Relative importance of top-down and bottom-up controls influencing Turbinaria ornata biomass varies with depth

Abstract

Tropical reefs have experienced declines in coral due to multiple human-caused stressors, often leading to algal domination. The stability of these emergent algal communities is influenced by anthropogenic changes to top-down control by herbivory and bottom-up control by nutrient availability. In the South Pacific, Turbinaria ornata has increased in abundance since the 1980s, particularly in shallow areas of the reef, likely in response to alterations of these controlling forces. We investigated whether herbivory, nutrient availability, habitat depth, and/or their interactions influence growth and physical defenses (toughness) of small (<3 cm) T. ornata as this stage is likely most vulnerable to alterations to these factors. To determine whether nutrient availability and herbivory impact T. ornata biomass and toughness differentially at two habitat depths, we conducted a transplant experiment manipulating nutrients (+/− Nutrients) and herbivory (+/− Cages), in both the shallow (0.25 m–1 m depth) Turbinaria-dominated crest of a fringing reef and below this zone (2.5 m–4 m depth) where Turbinaria was naturally absent. We used Aikake Information Criterion (AICc) to select generalized linear mixed models (GLMM) that best fit the experimental data. Herbivory strongly controlled biomass accumulation of T. ornata in the deeper but not the shallower zone, while cages that eliminated herbivores reduced thallus toughness but only in the shallow zone. In contrast, nutrients increased thallus toughness across all depths and herbivory treatments. Our results indicate that human impacts that reduce control by herbivores and/or nutrients may stabilize T. ornata populations, providing one possible mechanism for its recent range and habitat expansion.