Evidence for different thermal ecotypes in range centre and trailing edge kelp populations

Abstract

Determining and predicting species' responses to climate change is a fundamental goal of contemporary ecology. When interpreting responses to warming species are often treated as a single physiological unit with a single species-wide thermal niche. This assumes that trailing edge populations are most vulnerable to warming, as it is here where a species' thermal niche will be exceeded first. Local adaptation can, however, result in narrower thermal tolerance limits for local populations, so that similar relative increases in temperature can exceed local niches throughout a species range. We used a combination of common garden temperature heat-shock experiments (8-32 °C) and population genetics (microsatellites) to identify thermal ecotypes of northeast Atlantic range centre and trailing edge populations of the habitat-forming kelp, Laminaria digitata. Using upregulation of hsp70 as an indicator of thermal stress, we found that trailing edge populations were better equipped to tolerate acute temperature shocks. This pattern was consistent across seasons, indicating that between-population variability is fixed. High genetic structuring was also observed, with range centre and trailing edge populations representing highly distinct clusters with little gene flow between regions. Taken together, this suggests the presence of distinct thermal ecotypes for L. digitata, which may mean responses to future warming are more complex than linear range contractions.

Impacts of ocean warming on kelp forest ecosystems

“…In addition to the structural changes described here, it is highly likely that ocean warming has affected key processes, including rates of primary productivity (Pessarrodona et al, 2019), the capture and export of carbon to adjacent habitats , resilience to physical disturbance (Wernberg et al, 2010), and habitat provision (Teagle & Smale, 2018 Moreover, several kelp deforestation events have been attributed to non-climatic stressors, such as overgrazing by sea urchins (Ling et al, 2015) or decreased water quality in coastal habitats (Connell et al, 2008). While range edge populations are likely to be particularly vulnerable to warming, recent evidence suggests that local thermal adaptation in kelps and other marine macrophytes may be commonplace (King et al, 2019). Typically, equatorward trailing range edge populations are more likely to undergo local extinctions and range contractions, whereas poleward leading-edge populations are more likely to proliferate and extend their range.…”

“…This general pattern will allow for explicit predictions of how and where future warming impacts will occur. While range edge populations are likely to be particularly vulnerable to warming, recent evidence suggests that local thermal adaptation in kelps and other marine macrophytes may be commonplace (King et al, 2019). As such, intra-specific variability in thermal tolerance may render some midrange populations susceptible to current and future warming trends (Bennett et al, 2015a).…”

Intra‐Annual Variability in Responses of a Canopy Forming Kelp to Cumulative Low Tide Heat Stress: Implications for Populations at the Trailing Range Edge

“…King et al. () observed reduced genetic diversity but distinct adaptive ecotypes at the trailing edge of Laminaria digitata . Therefore, while climate induced range contractions may not compromise L. digitata 's genepool the loss of unique phenotypic variation may still threaten the adaptive potential of a species as a whole (Reed and Frankham ).…”

Photophysiological Responses of Canopy-Forming Kelp Species to Short-Term Acute Warming

“…However, it should be noted that this study was conducted on single populations over a single event and did not account for any local adaptation that could lead to between-population variability in thermal tolerance. Indeed, thermal divergence between populations has been shown for several kelp species (Gerard and Du Bois, 1988;King et al, 2019) and may be commonplace in marine macrophyte species more generally (King et al, 2018a). Clearly, further work comparing photophysiological responses to temperature across multiple populations within these species' ranges and in response to varied temperature regimes is needed to confirm the photophysiological resilience of these species to the chronic and acute ocean warming projected for the coming decades.…”

“…At the physiological level, the temperature range within which kelps can carry out critical processes such as photosynthesis varies between species, life stages and the processes themselves (Delebecq et al, 2016), so that determining the thermal niche is complex. Even so, the extreme temperatures that macroalgal populations are exposed to during MHWs can exceed thermal thresholds, inducing protective responses (e.g., heat shock proteins, see King et al, 2019) and leading to impaired performance (Hargrave et al, 2017), altered physiology (Gouvêa et al, 2017), and ultimate mortality (Wernberg et al, 2013). Inter-specific variability in photophysiological mechanistic responses to short-term acute warming events remains unclear, but would allow for improved predictions of the future effects of ocean warming on kelp populations and the communities they underpin.…”