Читать книгу Marine Mussels - Elizabeth Gosling - Страница 55

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. Additive effects of low pH and hypoxia were evident for all parameters, and low pH appeared to elicit a stronger effect than hypoxia (2 mg L⁻¹). Overall, the study showed that hypoxia can aggravate the effects of acidification and that its impacts on physiology and SFG may have ecological and economic ramifications in the short term.

Sui et al. (2017) investigated the combined effects of hypoxia and OA on the defence responses of adult M. coruscus in the East China Sea, one of the largest coastal hypoxic zones observed in the world (Chen et al. 2007). Mussels were exposed to three pH/pCO₂ levels (7.3/2800 μatm, 7.7/1020 μatm and 8.1/376 μatm) at two DO concentrations (6 and 2 mg L⁻¹) for 72 hr. Results showed that byssus thread parameters (e.g. number, diameter, attachment strength, plaque area) were reduced by low DO, and shell‐closing strength was significantly weaker under both hypoxia and low‐pH conditions. Expression patterns of genes related to mussel byssus proteins were affected by hypoxia. Generally, hypoxia reduced expressions of some of these and increased expression of others. Overall, both hypoxia and low pH induced negative effects on mussel defense responses, with hypoxia being the main driver of change. In addition, significant interactive effects between pH and DO were observed on shell‐closing strength. Depressed byssus attachment strength and shell‐closing strength may enable predators and water currents to remove mussels from the substrate more easily. In a similar study, the effects of hypoxia and salinity on antipredator responses were examined in juvenile P. viridis from Hong Kong coastal waters (Wang et al. 2012). In hypoxic and low‐salinity conditions, P. viridis produced fewer byssal threads and had a thinner shell and adductor muscle, indicating that hypoxia and low salinity are severe environmental stressors for self‐defence of mussels, and that their interactive effects further increase the predation risk.

Growing human pressures, including climate change, are having profound and diverse consequences for marine ecosystems (Doney et al. 2012). Rising atmospheric carbon dioxide is one of the most critical problems, because its effects are globally pervasive and irreversible on ecological time scales. The primary direct consequences are increasing ocean temperatures and acidity. Also, both warming and altered ocean circulation act to reduce subsurface oxygen concentrations. Furthermore, climbing temperatures create a multitude of additional changes, such as rising sea level, increased ocean stratification, decreased sea‐ice extent and altered patterns of ocean circulation, precipitation, freshwater input and extreme weather events (reviewed in Doney et al. 2012; Howard et al. 2013; USGCRP 2017). In recent decades, the rates of change have been rapid, perhaps exceeding the current and potential future tolerances of many organisms to adapt. In addition, the rates of physical and chemical change in marine ecosystems will almost certainly accelerate over the next several decades in the absence of immediate and dramatic efforts toward climate mitigation.