Marine protection and environmental forcing influence fish‐derived nutrient cycling in kelp forests

Abstract Animal consumers excrete dissolved inorganic nitrogen, an essential nutrient for regulating primary production and macroalgal growth in the ocean. Often overlooked in attempts to explain kelp forest productivity, relatively little is known about the role of consumers in nutrient cycling in temperate systems, such as the magnitude of nutrients excreted and the factors that influence spatial and temporal variability in consumer‐derived nutrients. To investigate the supply of ammonium (NH 4 + ) excreted by the dominant members of the nearshore rocky reef fish community, we combined empirically‐measured relationships between excretion rate and body mass with data on fish density and size structure from visual SCUBA surveys conducted from 2005 to 2018 in the northern Channel Islands, California, USA. The fish community excreted a substantial amount of ammonium to the kelp forest (mean: 95.5–131.3 μmol·m −2 ·h −1 ), however excretion varied substantially among sites and over time (range: 23.1–247.9 μmol·m −2 ·h −1 ). The ammonium supply was influenced by the establishment of marine protected areas (MPAs) that restricted fishing activities and environmental characteristics that influenced geographic and temporal shifts in the overarching fish community structure. Fish‐derived ammonium excretion rates were 50%–80% greater inside MPAs compared to areas open to fishing, while environmental and habitat characteristics (e.g. habitat vertical relief, kelp biomass, wave exposure, chlorophyll a ) explained 85% of the spatial variation in community excretion rates. In contrast, large‐scale oceanographic phenomena (e.g. North Pacific Gyre Oscillation index) and past patterns of fish recruitment explained 57% of the temporal variation in nutrient excretion over the 14‐year time series. Results suggest that fish‐derived ammonium may provide an important and underappreciated nutrient source to kelp beds, particularly during low‐nutrient periods (e.g. seasonal reductions in upwelling or El Niño‐Southern Oscillation events), and that fishing disrupts these nutrient cycling pathways. Fishes likely play a critical role in supporting the resiliency of kelp forest ecosystems by supplying a relatively constant source of reduced nitrogen that can be utilized by giant kelp and other macroalgae to fuel primary production of biogenic habitat. Read the free Plain Language Summary for this article on the Journal blog.