MEPS prepress abstract   -  DOI: https://doi.org/10.3354/meps14548

ABSTRACT: Kelp forests are among the most abundant coastal marine habitats but are vulnerable to climate change. The Northeast Pacific has experienced recent large-scale changes in kelp abundance and distribution, but little is known about changes north of the British Columbia (B.C.) - Washington border. Here, we assess whether and how floating canopy kelp (Macrocystis, Nereocystis) distributions have changed in recent decades along the extensive coast of B.C. We assembled and analysed available kelp distributional data, comparing snapshots of kelp linear extent from 1.5 - 3 decades ago (1994 – 2007) to recently collected data (2017 – 2021) across 11 different subregions spanning the province. We then leveraged timeseries, where available (n = 7 datasets), to contextualise patterns of change. In aggregate, these data suggest that kelp forests have declined considerably in some parts of the province, but with variable patterns of change across B.C. In the warmest areas (southern B.C.), kelp persistence was negatively correlated with mean summer sea temperatures that at times exceeded known thermal tolerances. In contrast, in northern subregions, top-down control by sea urchins and otters appeared to modulate kelp dynamics, with declines occurring in two subregions despite cool ocean temperatures. Timeseries data suggest that many declines occurred around the 2014 – 2016 marine heatwave, an event associated with sustained warming and altered trophic dynamics. Our results suggest that the extent of British Columbia’s kelp forests has declined in some places in recent decades, but that regional and local-scale factors influence their responses to environmental change.