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

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 stressor on reefs worldwide, is relatively unknown. We used a 2-factor caging experiment to evaluate the effects of varying sediment depth and presence of herbivorous fish on algal turf height on a fringing reef in Mo'orea, French Polynesia. Without herbivory, 2 mm of sediment reduced turf growth by similar to 50% compared to sediment removal treatments; in contrast, growth with 4 mm of sediment was low or negligible regardless of herbivory treatment. Negative effects of sediment were linked to the development of black basal layers of sediment, indicating accumulation of hydrogen sulfide. Black sediment occurred in 60 to 70% of all 4 mm plots and in 43% of caged 2 mm plots but was not found in open 2 mm plots, implying that grazing ameliorated development of black sediment under 2 mm loads. Sediment levels of 2 mm did not deter herbivory, evidenced by the significant 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 across 4 mm plots but with differing underlying mechanisms. Field surveys on other sedimented reefs with healthy herbivore communities confirmed an increase in the presence of black sediment at depths over 3 mm. Thus, deeper sediment depths inhibit turf growth, yet under moderate levels of sedimentation, intact herbivorous fish communities may maintain closely cropped, healthy turf communities by preventing the negative effects of black sediment.