Effects of sediment depth on algal turf height are mediated by interactions with fish herbivory on a fringing reef

Abstract

ABSTRACT: Closely cropped algal turfs support key ecosystem functions on healthy coral-dominated reefs, yet how this important reef component is affected by sedimentation, a key stres-sor on reefs worldwide, is relatively unknown. We used a 2-factor caging experiment to evaluatethe effects of varying sediment depth and presence of herbivorous fish on algal turf height on afringing reef in Mo’orea, French Polynesia. Without herbivory, 2 mm of sediment reduced turfgrowth by ~50% compared to sediment removal treatments; in contrast, growth with 4 mm of sed-iment was low or negligible regardless of herbivory treatment. Negative effects of sediment werelinked to the development of black basal layers of sediment, indicating accumulation of hydrogensulfide. Black sediment occurred in 60 to 70% of all 4 mm plots and in 43% of caged 2 mm plotsbut was not found in open 2 mm plots, implying that grazing ameliorated development of blacksediment under 2 mm loads. Sediment levels of 2 mm did not deter herbivory, evidenced by thesignificant decrease in turf height in open compared to caged plots. Under 4 mm of sediment,black sediment inhibited both growth and herbivory where it occurred. Without black sediment,however, fish grazing balanced algal growth, resulting in negligible algal height changes across4 mm plots but with differing underlying mechanisms. Field surveys on other sedimented reefswith healthy herbivore communities confirmed an increase in the presence of black sediment atdepths over 3 mm. Thus, deeper sediment depths inhibit turf growth, yet under moderate levels ofsedimentation, intact herbivorous fish communities may maintain closely cropped, healthy turfcommunities by preventing the negative effects of black sediment.KEY WORDS: Sediments · Algal turf · Herbivory · Fringing reef · Tropical algae