UC Santa Barbara

Globally, human impacts like habitat destruction, overfishing, and climate change are reducing or redistributing the abundances of marine consumers and altering ecosystems through modification of trophic or “top-down” interactions. Less considered are how the impacts of global change disrupt important “bottom-up” processes that these consumers influence, like nutrient recycling, which are critical for ecosystem functioning. Consumer-mediated nutrient dynamics (CND) are now considered integral biogeochemical components of most ecosystems, but lacking long-term studies it is difficult to predict how CND will respond to accelerating disturbances in the wake of global change. To aid in such predictions, we completed multiple studies on CND in kelp forests, which are highly dynamic, diverse, and among the most productive ecosystems on Earth. Nitrate is the major form of nitrogen believed to fuel this high productivity, yet its availability varies greatly among seasons and years. Forms of nitrogen, like ammonium, recycled by consumers are believed to sustain kelp growth during low nitrate availability, however the extent to which changes to CND in these temperate systems has received little attention. To address these gaps, we paired data of ammonium excretion rates of common fishes and macroinvertebrates with three different time series of their population dynamics in southern California kelp forests. First, we assessed how fishing, ocean warming, and disease altered the recycling of ammonium by reef macroinvertebrates in five kelp forests over an 18-year period. We discovered that California spiny lobster (Panulirus interruptus) became the dominant source of ammonium among reef invertebrates, following region-wide mass mortalities of sea stars. Following up on this finding, we evaluated how fishing disrupts the ability of lobster aggregations to form localized and reliable sources of ammonium, i.e. “hot spots”, that influence surrounding benthic communities. Finally, we examined how the loss of giant kelp as a foundation species may alter community-wide CND and the demand for nitrogen by kelp forest macroalgae by taking advantage of a 10-year kelp forest removal experiment that mimics frequent wave disturbance. Our findings from all three studies suggest that kelp forest consumers are resilient to disturbances and serve as reliable sources of ammonium, but are vulnerable to prolonged impacts from fishing, ocean warming, and loss of habitat.