Temporal variation in macroalgal removal: insights from an impacted equatorial coral reef system

Abstract

Macroalgal removal is a critical ecosystem function yet few studies have considered its temporal variability, especially on impacted reefs with limited herbivorous fish biodiversity. To address this, we quantified macroalgal removal and mass-standardised bite rates of herbivorous fishes monthly from July 2016 to June 2017 using a series of transplanted Sargassum ilicifolium assays and underwater video cameras on three degraded coral reefs in Singapore: Pulau Satumu, Kusu Island, and Terumbu Pempang Tengah. Our results revealed a distinct temporal pattern in macroalgal herbivory (proportion of biomass removed and mass-standardised bite rates) rates across all sites, increasing from July and decreasing from January, with the highest rates recorded in December (28.10 ± 3.05 g 3.5 h?1; 208.24 ± 29.99 mass-standardised bites 3.5 h?1) and the lowest in May (0.86 ± 0.17 g 3.5 h?1; 9.55 ± 3.19 mass-standardised bites 3.5 h?1). These coincided with the S. ilicifolium growth cycle, confirming previous evidence that herbivory rates are closely linked to macroalgal condition. Video analyses revealed nine species feeding over a year (31,839 bites; 8702.89 mass-standardised bites), with Siganus virgatus responsible for ? 80% of the total mass-standardised bites. Siganus virgatus took the largest proportion of bites monthly, except between April and June, when Scarus rivulatus was dominant, suggesting temporal constraints in functional roles. © 2020, The Author(s).