Growth, mortality and recruitment rates of the brown alga Lobophora variegata in relation to herbivory and nutrients on Bonaire

Abstract:

Declines in herbivory and increases in nutrient concentrations on tropical coral reefs are considered two of the major drivers of coral-algae phase shifts, but little is known about how growth and mortality of the dominant macroalgae in such shifts is regulated exactly. This study quantified growth and mortality rates at a small-scale and at high resolution in Lobophora variegata populations growing under two grazing intensities and at two different water depths on Bonaire (Southern Caribbean). Net relative growth rates were 2.2 times higher in absence of herbivores (caged treatment) compared to controls (non-caged) at 21m and 14m water depth, confirming that herbivory exerts a degree of top-down control on algae growth. Despite positive net growth rates for all treatments, high mortality rates exceeding 50% of leaves in 4 months balanced off the positive net growth rates keeping algae cover relatively stable. Only when herbivores were excluded an increase in cover of 50% was observed over 4 months. Direct evidence for mortality by grazing was only found for 6% of leaves, though there are indications that this percentage is underestimated. Recruitment of new Lobophora was positively influenced by the density of the already present Lobophora vegetation, and negatively influenced by herbivore presence. Although nutrient concentrations at the experimental sites were likely eutrophic, nutrient limitation experiments using Nutrient induced Fluorescence Transients (NIFTs) showed that L. variegata was limited by both phosphate and ammonium. This study shows that on Bonairean reefs herbivory still plays a strong role in controlling the local abundance and growth of macroalgae and their recruits, despite recent increases in fishing pressure. Secondly, our results demonstrate that even under eutrophic conditions L. variegata can be nutrient limited. This implies that a further increase in nutrients concentrations and fishing pressure could result in higher than present Lobophora growth rates and possibly faster degradation of coral reefs.