Community dominance by a canopy species controls the relationship between macroalgal production and species richness

By field manipulations of nutrients, propagules, and canopy cover in a shallow rocky subtidal, we show that dominance by a leathery macrophyte (Fucus vesiculosus) controls net production and the propagation of nutrient effects on understory algal species richness. On substrates with propagules, canopy cover prevented a significant loss of diversity from nutrient enrichment demonstrated outside canopy cover. On sterile substrates, negative canopy effects on diversity were counteracted by nutrient enrichment. Experimental manipulations produced a significant hump‐shaped relationship between species richness and log net production, with low richness at low or high biomass production and highest richness at intermediate biomass production. By strong control of net primary production, canopy cover changed the algal understory community to an alternative production state and thereby generated a switch in the relationship between net primary production and species richness from negative outside canopy cover to positive inside. This demonstrates that species contributing to biological structure and habitat diversity can determine the responses of a coastal ecosystem to resource loading.