Abstract
Open–ocean observations have revealed gradual changes in seawater carbon dioxide (CO2) chemistry resulting from uptake of atmospheric CO2 and ocean acidification (OA), but, with few long–term records (>five years) of the coastal ocean that can reveal the pace and direction of environmental change. In this paper, observations collected from 1996 to 2016 at Harrington Sound, Bermuda, constitute one of the longest time–series of coastal ocean inorganic carbon chemistry. Uniquely, such changes can be placed into the context of contemporaneous offshore changes observed at the nearby Bermuda Atlantic Time-series Study (BATS) site. Onshore, surface dissolved inorganic carbon (DIC) and partial pressure of CO2 (pCO2; >10% change per decade) have increased and OA indicators such as pH and calcium carbonate (CaCO3) saturation state (Ω) decreased from 1996–2016 at a rate of two to three times that observed offshore at BATS. Such changes, combined with reduction of total alkalinity over time, reveal a complex interplay of biogeochemical processes influencing Bermuda reef metabolism, including net ecosystem production (NEP=gross primary production–autotrophic and heterotrophic respiration) and net ecosystem calcification (NEC=gross calcification–gross CaCO3 dissolution). These long–term data show ae seasonal shift between wintertime net heterotrophy and summertime net autotrophy for the entire Bermuda reef system. Over annual time-scales, the Bermuda reef system does not appear to be in trophic balance, but rather slightly net heterotrophic. In addition, the reef system is net accretive (i.e., gross calcification > gross CaCO3 dissolution), but there were occasional periods when the entire reef system appears to transiently shift to net dissolution. A previous five–year study of the Bermuda reef suggested that net calcification and net heterotrophy have both increased. Over the past twenty years, rates of net calcification and net heterotrophy determined for the Bermuda reef system have increased by ~30%, most likely due to increased coral nutrition occurring in concert with increased offshore productivity in the surrounding subtropical North Atlantic Ocean. Importantly, this long–term study reveals that other environmental factors can mitigate against the effects of ocean acidification on coral reef calcification, at least over the past couple of decades.
Highlights
The human–mediated increase in atmospheric carbon dioxide (CO2) and its uptake by the global ocean (Sabine et al, 2004; Khatiwala et al, 2013) has resulted in substantive changes in ocean chemistry and the marine CO2–carbonate system over the past few decades (e.g., Dore et al, 2009; Bates, 2012; Bates et al, 2014)
The observed gradual decline in ocean pH and saturation states of calcium carbonate (CaCO3) minerals (i.e., ) that results from ocean acidification (i.e., OA; Broecker et al, 1971, 1979; Caldeira and Wickett, 2003) and future end–of– century projections of ocean chemistry (e.g., Orr et al, 2005), constrained by emission scenarios of anthropogenic CO2, raise profound concerns about the potential consequences for marine calcifiers and ecosystems, in particular (e.g., Kleypas et al, 1999; Royal Society, 2005; Doney, 2006; Fabry et al, 2008; Doney et al, 2009)
In the milieu of environmental issues that beset coral reef health and future viability, ocean acidification comingles with variability of natural conditions to influence coral reef biogeochemical processes such as calcification, photosynthesis, respiration and CaCO3 dissolution
Summary
The human–mediated increase in atmospheric carbon dioxide (CO2) and its uptake by the global ocean (Sabine et al, 2004; Khatiwala et al, 2013) has resulted in substantive changes in ocean chemistry and the marine CO2–carbonate system over the past few decades (e.g., Dore et al, 2009; Bates, 2012; Bates et al, 2014). There is a growing appreciation of the complex synergies, interactions and feedbacks that exist between ocean biogeochemistry and individual species/entire ecosystem response and resilience across multiple time—and spatial scales (e.g., Ries et al, 2009; Andersson et al, 2014; Breitburg et al, 2015; Gaylord et al, 2015) This includes the interaction and interplay between warming, deoxygenation and OA, and other environmental changes such as nutrient pollution (e.g., Anthony et al, 2008; Reid et al, 2009; Atewerbehan et al, 2013; Bijma et al, 2013; Bozec and Mumby, 2015; Malone et al, 2016). The simplification of relationships between coral calcification and CO2–carbonate chemistry has many predictive uses and value for OA research (e.g., Kleypas et al, 1999; Orr et al, 2005; Dove et al, 2013; Evenhuis et al, 2015), but as Jokiel and others have pointed out (e.g., Jokiel et al, 2008; Jokiel, 2011a,b, 2016; Comeau et al, 2013; Andersson et al, 2014; Bach, 2015), there may not be simple relationships or thresholds in ocean chemistry that are insightful for future projections
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