Buffer Capacity, Ecosystem Feedbacks, and Seawater Chemistry under Global Change

Abstract

Ocean acidification (OA) results in reduced seawater pH and aragonite saturation state (Ω arag ), but also reduced seawater buffer capacity. As buffer capacity decreases, diel variation in seawater chemistry increases. However, a variety of ecosystem feedbacks can modulate changes in both average seawater chemistry and diel seawater chemistry variation. Here we model these effects for a coastal, reef flat ecosystem. We show that an increase in offshore pCO 2 and temperature (to 900 µatm and + 3 °C) can increase diel pH variation by as much as a factor of 2.5 and can increase diel pCO 2 variation by a factor of 4.6, depending on ecosystem feedbacks and seawater residence time. Importantly, these effects are different between day and night. With increasing seawater residence time and increasing feedback intensity, daytime seawater chemistry becomes more similar to present-day conditions while nighttime seawater chemistry becomes less similar to present-day conditions. Recent studies suggest that carbonate chemistry variation itself, independent of the average chemistry conditions, can have important effects on marine organisms and ecosystem processes. Better constraining ecosystem feedbacks under global change will improve projections of coastal water chemistry, but this study shows the importance of considering changes in both average carbonate chemistry and diel chemistry variation for organisms and ecosystems. OPEN ACCESSWater 2013, 5 1304

Coral Reef Carbonate Chemistry Variability at Different Functional Scales

“…These changes will most likely increase the difference between the daily extreme values relative to the mean pH due to increasing dominance by NCP over NCC, although, nighttime pH could be partially buffered due to increasing CaCO 3 dissolution (Page et al, 2016). Furthermore, increased CO 2 due to ocean acidification will lead to larger diel cycles in pH due to decreased buffering capacity of seawater (Jury et al, 2013;Shaw et al, 2013;Takeshita et al, 2015).…”

Net Community Metabolism and Seawater Carbonate Chemistry Scale Non-intuitively with Coral Cover

“…Understanding how coral reef metabolism will change in response to decreasing coral cover and ocean warming and acidification is critical because it relates to a reef 's ability to accrete CaCO 3 and its ability to locally alleviate or exacerbate OA Kleypas et al, 2011;Jury et al, 2013;Andersson et al, 2014;Page et al, 2016). Coral reef metabolism can have a significant influence on the local seawater carbonate chemistry (Bates, 2002;Bates et al, 2010;Hofmann et al, 2011;Andersson and Mackenzie, 2012;Shaw et al, 2012;Drupp et al, 2013), and diel and seasonal variations in coral reef CO 2 parameters (e.g., pCO 2 , pH, and a ) are much greater than in the open ocean.…”

Impact of Ocean Acidification on Marine Organisms—Unifying Principles and New Paradigms

“…We are now forewarned that that daily variations in seawater chemistry may increase on tropical coral reefs because ocean acidification causes a marked reduction in the buffering capacity of seawater on the reefs at night [4], but at least by acting on local stressors (e.g., eutrophication and overfishing) managers may be able to "buy time" as we grapple with international agreements on reducing CO2 emissions [10]. The finding that simulated ocean acidification had no significant effects on the growth and skeletal structure of the adult stages of two widespread cold-water corals [3], but can be particularly severe for organisms which start to calcify in their larval and/or juvenile stages [1] and there may also be indirect effects through loss of food quality since rising CO2 levels affect the biochemistry of phytoplankton [7,8] highlights the fact that there is likely to be significant variations in species' responses.…”

“…As part of his PhD, Christopher Jury worked with colleagues at the University of Hawaii, including Marlin Atkinson, who is now sadly deceased, on a modelling study that concerns the capacity of coastal waters to buffer reductions in pH [4]. They produced a model of a coastal tropical coral reef that incorporates the fact that as the buffer capacity of seawater decreases, daily variations in chemistry increase.…”