Warming Alters the Relationship Between Benthic Cover and Herbivores on Hawaiian Reefs

Shawna A Foo,Christopher H Teague,Gregory P Asner

doi:10.3389/fmars.2022.787314

Shawna A Foo, Christopher H Teague + Show 1 more

Open Access

https://doi.org/10.3389/fmars.2022.787314

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Abstract

Increases in sea surface temperature impact animal metabolism, which in turn could influence benthic structure and resulting algal-coral balance. We utilized a long-term coral reef dataset from the west coast of Hawai‘i Island to investigate impacts of annual positive and negative sea surface temperature anomalies (SSTA) on benthic cover [algal turf, macroalgae, crustose coralline algae (CCA), and coral], herbivore density (sea urchins, grazers, browsers, and scrapers) and the relationship between benthic cover and herbivore density. Results showed significantly lower coral cover, but higher CCA cover with positive SSTA. Additionally, the density of sea urchins, grazers and browsers increased with increasing SSTA. Warming disrupted the normal relationship between herbivores and benthic cover on reefs, particularly for grazers where higher densities were coupled with lower algal turf cover only during negative SSTA. The direction of the relationship between benthic cover and herbivore type changed with positive SSTA, where increased algal turf cover was associated with increased herbivore density. Here, herbivores are likely responding accordingly to increases in food availability due to increased metabolism under warming. Despite herbivore populations increasing in density over the past two decades, algal turf cover remains on an upward trajectory. These results indicate that warming can alter herbivore-algal dynamics, where greater herbivore densities may be required to cause a reduction in algal turf cover. Protection of herbivores in addition to reducing nutrient input onto reefs will be essential in driving a reduction in algal turf cover on Hawaiian reefs.

Highlights

The worldwide loss of live coral cover and phase shifts from coral- to algal-dominated reefs has sparked various efforts to facilitate coral reef persistence and resilience (Hughes et al, 2018a; National Academies of Sciences, Engineering, and Medicine [NAS], 2019)
The objectives of this study were to: (1) investigate the impacts of positive and negative sea surface temperature anomalies (SSTA) on benthic cover and herbivore density, (2) test how positive and negative SSTA affect the relationship between benthic cover and herbivore density
The dataset utilized in this study was part of the long-term, daytime monitoring efforts of coral reefs in West Hawai‘i conducted by the Hawai‘i Division of Aquatic Resources (DAR) at 23–25 permanent sites from 1999 to 2019 spanning over 140 km of the West Hawai‘i coastline (Supplementary Figure 1)

Summary

Introduction

The worldwide loss of live coral cover and phase shifts from coral- to algal-dominated reefs has sparked various efforts to facilitate coral reef persistence and resilience (Hughes et al, 2018a; National Academies of Sciences, Engineering, and Medicine [NAS], 2019). A key indicator of coral reef health and area of research includes preventing the transition from coral to macroalgal dominance (Hughes, 1994), where overfishing of herbivorous fish and eutrophication favor algal growth (Pandolfi et al, 2005). Algae are major space competitors with corals (McCook, 2001; McManus and Polsenberg, 2004), where the control of algal cover is important in preventing suffocation of corals, allowing growth of crustose coralline algae (CCA), and providing space for juvenile corals to settle (Edmunds and Carpenter, 2001; Craggs et al, 2019). Algal dominance is controlled through grazing by herbivorous fish and sea urchins (Carpenter and Edmunds, 2006; Johansson et al, 2010). Algal growth is influenced by a variety of bottom-up factors, such as light, salinity and nutrient concentration, where these impacts can sometimes outweigh those of herbivorous fish (Gilby et al, 2015)

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