Recovery of Intertidal Mussel Stands Three Years after the Severe 2021 Heatwave in British Columbia, Canada

Abstract

In the early summer of 2021, an intense heatwave killed millions of intertidal mussels in British Columbia, Canada. Using photographs taken three years later (June 2024), this article provides visual evidence of active recovery of intertidal mussel stands in the Vancouver region, revealing the resilience of these intertidal systems. Future monitoring should evaluate their ability for long-term persistence in light of the ongoing climate change, which is expected to increase the frequency and intensity of heatwaves.

In the early summer of 2021, an intense heatwave affected southwestern Canada and the northwestern contiguous United States [1,2]. This heatwave has been attributed to the ongoing climate change [3] and broke historical records of maximum air temperature. Rocky intertidal habitats on the Salish Sea coast experienced temperatures above 50 °C at low tide under direct sunlight, which caused widespread mortality in intertidal organisms [1]. This was particularly noticeable in mussels (Mytilus trossulus) due to their traditionally high abundance at mid-intertidal elevations. Intertidal mussels are important foundation species on many temperate rocky shores, where they often form extensive stands of densely packed individuals that host a diversity of small species [4]. Thus, mussel losses are of concern not only because of negative effects up the food chain but also because of the wider impact on coastal biodiversity [5,6,7].

A few days after the 2021 heatwave, a field survey near Vancouver (British Columbia, Canada) revealed an average mortality rate of 74% in intertidal mussels [1]. Specifically, adult mussels suffered a near total mortality on many south-facing intertidal surfaces. On north-facing intertidal surfaces and on intertidal substrates covered with fucoid algae during low tides, dead mussels were uncommon [1], suggesting that those habitats were spared the intense heat.

In light of the ecological importance of mussels, evaluating their recovery after mass losses caused by environmental extremes is relevant. This article provides visual evidence of active recovery as seen in June 2024, three years after the devastating 2021 heatwave. At low tide, I surveyed rocky intertidal habitats on 6 June 2024 at Point Atkinson (49.3309° N, 123.2663° W), just north of Vancouver, and on 7 June 2024 at Kitsilano Point (49.2777° N, 123.1523° W), in Vancouver. Point Atkinson exhibits rocky intertidal habitats composed mainly of stable bedrock (Figure 1), while Kitsilano Point is a field of rocky boulders of various sizes, including large ones (Figure 2).

At Point Atkinson, in June 2024, stands composed exclusively of juvenile mussels were common at mid-intertidal elevations on south-facing intertidal substrates. These stands extended more or less continuously for meters along the coastline, mainly interrupted only by coastal topography. Visual age estimates based on annual growth rings on the shells [8,9] indicated that these mussels were three years old or younger. These stands were essentially monolayered, with high densities of individuals averaging almost 220 individuals dm⁻² (Figure 3).

At Kitsilano Point, in June 2024, observations focused on large boulders, as they offer a stable substrate for intertidal life. Stands composed exclusively of juvenile mussels aged three years old or less were also common at mid-intertidal elevations. These stands were patchier than those at Point Atkinson but were also basically monolayered with high densities of individuals averaging about 245 individuals dm⁻² (Figure 4). Interestingly, at lower (mid-to-low) intertidal elevations on north-facing substrates on large boulders, patches of adult mussels older than three years were common, exhibiting several attached barnacles on their shells (Figure 5). Occurrence at lower elevations implies longer submergence times, while occurrence on north-facing surfaces implies lower solar radiation, possibly explaining the survival of adult mussels in such environments. The size difference between the juvenile mussels in the recovering stands and the adult mussels in the protected mature stands is shown most clearly in Figure 6. Before the 2021 heatwave, adult mussels were widely common at mid-intertidal elevations on these boulders regardless of cardinal orientation, as exemplified by photos taken on 23 June 2010 (Figure 7).

Overall, the surveys in 2024 found evidence of active recovery of intertidal mussel stands following the deadly 2021 heatwave. The visual age estimates for the juvenile stands found in 2024 are consistent with mass recolonization after the mass mortality of adults experienced three years earlier. This process is perhaps most clearly seen with the succession of visual records available for Kitsilano Point, as adult mussels were very abundant at the mid-intertidal zone before 2021 (Figure 7) and then experienced extensive mortality right after the 2021 heatwave (see Figure 1b in ref. [2]), followed by an extensive recolonization by juveniles as seen in 2024 (Figure 3). The findings described in this article speak of the resilience of these intertidal ecosystems. With the ongoing climate change, however, extremes in temperature are predicted to increase in frequency and intensity [10], which calls for regular monitoring of these systems to assess their long-term persistence. Monitoring could include quantitative surveys on north- and south-facing intertidal substrates along the elevation gradient. The variables to quantify could include mussel population density, growth rates, gonadal condition and reproductive output, and, importantly, the many species of invertebrates that live among the mussels and thus depend on mussel bed integrity [4].