Harmful algae on tropical coral reefs: Bottom-up eutrophication and top-down herbivory

A conceptual paradigm, the “Relative Dominance Model”, provides the perspective to assess the interactive external forcing-mechanisms controlling phase shifts among the dominant benthic functional groups on tropical coral reefs [i.e., microalgal turfs and frondose macroalgae (often harmful) versus reef-building corals and calcareous coralline algae (mostly beneficial due to accretion of calcareous reef framework)]. Manipulative experiments, analyses of existing communities and bioassays tested hypotheses that the relative dominances of these functional groups are mediated by two principal controlling factors: nutrients (i.e., bottom-up control) and herbivory (i.e., top-down control). The results show that reduced nutrients alone do not preclude fleshy algal growth when herbivory is low, and high herbivory alone does not prevent fleshy algal growth when nutrients are elevated. However, reduced nutrients in combination with high herbivory virtually eliminate all forms of fleshy micro- and macro-algae. The findings reveal considerable complexity in that increases in bottom-up nutrient controls and their interactions stimulate harmful fleshy algal blooms (that can alter the abundance patterns among functional groups, even under intense herbivory); conversely, elevated nutrients inhibit the growth of ecologically beneficial reef-building corals. The results show even further complexity in that nutrients also act directly as either limiting factors (e.g., physiological stresses) or as stimulatory mechanisms (e.g., growth enhancing factors), as well as functioning indirectly by influencing competitive outcomes. Herbivory directly reduces fleshy-algal biomass, which indirectly (via competitive release) favors the expansion of grazer-resistant reef-building corals and coralline algae. Because of the sensitive nature of direct/indirect and stimulating/limiting interacting factors, coral reefs are particularly vulnerable to anthropogenic reversal effects that decrease top-down controls and, concomitantly, increase bottom-up controls, dramatically altering ecosystem resiliencies.