Context-Dependent Top-Down and Bottom-Up Regulation of Algal Growth: A Global Meta-Analysis and Experimental Evidence from a Nutrient-Enriched Coral Reef

Coral reefs face increasing risks of phase shifts from coral to algal dominance due to anthropogenic pressures such as nutrient enrichment, overfishing, and climate change. Algal dominance can be promoted by top-down (overfishing of herbivores) and bottom-up (nutrient enrichment) factors. However, variation in their relative importance is still poorly understood. We tested the dependence of top-down control of algal growth on algal groups and herbivore size. We first conducted a controlled field experiment in a marine protected area along a coastal area of Barbados exposed to high nutrient enrichment. Exclusion cages of varying mesh sizes were used to assess the role of herbivore size in controlling macroalgae. Algal biomass was significantly higher in both large and small mesh cages compared to open and partial controls, highlighting the critical role of large herbivores in algal suppression. A global meta-analysis of 93 field experiments further revealed that both nutrient enrichment (bottom-up control) and herbivore exclusion (top-down control) significantly increase primary producer abundance, with the strongest effects observed when both factors co-occurred. While cyanobacteria showed synergistic responses to these combined pressures, macrophytes and crustose coralline algae exhibited antagonistic effects. These findings suggest herbivory can regulate both the net growth and community structure of primary producers and mitigate the impacts of nutrient enrichment on coral reef ecosystems.

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Primary Presenter: Alexandra McCallum, McGill University (alexandra.mccallum@mail.mcgill.ca)

Authors:

Alexandra McCallum, McGill University  (alexandra.mccallum@mail.mcgill.ca)