Mechanisms controlling lower trophic ecosystem response to ocean outfall discharges: Role of nitrogen form and freshwater volume

Ocean outfalls, a primary means of municipal and industrial wastewater disposal in coastal communities around the world, are designed to take advantage of transport by the ocean circulation for wastewater dispersal. However, studies that comprehensively characterize the effect of anthropogenic nitrogen discharged via ocean outfalls on coastal eutrophication remain limited. Here, we investigate the effects of the following individual component of outfall discharge on net primary production (NPP): (1) inorganic nitrogen input, (2) freshwater input, and (3) inorganic nitrogen form. To this end, we develop a series of idealized scenarios utilizing an ocean biogeochemical model of the Southern California Bight, a highly urbanized coastal region affected by wastewater discharge. We find that a greater volume of freshwater input via wastewater expands the spatial footprint of NPP changes, allowing for a greater spatial dispersion of nutrients, but does not decrease the NPP averaged over a coastal band. In contrast, introducing nutrient loads in a more concentrated wastewater volume causes a patchier distribution of NPP, and can result in higher NPP in embayments characterized by longer water residence times. Ammonium-dominated wastewater stimulates 40% more productivity than nitrate-dominated wastewater for the same DIN addition. These changes are modulated by the seasonal cycle, with greatest impacts during the weakly-stratified regime in wintertime. By using a realistic ocean biogeochemical model, we quantify changes to nearshore nutrients and productivity at horizontal, vertical, and temporal scales that remain challenging to monitor by in situ or remote sensing techniques, offering new insights for water quality management.