Ocean Acidification and Direct Interactions Affect Coral, Macroalga, and Sponge Growth in the Florida Keys

Emily R Hall,Heather N Page,Clay Hewett,Hayden Tompkins

doi:10.3390/jmse9070739

Abstract

Coral reef community composition, function, and resilience have been altered by natural and anthropogenic stressors. Future anthropogenic ocean and coastal acidification (together termed “acidification”) may exacerbate this reef degradation. Accurately predicting reef resilience requires an understanding of not only direct impacts of acidification on marine organisms but also indirect effects on species interactions that influence community composition and reef ecosystem functions. In this 28-day experiment, we assessed the effect of acidification on coral–algal, coral–sponge, and algal–sponge interactions. We quantified growth of corals (Siderastrea radians), fleshy macroalgae (Dictyota spp.), and sponges (Pione lampa) that were exposed to local summer ambient (603 μatm) or elevated (1105 μatm) pCO2 seawater. These species are common to hard-bottom communities, including shallow reefs, in the Florida Keys. Each individual was maintained in isolation or paired with another organism. Coral growth (net calcification) was similar across seawater pCO2 and interaction treatments. Fleshy macroalgae had increased biomass when paired with a sponge but lost biomass when growing in isolation or paired with coral. Sponges grew more volumetrically in the elevated seawater pCO2 treatment (i.e., under acidification conditions). Although these results are limited in temporal and spatial scales due to the experimental design, they do lend support to the hypothesis that acidification may facilitate a shift towards increased sponge and macroalgae abundance by directly benefiting sponge growth which in turn may provide more dissolved inorganic nitrogen to macroalgae in the Florida Keys.

Highlights

IntroductionOpen ocean surface seawater pH is projected to decrease up to 0.29 units below present-day conditions by 2081–2100 under the most extreme representative concentration pathway (RCP) for greenhouse gases (RCP 8.5) [1]
Ocean acidification is the decrease in seawater pH and carbonate (CO3 2−) ions following oceanic uptake of anthropogenically generated atmospheric carbon dioxide (CO2 )
While there are abundant data describing the responses of individuals to acidification, While there are abundant data describing the responses of individuals to acidificafewer studies have examined the effects of acidification on species interactions, despite the tion, fewer studies have examined the effects of acidification on species interactions, deimportance of ecological processes to coral reef community structure, ecosystem function, spiteresilience

Summary

Introduction

Open ocean surface seawater pH is projected to decrease up to 0.29 units below present-day conditions by 2081–2100 under the most extreme representative concentration pathway (RCP) for greenhouse gases (RCP 8.5) [1]. This long-term acidification may be exacerbated across temporal scales in coastal environments in response to terrestrial influences [2]. The changes in seawater chemistry from ocean and coastal acidification ( termed “acidification”) is predicted to facilitate shifts in coral reef communities away from coral dominance in favor of non-calcifying macroalgae and sponges [4]. Net calcification of corals generally ( not always) decrease under exposure to acidification [5] while, in contrast, non-calcifying macroalgae and reef sponges maintain or accelerate their growth rates when exposed to elevated seawater pCO2 [6,7,8,9]

Methods

Results

Conclusion