Ocean Acidification Refugia of the Florida Reef Tract

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.

Calcification rates of the massive coral Siderastrea siderea and crustose coralline algae along the Florida Keys (USA) outer-reef tract

“…While stations very close to the main Keys did show elevated levels of nutrients (Boyer and Briceno 2010), dissolved inorganic nutrients have not shown convincing onshore-offshore patterns beyond the landward portion of Hawk Channel (Szmant and Forrester 1996). Recent monitoring of the carbonate system variables has shown onshore-offshore and seasonal patterns in the main Keys (Manzello et al 2012) and seasonal patterns in waters offshore of the Keys (Gledhill et al 2008), but few data exist for the Dry Tortugas area. Two studies so far have attempted to link water quality and coral growth on the FKRT.…”

The Future of Coral Reefs Subject to Rapid Climate Change: Lessons from Natural Extreme Environments

“…Daytime drawdown of CO 2 has resulted in seagrass meadows having elevated pH during the day across several regional locations, e.g., Spain (Invers et al, 1997), Florida (Manzello et al, 2012), the Cayman Islands, Indonesia, and the Seychelles (Camp et al, 2016a) that can create an inshore-to-offshore carbonate chemistry gradient (Manzello et al, 2012;Camp et al, 2016a). The capability of seagrasses to elevate local mean pH has led to them being described as potential refugia under ocean acidification (Manzello et al, 2012), since they buffer resident species from rising seawater acidity (Hofmann et al, 2011;Manzello et al, 2012;Hendriks et al, 2014). Unsworth et al (2012) proposed that the photosynthetic activity of seagrass meadows in the IndoPacific could enhance seawater physicochemical conditions to increase coral calcification downstream by as much as 18%.…”

“…Seagrass meadows have not typically been considered important coral habitats due to a lack of solid substrate (Jackson, 1986;Manzello et al, 2012). However, corals can recruit into seagrass systems; for example in the Cayman Islands where 14 coral taxa (primarily P. astreoides and Siderastrea sp.)…”

“…Furthermore, low night-time pH could result in chemical conditions where carbonate-sediment dissolution occurs (Yamamoto et al, 2012), increasing local A T and pH, thus creating a self-buffered system (Andersson et al, 2014). However the shallow nature of seagrass systems means they also experience variability in other parameters such as oxygen (Hendriks et al, 2014), salinity (Manzello et al, 2012), temperature (Manzello et al, 2012;Camp et al, 2016a,b), and light (Hendriks et al, 2014). Work from seagrass habitats in the Cayman Islands suggests that if future diel-oscillations in pH and temperature change at an equivalent magnitude as predicted for the open-ocean, seagrass systems could be limited in their ability to buffer resident species, due to the extreme low nighttime pH and elevated daytime temperatures they experience (Camp et al, 2016b).…”

“…Across global locations, there are reports of shallow in-shore reef systems being regularly exposed (albeit for different durations) to conditions predicted for the open-ocean under climate change by the end of the century (e.g., Hofmann et al, 2011;Price et al, 2012;Albright et al, 2013;Guadayol et al, 2014;Kline et al, 2015). The shallow water depth and reduced residency time of inshore waters can create an inshore to offshore carbonate chemistry gradient (Manzello et al, 2012;Shamberger et al, 2014;Camp et al, 2016a). Nutrient run-off and freshwater input can influence A T , while the metabolic activity of local benthos can change pCO 2 levels.…”