Natural Analogues in pH Variability and Predictability across the Coastal Pacific Estuaries: Extrapolation of the Increased Oyster Dissolution under Increased pH Amplitude and Low Predictability Related to Ocean Acidification.

Coastal-estuarine habitats are rapidly changing due to global climate change, with impacts influenced by the variability of carbonate chemistry conditions. However, our understanding of the responses of ecologically and economically important calcifiers to pH variability and temporal variation is limited, particularly with respect to shell-building processes. We investigated the mechanisms driving biomineralogical and physiological responses in juveniles of introduced (Pacific; Crassostrea gigas) and native (Olympia; Ostrea lurida) oysters under flow-through experimental conditions over a six-week period that simulate current and future conditions: static control and low pH (8.0 and 7.7); low pH with fluctuating (24-h) amplitude (7.7 ± 0.2 and 7.7 ± 0.5); and high-frequency (12-h) fluctuating (8.0 ± 0.2) treatment. The oysters showed physiological tolerance in vital processes, including calcification, respiration, clearance, and survival. However, shell dissolution significantly increased with larger amplitudes of pH variability compared to static pH conditions, attributable to the longer cumulative exposure to lower pH conditions, with the dissolution threshold of pH 7.7 with 0.2 amplitude. Moreover, the high-frequency treatment triggered significantly greater dissolution, likely because of the oyster's inability to respond to the unpredictable frequency of variations. The experimental findings were extrapolated to provide context for conditions existing in several Pacific coastal estuaries, with time series analyses demonstrating unique signatures of pH predictability and variability in these habitats, indicating potentially benefiting effects on fitness in these habitats. These implications are crucial for evaluating the suitability of coastal habitats for aquaculture, adaptation, and carbon dioxide removal strategies.

Predictable patterns within the kelp forest can indirectly create temporary refugia from ocean acidification.

Kelps are recognized for providing many ecosystem services in coastal areas and considered in ocean acidification (OA) mitigation. However, assessing OA modification requires an understanding of the multiple parameters involved in carbonate chemistry, especially in highly dynamic systems. We studied the effects of sugar kelp (Saccharina latissima) on an experimental farm at the north end of Hood Canal, Washington-a low retentive coastal system. In this field mesocosm study, two oyster species (Magallana gigas, Ostrea lurida) were exposed at locations in the mid, edge, and outside the kelp array. The Hood Head Sugar Kelp Farm Model outputs were used to identify dominating factors in spatial and temporal kelp dynamics, while wavelet spectrum analyses helped in understanding predictability patterns. This was linked to the measured biological responses (dissolution, growth, isotopes) of the exposed organisms. Positioned in an area of high (sub)-diel tidal fluxes with low retention potential, there were no measurable alterations of the seawater pH at the study site, demonstrating that the kelp array could not induce a direct mitigating effect against OA. However, beneficial responses in calcifiers were still observed, which are linked to two causes: increased pH predictability and improved provisioning through kelp-derived particulate organic resource utilization and as such, kelp improved habitat suitability and indirectly created refugia against OA. This study can serve as an analogue for many coastal bay habitats where prevailing physical forcing drives chemical changes. Future macrophyte studies that investigate OA mitigating effects should focus also on the importance of predictability patterns, which can additionally improve the conditions for marine calcifiers and ecosystem services vulnerable to or compromised by OA, including aquaculture sustainability.

Natural and Anthropogenic Drivers of Acidification in Large Estuaries.

Oceanic uptake of anthropogenic carbon dioxide (CO2) from the atmosphere has changed ocean biogeochemistry and threatened the health of organisms through a process known as ocean acidification (OA). Such large-scale changes affect ecosystem functions and can have impacts on societal uses, fisheries resources, and economies. In many large estuaries, anthropogenic CO2-induced acidification is enhanced by strong stratification, long water residence times, eutrophication, and a weak acid-base buffer capacity. In this article, we review how a variety of processes influence aquatic acid-base properties in estuarine waters, including coastal upwelling, river-ocean mixing, air-water gas exchange, biological production and subsequent aerobic and anaerobic respiration, calcium carbonate (CaCO3) dissolution, and benthic inputs. We emphasize the spatial and temporal dynamics of partial pressure of CO2 (pCO2), pH, and calcium carbonate mineral saturation states. Examples from three large estuaries-Chesapeake Bay, the Salish Sea, and Prince William Sound-are used to illustrate how natural and anthropogenic processes and climate change may manifest differently across estuaries, as well as the biological implications of OA on coastal calcifiers.