Examining climate-driven shifts in the ecological structure and functioning of Northeast Atlantic kelp forests

Abstract

Kelp species (i.e., large habitat-forming brown seaweeds) form extensive marine forests along rocky shores in temperate and subpolar regions. Kelps are widely distributed across the northeast Atlantic, where both cold-temperate and warm-temperate species co-occur. Recent and continuing ocean warming, combined with other environmental change factors, are likely impacting the ecological structure, and functioning of marine forests, but robust information on the nature and implications of such changes is lacking for most regions. For this thesis, a variety of field surveys were conducted in the southwest of the United Kingdom, in the mid-northeast Atlantic, to investigate species occurrences, spatiotemporal trends in habitat structure, population dynamics of key habitat-forming kelps, and the composition of associated faunal assemblages. This thesis examined temporal shifts in kelp habitat structure in a biogeographic transition zone and found some evidence to support the general hypothesis that changes in the occurrence and abundance of kelp species are congruent with recent ocean warming trends. Specifically, warm-adapted Lusitanian kelps, such as Saccorhiza polyschides and Laminaria ochroleuca, and an invasive non-native species, Undaria pinnatifida, have increased in abundance. In contrast, some cold-temperate Boreal species, such as Alaria esculenta and Saccharina latissima, have exhibited declines in occurrence and abundance, whereas others, namely Laminaria hyperborea and L. digitata, have remained relatively stable over time. Spatiotemporal variability in population structure of the warm-temperate kelp S. polyschides was investigated in more detail, as this species has proliferated markedly in the biogeographic transition zone in recent decades. S. polyschides dominated shallow rocky habitats in summer, with consistently high density and standing stock values recorded across survey sites. The structure of associated faunal assemblages was quantified to assess the ecological role of this species as a biogenic habitat former. Different morpho-structural components (e.g., the blade, stipe, and holdfast) offered distinct living space, shelter, nursery, and foraging grounds for associated fauna, including gastropod grazers and small coastal fishes. However, the short-lived sporophytes provided only temporary, highly seasonal habitats, with greatly reduced habitat availability in winter. With ongoing ocean warming, continued proliferation of S. polyschides within the region is expected, with replacement of some cold-adapted kelp species likely to induce wider changes in kelp habitat structure and functioning. The results presented in this thesis provide a robust baseline for further monitoring, which is needed to inform policy and management strategies aimed at protecting socioeconomically important marine forests.

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ORCID for Antony Jensen: orcid.org/0000-0002-8924-1198

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