Cumulative stress restricts niche filling potential of habitat‐forming kelps in a future climate

Abstract

Climate change is driving range contractions and local population extinctions across the globe. When this affects ecosystem engineers the vacant niches left behind are likely to alter the wider ecosystem unless a similar species can fulfil them.Here, we explore the stress physiology of two coexisting kelps undergoing opposing range shifts in the Northeast Atlantic and discuss what differences in stress physiology may mean for future niche filling.We used chlorophyll florescence (F v /F m) and differentiation of the heat shock response (HSR) to determine the capacity of the expanding kelp, Laminaria ochroleuca, to move into the higher shore position of the retreating kelp, Laminaria digitata. We applied both single and consecutive exposures to immersed and emersed high and low temperature treatments, replicating low tide exposures experienced in summer and winter.No interspecific differences in HSR were observed which was surprising given the species’ different biogeographic distributions. However, chlorophyll florescence revealed clear differences between species with L. ochroleuca better equipped to tolerate high immersed temperatures but showed little capacity to tolerate frosts or high emersion temperatures.Many patterns observed were only apparent after consecutive exposures. Such cumulative effects have largely been overlooked in tolerance experiments on intertidal organisms despite being more representative of the stress experienced in natural habitats. We therefore suggest future experiments incorporate consecutive stress into their design.Climate change is predicted to result in fewer ground frosts and increased summer temperatures. Therefore, L. ochroleuca may be released from its summer cold limit in winter but still be prevented from moving up the shore due to desiccation in the summer. Laminaria ochroleuca will, however, likely be able to move into tidal pools. Therefore, only partial niche filling by L. ochroleuca will be possible in this system as climate change advances. A plain language summary is available for this article.

Nitrogen sufficiency enhances thermal tolerance in habitat-forming kelp: implications for acclimation under thermal stress

“…Thermal plasticity is a mechanism by which populations rapidly acclimate to warming and to extreme high temperature-related events such as MHWs 40 . Our study is the first to describe the plasticity of temperature-dependant traits of the ecologically and economically important kelp Macrocystis.…”

“…Recent studies have highlighted the importance of plasticity and local adaptation in macroalgal responses to warming, suggesting that some ecotypes might be more resilient or vulnerable to high temperatures than others, depending on the conditions to which they are usually exposed 40,86,102 . Previous studies have illustrated the physiological plasticity of Macrocystis across its wide geographical distribution.…”

“…Kelps (Order Laminariales) form subtidal forests in temperate and polar waters, contributing to carbon storage, macronutrient dynamics, and the biodiversity of many associated species 35 . However, a clear decline (38%) in these habitat-forming species has been observed in the past 20 years across the world [36][37][38][39][40] . Climate change, along with other human impacts such as overfishing and direct harvest, are considered the main reasons for this decline 39 .…”

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. 2017b). Before all F v /F m measurements were taken, discs were dark adapted for 15 minutes.…”

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

“…Prolonged exposure to warmerthan-average sea temperatures can manifest in responses at the organism level, such as decreased growth (Hargrave et al, 2017), and at the population level, such as reproductive failure (Bartsch et al, 2013). Similarly, exposure to acute air temperature stress during periods of low-tide emersion can have impacts at the physiological and individual level (King et al, 2018b). Ultimately, warming can lead to the loss of kelp populations at the range edge, with subsequent shifts in the structure of communities (Voerman et al, 2013) and even ecosystems (Wernberg et al, 2016).…”