Recolonization of Intertidal Mussels in Nova Scotia (Canada) after Their Mass Disappearance Following the Severe 2023 Winter Cold Snap

Abstract

In February 2023, a severe cold snap took place in Atlantic Canada and was followed by the mass loss of mussels at mid-to-high intertidal elevations on the southeastern Nova Scotia coast. This loss was concerning because mussels sustain upper trophic levels in coastal food webs and because mussel stands enhance local biodiversity by sheltering many small invertebrate species. Using photographs taken in the second summer after that cold snap (July 2024), this article provides visual evidence of active ongoing recolonization of intertidal mussels on this coast, including the incipient formation of new stands. These are encouraging signs of ecological resilience. Reaching historical values of abundance will likely depend on the future occurrence of weather extremes, which are becoming more frequent with the ongoing climate change.

In February 2023, an unusually severe cold snap occurred in Atlantic Canada. On the southeastern Nova Scotia coast, air temperature dropped below -20 °C for the first time in at least 10 years and remained at those levels for several hours between 3 and 4 February [1]. This extreme weather event may have been related to the ongoing climate change, which is known to favor intrusions of cold Arctic air in winter in eastern North America [2]. This cold snap disrupted various human activities in the region [3,4]. Negative impacts were also observed in rocky intertidal habitats, as these environments are fully exposed to the air during low tides. Shortly after this cold snap, many intertidal seaweeds were completely bleached, possibly constituting lethal stress [5]. At mid-to-high intertidal elevations in wave-exposed habitats, mussels (Mytilus edulis and M. trossulus) commonly formed dense extensive stands before 2023 [6], but these stands disappeared soon after the 2023 winter cold snap [1]. This mass loss was concerning not only because mussels sustain upper trophic levels in coastal food webs but also because their stands shelter many invertebrate species, acting as important biodiversity reservoirs [7,8].

Mussels are common in temperate rocky intertidal habitats around the world [9], but declines in their abundance have occurred on various shores in recent years because of anthropogenic pressures [10,11,12,13]. Therefore, it is of interest to evaluate the resilience of these systems after severe disturbances. The present article provides visual evidence of active ongoing recolonization of intertidal mussels on the southeastern Nova Scotia coast as seen in late July 2024, two growth seasons after the 2023 winter cold snap. At low tide, we examined the mid-to-high intertidal zone of Western Head (43.9896 N, 64.6607 W) on 24 July 2024 and Duck Reef (44.4913 N, 63.5270 W) on 26 July 2024. These are wave-exposed locations (Figure 1) where mussel losses had been very high after the 2023 cold snap (Figure 2; see also [1]). Stable bedrock is the predominant intertidal substrate at these locations.

In July 2024, small juvenile mussels were common at the mid-to-high intertidal zone in the wave-exposed habitats surveyed at Western Head and Duck Reef. These juvenile mussels occurred in aggregations of various sizes, from groups including just a few individuals to patches including several tens of individuals (Figure 3 and Figure 4). Overall, these surveys revealed an active ongoing recovery of mussel populations after the 2023 cold snap that devastated them. The source of these juvenile mussels is unclear, although they may have originated from reproductive mussels at lower elevations, where the 2023 cold snap had a more limited influence [1].

Intertidal mussel populations can be resilient in the face of catastrophic environmental events, as also shown by populations on the Pacific coast of Canada that are presently recovering after a deadly heatwave in the summer of 2021 [14,15]. Whether mussels can regain their historical abundance levels over the years, however, will depend on the future occurrence of extreme weather events, which are becoming more frequent due to the ongoing climate change [16]. The increasing instability of abiotic conditions is already hampering the ability of natural ecosystems to recover from single disturbance events [17,18,19], highlighting the need to monitor affected ecosystems regularly [20]. Thus, the periodic monitoring of intertidal mussel stands will show how their health and overall extent relate to the expected environmental changes.