The roles of extreme and rising temperatures on individual, population and biogeographic responses of intertidal gastropods in the northeast Atlantic

Abstract

Anthropogenic climate change (ACC) is thermally altering atmospheric and marine ecosystems. Within intertidal environments, ACC exacerbates both emersed and immersed natural stressors experienced by organisms during daily tidal and seasonal cycles. Intertidal ectothermic gastropods are indicators of thermal change, as they occupy varying vertical levels of rocky shores, exist across multiple latitudes depending on thermal affinities, and are subject to extensive physiological and biogeographical research. Studies tracking biogeographic responses of intertidal species to rising sea surface temperatures (SSTs) around northeast Atlantic coastlines have shown poleward shifts of both leading and trailing range edges. Much less is known of species-specific biological mechanisms driving changes through life stages.
In this thesis, an observational study investigated current temperature regimes on northeast Atlantic rocky shores and how they were modulated by microhabitats serving as nursery grounds or providing refuges (Chapter 2). Temperature loggers were placed into multiple microhabitats at rocky shore sites throughout the coastal northeast Atlantic from northern Scotland to southern Portugal. Up to five years of collected in situ data was then compared with long-term satellite data to assess if calculated broad-scale extreme thermal events are reflected at fine scales. The results confirmed that rocky shore microhabitats are experiencing lethally extreme temperatures at higher frequencies, mostly reflecting broad-scale extreme temperature events in the northeast Atlantic. Refugia such as rockpools are currently providing the greatest thermal stability throughout all seasons in most regions, although will provide limited thermal protection as the planet progresses towards extreme ACC scenarios by 2100.
Laboratory experiments tested physiological responses of intertidal species to both current and end-of-the-century extreme atmospheric and marine temperatures (Chapter 3 and Chapter 4). Short-term laboratory experiments used locally-derived atmospheric and marine thermal extremes to explore lethal and sub-lethal (coma) responses of juvenile gastropod species, the trochids, Phorcus lineatus, Steromphala umbilicalis, Steromphala cineraria, and the littorinid, Littorina littorea (Chapter 3). A longer-term mesocosm experiment investigated lethal and sub-lethal (growth, gametogenesis) responses of northern and mid-range populations of the warm temperate species, S. umbilicalis, to variable SSTs under differing future ACC scenarios (Chapter 4). Boreal (cold temperate) mid to low-shore intertidal gastropod species may experience population declines, either through subtidal retreat or localised extinctions where thermal extremes are highest and differ most from localised seasonal conditions. Conversely, Lusitanian (warm temperate) intertidal populations at mid and northern biogeographic range limits will benefit under warming conditions from declining winter cold spells aiding survival. Milder winter SSTs are already enabling the early onset of spring growth and reproductive output rate. Considered together, these observational and experimental approaches have provided additional levels of detail to predict intertidal rocky shore gastropod population responses in a warming world.

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ORCID for Phillip Fenberg: orcid.org/0000-0003-4474-176X

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