From Nitrogen Loss to Retention: The Intensity and Duration of Organic Loading from Fish Farms Alter the Fate of Nitrogen in the Cohesive Surface Sediments

We experimentally demonstrate that organic matter from open-ocean fish farms, when deposited onto aphotic cohesive sediments, can significantly alter microbial nitrogen cycling. Initially, sediments act as sites of fixed-nitrogen removal via efficient nitrification–denitrification coupling. However, with continued organic loading, they shift to retaining fixed nitrogen and exporting ammonium. In lab-based sediment microcosms, we measured sediment–seawater fluxes of nitrate + nitrite (NO_x), ammonium (NH₄⁺), and dinitrogen (N₂) following short-term additions (1 and 7 days) of fresh fish waste across moderate (1.1–3.2 g C m^–2 d^–1) to heavy (11.2 g C m^–2 d^–1) loading scenarios. After 1 day, NH₄⁺ was efficiently converted to N₂. After 7 days, however, organic carbon accumulated and NH₄⁺ export dominated. 16S rRNA gene sequencing revealed that this shift in function was linked to reduced nitrification efficiency, increased dissimilatory nitrate reduction to ammonium (DNRA), and potential stimulation of nitrogen fixation. Our findings suggest that sustained organic enrichment from open-ocean aquaculture can disrupt benthic nitrogen removal processes and promote fixed-nitrogen retention with potential consequences for coastal nutrient dynamics. These results have important implications for the ecological sustainability of offshore aquaculture and can inform ecosystem-based management strategies.