Shallow coral reef free ocean carbon enrichment: Novel in situ flumes to manipulate pCO₂ on shallow tropical coral reef communities

Abstract

Given the severe implications of climate change and ocean acidification (OA) for marine ecosystems, there is an urgent need to quantify ecosystem function in present‐day conditions to determine the impacts of future changes in environmental conditions. For tropical coral reefs that are acutely threatened by these effects, the metabolism of benthic communities provides several metrics suitable for this purpose, but the application of infrastructure to manipulate conditions and measure community responses is not fully realized. To date, most studies of the effects of OA on coral reefs have been conducted ex situ, and while greater ecological relevance can be achieved through free ocean carbon enrichment (FOCE) experiments on undisturbed areas of reef, such approaches have been deterred by technical challenges (e.g., spatial scale and duration, stable maintenance of conditions). In this study, we describe novel experimental infrastructure called shallow coral reef (SCoRe) FOCE to overcome these challenges and present data from a proof of concept application in Mo'orea, French Polynesia. Our objectives were to (1) implement an autonomous system that could be deployed kilometers from shore, (2) regulate the chemical (pCO2) and physical properties of seawater over undisturbed, shallow (∼2–5‐m depth) coral reef over multiple weeks, and (3) measure the metabolic response of the coral community to the treatment conditions. We describe the design, function, and application of the SCoRe FOCE, and present data demonstrating its efficacy. This infrastructure has great potential for advancing ecologically relevant studies of the effects of changing environmental conditions on coral reefs.

Experimental Techniques to Assess Coral Physiology In Situ: Current Approaches and Novel Insights

“…Experiments at reef scale represent a key step forward in the in situ studies of coral responses in the natural environment [44,[150][151][152], but they also have provided the opportunity to quantify reef-scale responses to environmental perturbations. For example, manipulations to lower Wa by ~20% led to a 34% decrease in NCC consistent with the effect of lower pH on biocalcification [153], although this finding may have been influenced by a high abundance of crustose corallinae algae living in the reef community [152].…”

“…Indeed, a modification of such an experimental system, the coral-proto free ocean carbon enrichment system, allowed the short-term in situ study of the induced ocean acidification on coral reef organisms and diel changes of the seawater carbonate system [151]. Srednick et al [44] introduced another novel FOCE for spatial and temporal studies on shallow reefs, with the aim of studying a coral reef community in situ under controlled conditions of current and projected levels of pCO2. Such a system allowed a direct estimation of the hysteresis of seawater carbonate chemistry along a reef transect, using high accuracy and precision measurements of seawater pH, pCO2 and the biological responses of the reef community over high temporal resolution [44].…”

“…Srednick et al [44] introduced another novel FOCE for spatial and temporal studies on shallow reefs, with the aim of studying a coral reef community in situ under controlled conditions of current and projected levels of pCO2. Such a system allowed a direct estimation of the hysteresis of seawater carbonate chemistry along a reef transect, using high accuracy and precision measurements of seawater pH, pCO2 and the biological responses of the reef community over high temporal resolution [44]. Other high frequency sampling of coral reef carbonate dynamics and metabolic rates have used automated systems for measurements of alkalinity, pH and pCO2 at high frequency sampling [45,138].…”

“…But, our current understanding of coral stress responses is largely based on experimental manipulation studies in laboratory systems that are poor replicants of their natural habitats. In situ studies, by either SCUBA or automated sensors, could provide a better understanding of local and global impacts, but few studies have undertaken the logistical complexities of studying fine-scale physiological processes of corals in situ recently [44][45][46]. The primary objective of this review is to summarize the current strategies used to measure the metabolism of corals to highlight the benefits of non-destructive methodologies.…”

Experimental Techniques to Assess Coral Physiology In Situ: Current Approaches and Novel Insights

“…But our current understanding of coral stress responses is largely based on experimental manipulation studies in laboratory systems that are poor replicants of their natural habitats. In situ studies, by either SCUBA or automated sensors, could provide a better understanding of local and global impacts, but few studies have undertaken the logistical complexities of studying fine-scale physiological processes of corals in situ recently [49][50][51]. The primary objective of this review is to summarize the current strategies used to measure the metabolism of corals to highlight the benefits of non-destructive methodologies.…”