Ocean Acidification Refugia of the Florida Reef Tract

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 (Varag) values on inshore  patch reefs of the upper FRT. These waters can exhibit greater Varag than what has been modeled for the tropical surface  ocean during preindustrial times, with mean (6 std. error) Varag-values in spring = 4.69 (60.101). Conversely, Varag-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.