Diversity of Intertidal Macroalgae Increases with Nitrogen Loading by Invertebrates

Many ecological phenomena are characterized by context dependency, and the relationship between diversity and productivity is no exception. We examined the relationship between macroalgal diversity and nutrient availability by evaluating the effects of reduced nutrients and their subsequent replacement via local-scale nutrient loading in tide pools. Macroalgae in Oregon coast high-intertidal pools have evolved in a nitrate-rich upwelling ecosystem, but instead of settling on low-intertidal reefs (where algae are often immersed in nutrient-rich nearshore waters) these individuals have colonized high-zone pools, where they are isolated from the ocean for extended periods of time and are subjected to extended periods of nitrate depletion. In some pools, this nutrient stress was ameliorated by a positive interaction: the excretion of ammonium by invertebrates. We conducted experimental manipulations to quantify invertebrate-mediated ammonium loading and macroalgal ammonium uptake in high-intertidal pools. Variation in tide pool volume and invertebrate biomass created a gradient of local-scale nutrient inputs, allowing us to address the relationship between nitrogen loading and algal diversity. Slow-growing species tolerant of low nitrogen availability were joined by fast-growing species with higher nitrogen requirements in pools with higher ammonium loading rates. A fourfold increase in the ammonium loading rate was associated with a doubling in the number of macroalgal species, and macroalgal assemblages in more species-rich pools were characterized by higher rates of biomass-specific ammonium uptake. These patterns contrast with productivity–diversity relationships in terrestrial systems, where local-scale nutrient enrichments generally result in reduced producer diversity due to displacement of subordinate species by aggressive competitors. Our data suggest that the effect of enrichment on diversity is context-dependent. Each ecosystem has a critical level of nutrient availability, determined by the level of nutrients typically available in that system. Below this critical level, local-scale nutrient additions increase diversity, but above it, diversity declines with enrichment.