demonstrated that OA induces more ACC formation and less crystallographic control in mussels, suggesting that ACC is used as a repair mechanism to combat shell damage under OA. However, the resultant reduced crystallographic control in mussels raises concerns for shell protective function under predation and changing environments.
Gaylord et al. (2011) tested effects of OA on the larval stage of M. californianus, a critical community member on rocky shores of the northeastern Pacific. Larvae were cultured for eight days in seawater containing CO2 concentrations associated with a present‐day global‐mean atmospheric concentration (~380 ppm) contrasted against a ‘fossil fuel‐intensive’ projection (970 ppm) and a more optimistic prediction (540 ppm). The authors analysed the strength, size and thickness of larval shells and found that acidification of the seawater had a strong impact on shell strength. The shells of five‐day‐old larvae raised in 970 ppm CO2 were 20% weaker than those of larvae reared at the current CO2 level, while the shells of larvae reared at 540 ppm CO2 were only 13% weaker than those of control individuals. They also found that after eight days at 970 ppm CO2, the shells were up to 15% thinner and 5% smaller, and the body mass of the mussels within the shells were as much as 33% smaller than those of mussels grown at modern levels. Potential ecological consequences of OA are larvae weakened by rising CO2 levels, slow development, vulnerability to predation, susceptibility to stress and risk of desiccation, ultimately altering mussel survivorship and thereby overall community dynamics.
The following is a succinct summary of the current OA scenario:
Over the next decades, it is likely that ocean acidification will pose serious consequences for many marine and estuarine shelled molluscs. A comparison of the available literature to date suggests that while detrimental effects on adults remain uncertain, the most sensitive life‐history stage seems to be the larvae, with a large majority of studies on this critical stage of development revealing negative effects. Despite these obvious trends, our current understanding of the biological consequences of an acidifying ocean over the next century is still dominated by large uncertainties. This is because the majority of studies done to date have measured single‐species responses on one stage in the life cycle, without considering the synergistic effects of other stressors (i.e. temperature, hypoxia, food concentration) and have not considered the potential for species to adapt, nor the underlying mechanisms responsible for adaptation or acclimation. In order to fully understand the consequences of ocean acidification at the population and ecosystem level, multi‐generational and multi‐stressor experiments on multiple species from geographically distinct locations are needed to assess the adaptive capacity of shelled mollusc species and the potential winners and losers in an acidifying ocean over the next century.
(Gazeau et al. 2013, p. 2239)
The high economic value of bivalves for aquaculture has stimulated a number of studies to estimate their adaptation potential to future oceanic conditions (Parker et al. 2012, 2013, 2015; Sunday et al. 2011, 2014; Thomsen et al. 2017; Vargas et al. 2017). To illustrate, Thomsen et al. (2017) recorded the successful settlement of wild mussel larvae (M. edulis) in a periodically CO2‐enriched habitat. The larval fitness of the population originating from the enriched habitat was compared to the response of a population from a nonenriched one. The high CO2‐adapted population showed higher fitness under elevated pCO2 than the nonadapted cohort, demonstrating, for the first time, an evolutionary response of a natural mussel population to OA. To assess the rate of adaptation, the authors performed a selection experiment over three generations. Tolerance to CO2 differed substantially between the families within the F1 generation, and survival was drastically decreased in the highest – yet, realistic – pCO2 treatment. Selection of CO2‐tolerant F1 mussels resulted in higher calcification performance of F1 larvae during early shell formation but did not improve overall survival. Hence, the results reveal significant short‐term selective responses of traits directly affected by OA and long‐term adaptation potential in a key bivalve species. Because immediate response to selection did not directly translate into increased fitness, multigenerational studies need to take into consideration the multivariate nature of selection acting in natural habitats. Combinations of short‐term selection with long‐term adaptation in populations from CO2‐enriched versus nonenriched natural habitats represent promising approaches toward estimating the adaptive potential of organisms facing global change.
Hypoxia
Another serious consequence of global warming that has gained attention only in the last two decades is the decrease in dissolved O2 content of the world’s oceans (Keeling et al. 2010; Levin & Le Bris 2015). Global ocean deoxygenation is a direct effect of warming. A warmer ocean is more stratified because warm water is less dense than cold water, and strong density gradients reduce vertical mixing. The combined effects of reduced oxygen solubility in warmer water and increased thermal stratification create widespread oxygen reduction, termed ‘hypoxia’ or ‘deoxygenation’. In coastal and inland waters, nutrient input from wastewater treatment plants, runoff from farmland, urban and suburban areas and air pollution from cars and so forth exacerbates the problem, because nutrients can lead to the proliferation of primary production and consequently enhance reduction of dissolved oxygen (DO) by microbes (Jewett & Romanou 2017). Increased stratification leads to reduced mixing of oxygen into the ocean interior and accounts for up to 85% of global ocean oxygen loss (Helm et al. 2011). Effects of ocean temperature change and stratification on oxygen loss are strongest in intermediate or mode waters (nearly vertically homogeneous) at bathyal depths (generally 200–3000 m), and also nearshore and in the open ocean; these changes are especially evident in tropical and subtropical waters globally, in the eastern Pacific and in the Southern Ocean (Jewett & Romanou 2017).
Hypoxic zones are areas in the ocean where the oxygen concentration is so low that animals can suffocate and die, and as a result are often called ‘dead zones’ (Diaz 2013). The largest hypoxic zone in the United States, and the second largest worldwide, occurs in the northern Gulf of Mexico adjacent to the Mississippi River. Diaz & Rosenberg (2008) assembled a global database of over 400 dead zones, a number that has not increased much since (415 in 2020). They predicted that global warming could cause dead zones to grow by a factor of 10 or more by the year 2100. Notable dead zones include the Gulf of Mexico, the Atlantic coast of North America, the east China Sea and, in European waters, the Adriatic Sea, the German Bight, the Baltic Sea and parts of the Black Sea. However, it has been demonstrated that decreased nutrient loading strongly decreases the probability of hypoxic events (Mee et al. 2005; Kemp et al. 2009). Several mitigation programmes are currently underway to reduce nutrient loading into the Gulf of Mexico, Chesapeake Bay on the Atlantic coast of the United States, and the Baltic and Black Seas.
There is growing awareness that low‐oxygen regions of the ocean are also acidified as a result of warming‐enhanced biological respiration. While the effects of hypoxia on marine mussels are well documented (see Chapter 7), the combined effects of low oxygen and acidification were until recently largely unknown. This is now an area of active research in bivalves (Meire et al. 2013; Melzner et al. 2013) and marine mussels (Gu et al. 2019; Sui et al. 2017). As seen earlier, OA can pose negative effects for the physiological energetics of mussels (Navarro et al. 2013). Therefore, it may interact with hypoxia synergistically to cause much stronger effects than those of each in isolation. To test this, Gu et al. (2019) evaluated the interactive effects of elevated CO2 and hypoxia on physiological energetics in M. edulis. Adult mussels from the East China Sea were exposed to three pH levels (8.1, 7.7, 7.3) at two dissolved oxygen (DO) levels (6 and 2 mg L−1). Clearance rate, absorption efficiency, respiration rate, excretion rate, SFG and O:N ratio were measured during a14‐day exposure time. After exposure, all parameters except excretion rate were significantly reduced under low‐pH and hypoxic conditions, while excretion rate was significantly increased.
