Recruitment of biofouling assemblages in a coral reefs ecosystem: combined effects of reduced grazing and nutrient enrichment on functional groups

Abstract

Abstract While the coral reefs in the Persian Gulf have adapted to withstand some of the most challenging environmental conditions, overfishing and eutrophication can disrupt the structure of their associated epibenthic assemblage. In this study, we investigated the effects of reduced grazing pressure and nutrient pollution on the assemblage structure of biofouling functional groups in the coral reefs of Hengam Island in the Persian Gulf. We conducted a 21-month manipulative experiment using settlement tiles to assess these effects. A total of 40 biofouling taxa were identified, none of them belonged to reef-building corals or the key facilitators of coral settlement, such as coralline algae and calcareous sponges. Our results revealed that nutrients played a significant role in shaping the composition of biofouling assemblages associated with the reefs, suggesting a bottom-up control mechanism rather than top-down regulation in the coral ecosystem of the island. When grazers were excluded, biofouling cover percentage increased and biofilms and turf algae propagules dominated. However, the effect of grazers on cover percentage was overshadowed by nutrient enrichment. Furthermore, increased nutrients resulted in the proliferation of of foliose and frondose macroalgae. Our results suggest that herbivores can reduce the coral-competing functional groups only in the absence of other confounding factors such as nutrient pollution.