Wasted effort: Recruitment and persistence of kelp on algal turf

Abstract

Declines in kelp abundance over the past 3 decades have resulted in a shift from luxuriant kelp beds to extensive mats of turf-forming algae in Nova Scotia, Canada. With the reduced availability of open rocky substrate, kelps are increasingly recruiting to turf algae. At 3 sites near Halifax, we found that turf-attached kelp Saccharina latissima was generally restricted to smaller size classes (<50 cm length) than rock-attached kelp at 12 m depth. Turf-attached kelp allocated a greater proportion of biomass to the holdfast (anchoring structure), which differed morphologically from that of rock-attached kelp and had lower attachment strength. To assess how these differences affect survival, we monitored kelp in 2 m diameter plots at 11 m depth over 40 wk at 1 site. Smaller kelps were predominantly turf-attached and larger ones rock-attached in late summer and autumn, but there was near-complete loss of both turf- and rock-attached kelp over winter when wave action was greatest. In a concurrent manipulative experiment at 5 m depth at another site, we transplanted small boulders with turf- or rock-attached kelp to a wave-exposed or protected location. Survival was greater for rock-attached transplants at both locations after 12 wk, with a complete loss of turf-attached kelp in the wave-exposed treatment. Classification based on holdfast morphology showed that 76% of drift kelp within a depositional area at this site was once turf-attached. Low survival of kelps that recruit to turf algae, likely due to wave dislodgement, may represent an important feedback that increases resilience of a turf-dominated state and prevents reestablishment of kelp.