2-year survey determined N-fixation was influenced by seasonal precipitation, yet played a minor role in supplying N. During dry conditions porewater ammonium (NH4+) and nitrate (NO3-) were negatively related to N-fixation. In wet conditions, porewater salinity and N-fixation showed a negative relationship, despite simultaneously high NO3-. Only a small proportion of N-fixation variability was explained by these factors, possibly because salinity and NO3- typically co-vary and have conflicting relationships with N-fixation.

In a microcosm experiment, sediments were subjected to elevated NO3- and phosphate (PO43-) concentrations, alone and together. NO3- concentrations up to 2730μM significantly affected both processes; N-fixation decreased up to 40%, while denitrification increased up to 180-fold. However, denitrification was too low to compensate for high N inputs.

In a small, highly modified estuary, I determined N-fixation and denitrification rates under simulated low and high tide and NO3- enriched conditions. There were no differences in N-fixation or denitrification between ambient emerged and submerged conditions, suggesting rates do not vary across tidal cycles. Both processes responded significantly to N-enrichment; N-fixation decreased up to 9-fold and denitrification increased more than 100-fold.

Overall, N-fixation rates were comparable to other eutrophic mediterranean-type systems, though lower than in more oligotrophic systems. Denitrification was lower than in other eutrophic estuaries despite dramatic increases in activity under high NO3-. These results provide insight into how N is supplied internally in mediterranean-type estuaries with anthropogenic nutrient impacts and suggest additional nutrient management strategies are necessary to reduce anthropogenic eutrophication.

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