Effects of eutrophication and sedimentation on juvenile corals

Settlement of juvenile scleractinian corals was investigated from 1987 to 1990 on eutrophic and less eutrophic fringing reefs on the west coast of Barbados, West Indies. The number of coral recruits and number of recruiting coral species on cement blocks decreased with increasing eutrophication of the reefs. This may suggest lower settlement rates on eutrophic reefs, but could also liave resulted from higher post-settlement mortality, since blocks were examined only once after 3 yr of immersion. Coral settlement rates to artificial plates that were checked monthly were also lower on the more eutrophic reefs. This could result from lower local availability of larvae caused by fewer adult corals and/or lower reproductive rates of corals on eutrophic reefs. However, the ratio of coral recruits to adult coral abundance was considerably lower on eutrophic reefs, suggesting that local coral abundance alone can not explain lower settlement rates on eutrophic reefs. The lower rates on eutrophic reefs may result from a lower probability of coral larvae settlin when present, perhaps because of a limited availability of suitable settlement substrate. Colonization of settlement plates by non-coralline organisms was heavier on eutrophic reefs, and unoccupied space was lower, supporting the suggestion that suitable coral settlement substrate may be limiting on eutrophic reefs. Moreover, coralline algae, which facilitate metamorphosis and settlement of coral larvae, were less abundant on settlement plates on eutrophic reefs.