Giacomini Wetland Restoration Project: Restoration: What's the Long-Term Future of the Restored Wetlands?: Acidification of Coastal Waters

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Other changes that may affect the Tomales Bay estuary include the acidification of estuarine waters from deposition of carbon dioxide from the atmosphere, as well as over-fertilization of native habitats due to increased deposition of nitrogen from the atmosphere. Dissolved inorganic carbon (DIC) exists in seawater in three major forms: bicarbonate ion (HCO₃^-), carbonate ion (CO₃^2-), and aqueous carbon dioxide (CO₂), which here also includes carbonic acid (H₂CO₃). At a pH of 8.2, approximately 88 percent of the carbon is in the form of HCO₃^-; 11 percent is in the form of
CO₃^2-; and only 0.5 percent of the carbon is in the form of dissolved CO₂ (Fabry et al. 2008). When CO₂ dissolves in seawater, H₂CO₃ is formed: Most of the H₂CO₃ quickly dissociates into a hydrogen ion (H⁺) and HCO₃^- (Fabry et al. 2008). The atmospheric CO₂ value currently is ~100 ppm greater than the pre-industrial value (280 ppm), and roughly half (48%) of the CO₂ released by human activities between 1800 and 1994 is now stored in the ocean (Sabine et al. 2004). As a result, the average surface ocean pH has dropped by 0.1 unit, which is about a 30 percent increase in H⁺ (Fabry et al. 2008). Under the IPCC emission scenarios (Houghton et al. 2001), average surface ocean pH could decrease by 0.3 to 0.5 pH units from the pre-industrial values by the end of this century and by 0.3 to 1.4 units over the next 300 years, depending on the emissions scenario used (Caldeira and Wickett 2005).

This decrease in pH is important because many of the organisms in the ocean rely on calcium carbonate processes to form shells or body parts: when alkalinity drops below a certain level, shells are more likely to dissolve than to form. This phenomenon can be observed visually by pitting in shells or other morphological indicators (A. Russell, UC Davis, pers. comm.). The extent to which such organisms are affected depends largely upon the saturation state for either aragonite or calcite, two types of CaCO₃ commonly secreted by marine organisms: In regions where the saturation state of aragonite or calcite is >1.0, formation of shells and skeletons is favored (Fabry et al. 2008). For values <1.0, seawater is corrosive to CaCO₃, and, in the absence of protective mechanisms (e.g. Corliss and Honjo 1981; Isaji 1995 in Fabry et al. 2008), dissolution will begin. While many organisms have adapted to thermal fluctuations in the last few million years, the expected changes in pH are higher than any other pH changes inferred from the fossil record over the past 200 to 300 million years (Caldiera and Wickett 2005, Feely et al. 2004).

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Not only can this issue affect calcareous organisms, but it can have a cascade effect on the food web. Pteropods are an important component of the diet of pink and some other types of Pacific Northwest salmon. Pteropods form shells made of aragonite, which is approximately 50% more soluble in seawater than calcite (Mucci 1983 in Fabry et al. 2008). Owing to their highly soluble aragonitic shells, pteropods may be particularly sensitive to ocean acidification (Fabry et al. 2008). In a three-year study designed to examine the interannual variability of the feeding habits of juvenile pink salmon, Armstrong et al. (2005) found that pteropods accounted for ≥ 60 percent by weight of the juvenile salmon diet in two of three years. In a model study linking oceanic foodwebs to production and growth rates of pink salmon, Aydin et al. (2005) found that salmon expended less energy in the early spring when foraging on zooplankton than squid, although squid was more important later in the year. Other preliminary model results suggest that a 10 percent decrease in pteropod production could lead to a 20 percent drop in mature pink salmon body weight (Aydin, pers. comm. in Fabry et al. 2008), and body weight for salmon is positively correlated with survival at sea.

As part of her study, Russell has been re-evaluating the long-term dataset available for Tomales Bay from the LMER/BRIE program, because it includes alkalinity data. Between 1985 and 1996, pH of Tomales Bay waters varied seasonally throughout the 11-year dataset, with decreases in pH during the wet season (A. Russell, UC Davis, pers. comm.). A drop in pH was documented during the latter years of the program, but the cause of this shift is unknown and could potentially be related to easing of the below-average or drought conditions that occurred throughout most of the study period. Changes in pH and saturation levels of aragonite and calcite could prove important in an estuary that supports an active mariculture industry.

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At least one study predicted that mussel and oyster calcification rates could decline by as much as 25 percent and 10 percent, respectively, using the now seemingly optimistic IPCC scenarios (Gazeau et al. 2007). Another laboratory study by Bodega Marine Laboratory looked at calcification of larvae and juveniles produced by adult oysters from Tomales Bay (Gaylord 2010). Carbon dioxide concentrations in laboratory seawater were controlled to match present-day conditions in the oceans, 380 parts per million (ppm), as well as two carbon dioxide scenarios projected to occur by the year 2100 (540 and 970 ppm). The bottom-dwelling juveniles in the 970 ppm treatment grew 41 percent less than juveniles under control conditions, and the consequences persisted to a large degree, even after juveniles were returned returned to present-day CO₂ conditions, with juveniles still 28 percent smaller than those grown in control conditions.

The vulnerabily of estuaries such as Tomales to acidification impacts is still unclear. Some feel that estuarine and coastal ecosystems may be especially vulnerable to changes in water chemistry because their relative shallowness, reduced salinity, and lower alkalinity makes them inherently less buffered to changes in pH than in the open ocean. However, others note that estuaries-and the organisms that live there-may be less susceptible to the effects of acidification than those living in oceans, because pH is generally more variable in estuaries.

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What's the Long-Term Future of the Restored Wetlands?
Page 1. Introduction
Page 2. Sea Level Rise
Page 3. Precipitation, Run-Off, and Sedimentation
Page 4. Coastal Winds and Temperature
Page 5. Coastal Fog
Page 6. Coastal Upwelling
Page 7. Acidification of Coastal Waters
Page 8. Changes in Estuarine Salinity with Sea Level Rise
Page 9. Nitrogen Deposition
Page 10. Carbon Sequestration and Interactions of Carbon with Marsh Vegetation Communities
Page 11. Conclusions and References

 

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restoration includes:

Restoration Water, Water, and More Water How Long Will It Take For Tidal Wetlands To Develop? Two Years Later: Already Dramatic Improvements in Water Quality Giacomini Wetlands: A Haven for Ducks

Spring is in the Air--and in the Water Untangling the Food Web: Changes in Prey Base Following Restoration What's the Long-Term Future of the Restored Wetlands? How Can We Learn About the Restoration? How Can We Be A Part of the Restoration Process? Kids in Wetlands - Learning Through Experience Where Can We View the Restored Wetlands?

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