Cool-edge populations of the kelp Ecklonia radiata under global ocean change scenarios: strong sensitivity to ocean warming but little effect of ocean acidification

Abstract

Kelp forests are threatened by ocean warming, yet effects of co-occurring drivers such as CO 2 are rarely considered when predicting their performance in the future. In Australia, the kelp Ecklonia radiata forms extensive forests across seawater temperatures of approximately 7–26°C. Cool-edge populations are typically considered more thermally tolerant than their warm-edge counterparts but this ignores the possibility of local adaptation. Moreover, it is unknown whether elevated CO 2 can mitigate negative effects of warming. To identify whether elevated CO 2 could improve thermal performance of a cool-edge population of E. radiata , we constructed thermal performance curves for growth and photosynthesis, under both current and elevated CO 2 (approx. 400 and 1000 µatm). We then modelled annual performance under warming scenarios to highlight thermal susceptibility. Elevated CO 2 had minimal effect on growth but increased photosynthesis around the thermal optimum. Thermal optima were approximately 16°C for growth and approximately 18°C for photosynthesis, and modelled performance indicated cool-edge populations may be vulnerable in the future. Our findings demonstrate that elevated CO 2 is unlikely to offset negative effects of ocean warming on the kelp E. radiata and highlight the potential susceptibility of cool-edge populations to ocean warming.

Stronger effect of temperature on body growth in cool than in warm populations suggests lack of local adaptation

“…Changes in trait-temperature relationships due to thermal adaptation in natural populations are expected in response to climate warming (Angilletta 2009), and previous studies have shown that local adaptation in physiological traits can facilitate different thermal optima among populations (e.g., Atlantic cod (Righton et al 2010), kelp (Britton et al 2024), corals (Howells et al 2013) and invertebrates (Sanford and Kelly 2011)). However, adaptive capacities and the pace of thermal adaptation differ among species (Martin et al 2023) and depend on life-history trade-offs, underlying genetic variation, the potential for gene flow (Kirkpatrick and Barton 1997), and environmental conditions.…”

“…That is, to what extent populations conform to a global, species-wide thermal performance curve, versus having developed local thermal response curves with local temperature optima in order to have higher fitness in their local habitats. In other taxa, such as kelp (Britton et al 2024), corals (Howells et al 2013), invertebrates (Sanford and Kelly 2011), and phytoplankton (Thomas et al 2012), there is a growing evidence that local adaptation has lead to populations exhibiting different responses in growth (individual or population) to ocean warming. However, studies on fishes are scarcer (but see Neuheimer and Grønkjaer (2012) and Beaudry-Sylvestre et al…”