Abstract

Nitrogen is a critical nutrient limiting primary production in the ocean. Some forms of nitrogen, such as nitrates, are provided by upwelling of cold, deep water to the surface, characteristic of the west coast of most continents. These facts, provided in all marine biology textbooks, give rise to two fundamental questions. Why is productivity so high in nutrient-poor ecosystems like tropical coral reefs? And are nitrogen additions therefore unimportant for productivity in nutrient-replete ecosystems like temperate kelp forests? Over the past decade, we have tackled these questions empirically with field observations, experiments and meta-analyses in contrasting ecosystems: coral reefs of The Bahamas and kelp forests of British Columbia. We find that both questions are resolved by considering the input of excreted nitrogen by consumers such as fishes and invertebrates. In both ecosystems, consumer-mediated nutrients prove essential to mediating the growth of a range of primary producers. They also shed new light on nutrient-related effects of invasive species in these ecosystems. Taken together, our research suggests that bottom-up processes are important across marine ecosystems.