Elevated nutrients and herbivory negatively affect Dictyota growth dynamics

Abstract

Caribbean coral reefs are experiencing a shift to algal dominance at the expense of stony corals. Determining the factors leading to algal phase shifts is crucial for assuring the survival of Caribbean coral reefs. In this study, factors controlling the growth of the abundant brown macroalgae Dictyota spp. were investigated by varying herbivory pressure (caging) and nutrients (fertilizer addition) on coral reefs near St. Thomas (US Virgin Islands). Experiment 1 measured Dictyota heights and percent cover at 3 sites (11-20 m depth) and showed no growth response to nutrient addition and a weak negative response to herbivory. To confirm results of Experiment 1, a caging and nutrient manipulation (Experiment 2) was conducted at one site (14 m depth) using the dependent variable Dictyota biomass. A strong negative response of growth to nutrient addition was shown, presumably because of nutrient inhibition, and an equally negative response to herbivory (loss of ~50% biomass over 21 d). The inhibitory effect of fertilization on growth was confirmed in a third experiment that showed increasing biomass loss over 4 treatment levels of increasing fertilizer addition (0 [ambient], 5, 10, 20 g). Overall, Dictyota was not nutrient limited at any sites, and was weakly controlled by herbivore populations. Factors responsible for Dictyota abundance on Caribbean reefs may reflect decreased herbivory caused by overfishing and reductions in coral cover and do not appear to be affected by recent changes in nitrogen or phosphorus load. This study reinforces the need for conservation and management of herbivores in coral reef ecosystems, to mitigate the effects from anthropogenic stressors.