Habitat modification by a canopy-forming seaweed affects resource versus consumer control in a coastal benthic food web

The importance of the role played by habitat-modifying organisms in aquatic systems is increasingly recognized. In marine communities the presence of foundation species like canopy-forming seaweeds dramatically increases habitat heterogeneity. The activities of these organisms can affect the distribution and / or abundance of other species by modifying the physical environment, altering the availability of resources, and providing habitat for other organisms. However, how biotic contribution to habitat structure interacts with bottom-up and top-down processes, is less well understood. By field-manipulations of grazers, nutrients, and a dominating leathery macrophyte (Fucus vesiculosus) we tested for the effects of consumers and nutrient enrichment on biomass and elemental composition of benthic microalgae in the absence or presence of canopies or artificial shadings. Our results indicate that canopy cover controlled understory periphyton Chl a and AFDW both by limiting light and increasing disturbance by whiplash. Microphytobenthic biomass was antagonistically and independently affected by grazing and nutrient supply. Periphyton molar C:N ratios were significantly higher in ambient light. The presented study highlights the importance of incorporating habitat modification and facilitation into benthic food web theory and of extending the focus to understory benthic microalgae. We conclude that the net effect of canopy-forming algae on understory species does not simply depend on the degree of environmental stress but also on the niche requirements of the associated organism group.