Differences in top-down and bottom-up regulation of macroalgal communities between a reef crest and back reef habitat in Zanzibar

Coral reef ecosystems are subjected to intense pressure from growing coastal populations and subsequently increased nutrient loading and extraction of marine organisms. This development has altered top-down and bottom-up regulation of macroalgae in the reef system. The relative importance of these regulating forces is also influenced by environmental prerequisites, such as exposure to wave action and water motion. Thus, the present study tested the importance of top-down and bottom-up regulation, by manipulation of nutrient availability and grazer abundance, at one reef crest- and one back reef-site in Chwaka bay (Zanzibar, Tanzania). Wave action and water motion may regulate macroalgal communities by affecting the mobility of herbivores and availability of nutrients. The present study was conducted at the onset of the monsoon period, with a general decline of macroalgal cover and biomass in the region; positive effects on biomass development were therefore manifested in reduced decline and not in an actual increase. The experimental study showed that both caging and fertilization had significant impacts on macroalgal community composition but only caging showed any significant effects on biomass development. However, the influences of both these structuring forces were lower at the more exposed crest-site. This period was chosen as most similar studies have been conducted during growth season, often overlooking the studied period. Such previous studies have shown that herbivore exclusion increases macroalgal biomass, while the present study shows that it can also reduce biomass decline during the seasonal die-off by approx 50%. Together, these results suggest an overall larger macroalgal presence on the reef when herbivory is reduced. In general, our results propose that exposure to wave action and water motion functions as an important regulating factor, affecting macroalgal communities by influencing both top-down and bottom-up regulation. In turn, these results suggest that anthropogenic disturbances may have a greater impact on more sheltered coral reef habitats.

Research highlights
► We examine factors regulating coral reef macroalgae.
► Herbivore grazing had the largest influence on macroalgal biomass.
► Nutrient availability mainly influenced community composition.
► Hydrodynamic forces reduced the effects of treatments.
► Results suggest stronger effects of anthropogenic influences on sheltered reefs.