Abstract
Annual surveys of the abundance of intertidal invertebrates and macroalgae have been made at between 70 and 100 rocky intertidal time-series sites around the United Kingdom coastline since 2002 under the MarClim project. The data provide a unique opportunity to investigate the impacts of both pervasive climate change and their punctuation by extreme events on intertidal species. After the extreme storm events in the 2013/2014 winter season and the record heatwaves in the summers of 2018 and 2020, MarClim surveys recorded both physical and biological changes to rocky shore habitats. Subsequent surveys reassessed the effects on community structure via analysis of those species that resisted storm damage, those species that returned after the extreme storm events, and species that opportunistically occupied vacant habitat after storm-induced species loss. In addition, biannual storm damage surveys documenting communities recovery were carried out in the spring and winter of each year from 2014 to 2020 at three MarClim sites in north Cornwall (Crackington Haven, Trevone, and St. Ives), which experienced different types of abiotic and biotic damage resulting from these storms. Impacts of heatwaves and cold spells on the abundance of species were determined by regression on frequencies of event per year. Species of invertebrates and macroalgae generally declined in years of more frequent winter cold spells and summer heatwaves, while winter heatwaves and summer cold spells had similar numbers of positive and negative effects across species. Winter warm spells tended to have a more negative effect on cold-affinity species than on warm-affinity species. No abrupt shift was recorded after the 2013/2014 storms. Whilst a short-term change in some species was recorded in quantitative quadrat surveys, the biological communities returned to the long-term species composition and abundance within 2 years. The heatwave events caused sublethal heat damage in macroalgae, evidenced as dried areas of tissue on many individuals, with mortality-induced reductions in the abundance of only a few invertebrate species, recorded in Scotland and southwest England after the heatwave events in 2018 and 2020. MarClim and storm-damage surveys indicate that there have been no sustained impacts from either extreme thermal or storm events across the rocky intertidal communities, and biodiversity has not been significantly altered as a result. The abundance and biogeographical distributions of rocky intertidal species and communities around the United Kingdom are being driven by longer-term, large scale, pervasive change in environmental conditions, with a gradual shift towards dominance of Lusitanian species from the early 2000s in responses to warming of the marine climate.
Highlights
Storms and heatwaves are becoming more frequent as a result of climate warming (Oliver et al, 2018; IPCC, 2019) with significant impacts on the coastal ecosystems of the world (Smale et al, 2019)
Known as the “Beast from the East,” anticyclone Hartmut coincided with storm Emma to cause some of the worst weather conditions in decades. These events caused a mass mortality of subtidal biota along the east coast of England (Pinnegar et al, 2020), no observed changes in the abundance of any of the 50 species of invertebrate surveyed by MarClim were recorded at any long-term monitoring site along the North Sea coastline during 2018 (Mieszkowska et al, 2020)
Year-to-year changes in occupancy-derived measures of abundance for rocky intertidal species are a sensitive metric with which to track the vulnerability of species to extreme thermal events occurring in the marine climate
Summary
Storms and heatwaves are becoming more frequent as a result of climate warming (Oliver et al, 2018; IPCC, 2019) with significant impacts on the coastal ecosystems of the world (Smale et al, 2019). Long-term sustained observing programmes have shown evidence of community-wide shifts in composition from cold to warm affinity species (Simpson et al, 2011; Burrows et al, 2019) as long-term trends, but such studies neglect the short-term effects of extreme events. The warming trend has slowed down over the last decade (NOAA OISST; Reynolds et al, 2007) due to the climate-driven slowdown in the Atlantic Meridional Overturning Circulation (AMOC) causing a colder patch of surface water to the northwest of the United Kingdom This has been reflected in changes in intertidal communities (Burrows et al, 2020). Despite the recent slowing of the warming trend in the United Kingdom, the impacts of climate change are still being experienced, especially the punctuation of pervasive climate change by extreme events (Firth et al, 2015). It is important to improve understanding of the impacts of climate by extreme events, or climate-driven disturbances on the natural environment and the resultant persistence of the associated communities, rather than just focusing on average annual sea temperatures
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