Marine Mussels. Elizabeth Gosling

Figure 3.16 Global atmospheric carbon dioxide (CO₂) concentrations in parts per million (ppm) for the past 800 000 years. The peaks and valleys track ice ages (low CO₂) and warmer interglacials (higher CO₂). During these cycles, CO₂ was never higher than 300 ppm. In 2018, it reached 407.4 ppm. On the geologic time scale, the increase (dashed line) looks virtually instantaneous.

      Source: Data from Lindsey (2020).

So, what are the consequences of climate change for marine ecosystems? Mora et al. (2013), using global climate models, have shown that in the next 100 years, the entire world’s ocean surface will be simultaneously impacted by varying intensities of ocean warming, acidification, oxygen depletion or shortfalls in productivity. In contrast, only a very small fraction of the world’s ocean surface, mostly in polar regions, will experience increased oxygenation and productivity, and almost nowhere will there be cooling or pH increase. From a compiled list of 32 marine habitats and biodiversity hot spots, Mora et al. (2013) found that all would experience simultaneous exposure to changes in multiple biogeochemical parameters, which will demand multiple physiological adjustments from marine biota. However, regional‐scale differences in response to climate change can often be more relevant than global averages. For example, a study of SST change in 63 global large marine ecosystems (LMEs) over a 50‐year period (1957–2006) revealed strong regional variation, with the Subarctic Gyre, European Seas and East Asian Seas warming at two to four times the global mean rate (Belkin 2009). The Subarctic Gyre warming is likely caused by natural variability in relation to the North Atlantic Oscillation, a climatic phenomenon which varies over time but has no particular periodicity. The most rapid warming was observed in the land‐locked or semi‐enclosed European and East Asian Seas (Baltic and North Seas, Black Sea, Japan Sea/East Sea and East China Sea), and also over the Newfoundland–Labrador Shelf. The proximity of the European and East Asian Seas to major industrial/population agglomerations suggests a possible direct anthropogenic effect. In a comparable study, Alexander et al. (2018) examined changes in SSTs in 18 LMEs adjacent to North America, Europe and the Arctic Ocean. The annual SST trends over 1976–2099 in all 18 were positive, ranging from 0.05 to 0.5 °C per decade. SST changes by the end of the 21st century will primarily be due to a positive shift in the mean, with only modest changes in the variability in most LMEs, resulting in a substantial increase in warm extremes and decrease in cold extremes. The shift in the mean is so large that in many regions SSTs during 2070–2099 will always be warmer than the warmest year during 1976–2005. The SST trends are generally stronger in summer than in winter, which amplifies the seasonal cycle of SST over the 21st century. In the Arctic, the mean SST and its variability increase substantially during summer, when it is ice‐free, but not during winter, when a thin layer of ice reforms and SSTs remain near the freezing point. While basin‐wide changes in the ocean are expected (Alexander et al. 2018), it is critical to examine temperature changes along continental margins, which supply more than 75% of the world’s marine fish catch. Lima & Wethey (2012) explored global and monthly warming patterns along 19 276 coastal locations between 1982 and 2010. They demonstrated that 46% of the coastlines had experienced a significant decrease in the frequency of extremely cold events, while extremely hot days were becoming more common in 38%. They further showed that the onset of the warm season was advancing significantly earlier in the year in 36% of the temperate coastal regions. More importantly, it is now possible to analyse local patterns within the global context, which is useful for a broad array of scientific fields, policymakers and the general public. Li et al. (2019) similarly conducted a global analysis of SST at 26 locations in Chinese coastal waters.

      Climate Warming

      Latitudinal distributions of many organisms are limited by temperature. One major response is a shift in distribution, usually poleward (Root et al. 2003). Physiological processes that set thermal tolerance limits are thought to determine, or at least contribute to, some of the shifts that have been observed (Tomanek 2008 and references therein). As already mentioned, seasonal air and water temperatures since 1960 have increased along the eastern US seaboard, and south of Lewes, Delaware (38.8 °N) summer SST increases have exceeded the upper lethal limits (32 °C) of M. edulis (Jones et al. 2010), resulting in geographic contraction of its southern, equatorward range edge approximately 350 km north, or ~7.5 km per year (Somero 2012). At the southern part of the range, high water and air temperatures cause mass mortality events, while along the more northerly portion, mortality is caused by high temperatures during aerial exposure. Ultimately, water temperatures in excess of thermal tolerances have caused contraction of the mussel’s biogeographic range (Jones et al. 2010).

      Range shifts vary greatly between species, and the distributions of Mytilus populations all over the world are responding differently to climate change. For example, Harley et al. (2011) compared distributions of M. californianus from 2009 to 2010 to a historical data set from 1957–1958. Sampling sites were believed to be within ~30 m of the original survey sites. The 52 years separating the two sampling intervals span a period of climatic warming. During the latter half of the 20th century, maximum air temperatures near the eastern and western ends of the Strait of Juan de Fuca near Washington state increased by ~0.2 and ~0.13 °C per decade, respectively, and mean annual water temperatures along the southern and western Vancouver Island coast warmed by ~0.08–0.11 °C per decade. Average daily maximum air temperatures during the summer, which are particularly relevant to thermal stress experienced in the intertidal zone, have warmed even more rapidly. At Victoria, on the southern end of Vancouver Island, summer average daily maxima have risen approximately linearly at a rate of 0.654 °C per decade since 1950, corresponding to an increase of 3.48 °C over 52 years, which has caused a 51% decrease in the vertical distribution of M. californianus, providing strong evidence that this change is linked to global warming. Associated with this decrease are local extinctions of M. californianus at 13% of the sites resurveyed between 2009 and 2010. Historical data for M. trossulus were only available for one site (former vertical range = 49 cm). By 2010, M. trossulus had been completely eliminated from that site, with the exception of three small juveniles found under a single rock.

From 1995 to 1999, the poleward movement of M. galloprovincialis showed a reversal concomitant with a cooling phase of the PDO (Hilbish et al. 2010). M. galloprovincialis has declined in abundance over the northern third of its geographic range (~540 km) and has become rare or absent across the northern 200 km of the range it previously colonised during its initial invasion. The distribution of the native species M. trossulus has, however, remained unchanged over the same time period. The difference in SST between warm and cold phases of the PDO is small (~1 °C), but Hilbish et al. (2010) deduced that even this minor decrease in temperature during the cold phase of the PDO may be enough to retard larval development in M. galloprovincialis, such that recruitment is handicapped