The contribution of autochthonous versus allochthonous inputs to productivity is an important determinant of ecosystem function across multiple habitats.  In coastal marine systems, nutrients are thought to come primarily from the upwelling of deep, nutrient rich water.  Using experimental manipulations of a dominant tidepool animal, the mussel Mytilus californianus, I show that the presence of mussels greatly increases the supply of inorganic nitrogen and phosphorus on the rocky shores of the Washington coast.  Mussels further had a direct effect on productivity:  benthic microalgal abundance increased by a factor of 4 to 8, while the growth of a red alga was 4 times greater in the presence of mussels.  The increase in nitrite and nitrate associated with mussels further suggests nitrifying activity by microbes.  Nutrient censuses from Tatoosh Island showed elevated levels immediately adjacent to the Island, further suggesting strong local effects of marine animals on nutrient dynamics and cycling in coastal ecosystems.  These findings have broad implications for coastal marine systems, including that regenerated nutrients may contribute more to productivity than previously recognized and that the presence of animal-generated nutrients sets the stage for numerous positive interactions among species.