Along upwelling marine shorelines, much attention is focused on the supply of nitrate to coastal organisms and how it is regulated by wind-driven upwelling processes. However, regenerated nitrogen is available through the metabolic activities of animals and might also be important for local communities. I investigated how regenerated nitrogen from the abundant mussel, Mytilus californianus, locally enhances ammonium levels and seaweed productivity on the outer coast of Washington through experimental manipulation, stable isotope analysis, nutrient sampling and known rates of excretion and abundance of mussels.

Results/Conclusions

The experimental manipulation of mussels in tidepools showed that algal growth was enhanced by the presence of mussels and the increased ammonium supply. Stable isotope analyses of seagrasses demonstrated an enriched δ¹⁵N signal in seagrasses in proximity to mussels compared with those away from mussels. The importance of regenerated nitrogen at larger scales was supported by water sampling near Tatoosh Island, Washington, where ammonium peaks nearshore and rapidly declines at distances of 50 to 500 m away. Estimations from published research on mussel physiology and abundance suggest that the mussel contribution to regenerated nitrogen is several grams per m² per day. Further, analysis of the nitrogen dynamics in the nearshore suggests that this animal-regenerated ammonium provides a resource for a microbial community whose role in the nitrogen cycle is relatively undescribed.