Wave exposure affects the persistence of kelp beds amidst outbreaks of the invasive bryozoan Membranipora membranacea

Abstract

In certain regions of the northwest Atlantic, rocky subtidal habitats have transitioned from luxuriant kelp beds to turf dominated ecosystems. Encrustation by the invasive epiphytic bryozoan Membranipora membranacea has accelerated the defoliation of macroalgal stands, particularly on the southwestern shore (SWS) of Nova Scotia. In contrast, at the Eastern Shore Islands (ESI) in Nova Scotia, a temperate island archipelago with relatively high wave exposure, kelp beds persist. We investigated how spatial and temporal dynamics of the dominant kelps (Saccharina latissima and Laminaria digitata) and M. membranacea relate to wave exposure. We examined canopy cover and density of kelps using video transects and quadrat sampling at 6 sites of different wave exposures at ESI and a highly wave exposed site on the SWS (Shag Rock), where kelp also persists, in summer and autumn 2018-2020. We measured percent cover of adult colonies of M. membranacea on kelps from autumn 2018 to autumn 2020, and density of settlers in 2019. Colony cover of M. membranacea was comparable across the range of wave exposures at ESI, whereas settler density peaked at intermediate wave exposure. Despite annual outbreaks of the bryozoan, kelp beds at ESI persisted over multiple years, in contrast to a declining trend at Shag Rock. Overwinter recovery of kelps at ESI following bryozoan-induced seasonal declines the preceding autumn indicate that, in contrast to sites on the SWS, these colder more wave exposed kelp beds may be resilient to defoliation.