# Environmental Drivers of Genetic Structure and Local Adaptation in a Marine Foundation Species

## TL;DR

This study explores how environmental factors influence genetic diversity and adaptation in a kelp species across a large geographic range, revealing patterns of local adaptation and genetic differentiation.

## Contribution

The study provides new insights into the genetic structure and local adaptation of Eisenia arborea in response to environmental gradients and climate change.

## Key findings

- Strong genetic differentiation was found between northern and southern populations of Eisenia arborea.
- Environmental association analyses identified SNPs correlated with sea surface temperature and depth, indicating local adaptation.
- Southern populations showed low genetic diversity and high inbreeding, suggesting vulnerability to climate change.

## Abstract

Predicting how species will respond to global change requires understanding how environmental drivers shape both neutral and adaptive genetic variation across space. The kelp
Eisenia arborea
is a thermally tolerant foundation species spanning more than 3000 km of coastline and a broad latitudinal temperature gradient in the Northeast Pacific, yet how environmental and demographic processes influence genomic and population structure remain unclear. We used genome‐wide ddRAD sequencing to investigate patterns of genetic diversity, connectivity and local adaptation in
E. arborea
across two depths and ~2700 km of coastline. We detected strong genetic differentiation between northern (British Columbia, Canada) and southern (Baja California, Mexico) populations, indicating limited gene flow across the species' broad range. Southern populations also had the lowest genetic diversity and highest inbreeding, broadly consistent with expectations for populations occupying environmentally marginal portions of a species' range. However, the two southernmost populations (~200 km apart) were highly similar and well connected, whereas mid‐range sites were more differentiated, indicating that the geographic range edge population was not genetically isolated as is often hypothesised. Environmental association analyses identified SNPs correlated with both sea surface temperature and depth, revealing signals of local adaptation to broad climatic gradients and fine‐scale habitat variation. The combination of high inbreeding, restricted connectivity and local adaptation highlights both the vulnerability and potential conservation value of distinct genetic units, especially warm‐adapted southern populations, for maintaining the resilience of these Eisenia forests under ocean warming.

We used genome‐wide SNP data to investigate how environmental gradients shape genetic structure, connectivity and local adaptation in the kelp
Eisenia arborea
across ~2700 km of coastline and contrasting depth environments. We found strong latitudinal genetic structure, evidence for local adaptation to temperature and depth despite high connectivity within locations, and demographic signatures consistent with marginality at the warm range edge. Together, these results highlight how neutral and adaptive processes interact across spatial scales in a marine foundation species, with implications for conservation under ongoing climate change.