Ocean Acidification Refugia of the Florida Reef Tract

Derek P Manzello,Dwight K Gledhill,Nelson Melo,Elizabeth M Johns,Ian C Enochs,Richard K F Unsworth

doi:10.1371/journal.pone.0041715

Abstract

Ocean acidification (OA) is expected to reduce the calcification rates of marine organisms, yet we have little understanding of how OA will manifest within dynamic, real-world systems. Natural CO2, alkalinity, and salinity gradients can significantly alter local carbonate chemistry, and thereby create a range of susceptibility for different ecosystems to OA. As such, there is a need to characterize this natural variability of seawater carbonate chemistry, especially within coastal ecosystems. Since 2009, carbonate chemistry data have been collected on the Florida Reef Tract (FRT). During periods of heightened productivity, there is a net uptake of total CO2 (TCO2) which increases aragonite saturation state (Ωarag) values on inshore patch reefs of the upper FRT. These waters can exhibit greater Ωarag than what has been modeled for the tropical surface ocean during preindustrial times, with mean (± std. error) Ωarag-values in spring = 4.69 (±0.101). Conversely, Ωarag-values on offshore reefs generally represent oceanic carbonate chemistries consistent with present day tropical surface ocean conditions. This gradient is opposite from what has been reported for other reef environments. We hypothesize this pattern is caused by the photosynthetic uptake of TCO2 mainly by seagrasses and, to a lesser extent, macroalgae in the inshore waters of the FRT. These inshore reef habitats are therefore potential acidification refugia that are defined not only in a spatial sense, but also in time; coinciding with seasonal productivity dynamics. Coral reefs located within or immediately downstream of seagrass beds may find refuge from OA.

Highlights

Ocean acidification (OA) is the global decline in seawater pH due to the uptake of carbon dioxide (CO2) by the surface ocean [1]
During periods of heightened productivity, there is a net uptake of total CO2 (TCO2) which significantly increases aragonite saturation state (Varag) values on inshore patch reefs of the upper Florida Reef Tract (FRT)
Inshore waters at all sites were depleted in both TCO2 and total alkalinity (TA) relative to offshore during spring and summer (Fig. 2) with the pattern generally reversing in autumn and winter (Fig. 3)

Summary

Introduction

Ocean acidification (OA) is the global decline in seawater pH due to the uptake of carbon dioxide (CO2) by the surface ocean [1]. Coral reef ecosystems are especially vulnerable as their continued persistence is dependent on the deposition of CaCO3 exoskeleton by scleractinian corals [4]. Despite these concerns, we still have only a rudimentary understanding of the spatial and temporal variability of carbonate chemistry within reef environments. There is a pressing need to ascertain which locations, habitats or regions may be relatively susceptible or even resilient to OA This is a challenging undertaking which will take years, if not decades to unravel, as a given areas risk to OA will be a function of localized biogeochemical feedbacks that may locally alter the rates of OA [5], differing species-specific susceptibilities, and interactions with other stressors. Regardless, areas that act as natural CO2 sinks may serve as OA refugia because calcareous organisms will experience higher Varag relative to the open ocean

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Conclusion