The Effects of Sediment Runoff on the Foraging Behavior of Herbivorous Coral Reef Fishes
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2024
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Increased sediment and nutrient runoff – a consequence of localized and global anthropogenic impacts – is a primary threat to tropical coral reefs worldwide. Excessive sediment loads smother and kill coral, while algae flourish in the same nutrient-rich habitats. Herbivorous reef fishes are crucial for their role in top-down algal biomass control that maintains coral dominance. However, sediment-induced turbidity is known to affect fishes in a variety of ways, leaving questions about the ramifications of escalated sediment loads on herbivore foraging as an essential ecological function on impacted reefs. To assess broad foraging responses of herbivorous fishes to sediment-induced turbidity, foraging rates were quantified across herbivore functional traits, including reef fish that scrape algae off hard substrates (i.e., scrapers), turf croppers (i.e., grazers), and macroalgae consumers (i.e., browsers). Snorkel-based behavioral surveys focused on two scraper species (Chlorurus spilurus and Scarus psittacus), three grazer species (Acanthurus blochii, A. triostegus, and Zebrasoma flavescens) and three browser species (Naso lituratus, N. unicornis, and Z. velifer) along three independent turbidity gradients in Kāneʻohe Bay, Oʻahu Island, Hawaiʻi over the span of 13 months. Monthly surveys of benthic composition (i.e., branching coral, plate coral, macroalgae, turf, crustose coralline algae, sand, coral rubble, other) and water quality parameters (i.e., turbidity, salinity, dissolved oxygen, temperature, nitrate, visibility) were coupled with in situ behavioral observations of individual foraging rates, dietary choices, and consumption of substrates with or without settled sediment (SeS) across fish sizes (visually estimated total length). Subsequent in vivo foraging experiments were conducted with Z. velifer and inorganic sediment to tease apart the relative effects of sediment-induced turbidity versus SeS. My findings revealed a strong correlation between high macroalgae cover and turbid reefs, as well as clear functional trait- and species-specific foraging responses to turbidity gradients. Contrary to predictions, scrapers and grazers did not significantly alter foraging rates except for adult A. blochii, which showed a two-fold increase in foraging rate in response to turbidity. Despite high abundance on nearby clear-water reefs, Naso spp. and larger individuals of most scraper and grazer species were virtually absent from turbid reefs throughout the study. Zebrasoma velifer, which functions as a browser on clear-water Hawaiian reefs, avoided macroalgae relative to availability in high turbidity sites, instead showing a greater consumption of turf, denoting a high dietary plasticity in this species. There was also a significant difference in foraging responses between juvenile and adult Z. velifer, with juveniles tripling their foraging rate and adults slightly decreasing their rate across a turbidity gradient. While all species observed in the field preferred SeS-laden substrates, Z. velifer avoided inorganic SeS during in vivo foraging experiments, suggesting access to organic matter may be a driver for foraging on sedimented reefs. Overall, these results highlight divergence in tolerance to sediment-induced turbidity both within and between herbivorous fish guilds, indicating that sediment runoff causes a cascading effect whereby browsers and larger individuals disappear, and traditional top-down algal control by a variety of herbivores is diminished.
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Ecology, Behavioral sciences, Biology, Functional trait, Grazing, Land use, Nutrients, Turbidity, Water quality
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72 pages
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