Invasive Fucus serratus (Phaeophyceae) replaces shallow water kelp community in southwestern Nova Scotia, Canada

Along the Atlantic coast of Nova Scotia, recent observations in 2020-2021 of the intertidal zone showed that the European native fucoid Fucus serratus was becoming more widely distributed, both geographically and ecologically. Here, we extended these observations to the shallow subtidal zone (to about 5 m below chart datum) in southwestern Nova Scotia. We used semi-quantitative and qualitative sampling via snorkeling at selected mainland and offshore island sites, observed using both transects (50 m) and spot sampling. At all sites, the expected populations of kelp species (i.e., Laminaria digitata, Saccharina latissima, and Alaria esculenta) were absent to rare, and extensive populations of F. serratus were present with > 50% cover that formed the dominant canopy species. The decline in kelp species mirrors previous accounts of kelp population collapse in St. Margaret’s Bay and Lunenburg Bay in recent decades but differs in that the canopy species is Fucus serratus rather than red algal turfs. While ocean warming may be playing a role in regional kelp decline, significant warming did not occur in Nova Scotia until the late 1990s. We suggest that the commercial harvesting of Chondrus crispus by the 1970s played a key role in the replacement of shallow water kelp beds by Fucus serratus. Keywords: Climate change, Fucus serratus, irish moss, kelp, Nova Scotia, seaweed harvesting