Low recruitment, high tissue loss, and juvenile mortality limit recovery of kelp following large-scale defoliation

Low recruitment due to limitations of propagule supply or post-settlement survival reinforces dominance of turf algal assemblages that replace canopy algae following large-scale losses. However, post-recruitment processes that hinder juvenile growth and survival (epiphytic overgrowth, grazing, physical stress) also could impede recovery. To evaluate the contribution of recruitment, growth, and survival of young sporophytes to recovery of degraded kelp populations and key factors driving post-recruitment tissue loss and mortality, we followed cohorts of juvenile kelp Saccharina latissima at two defoliated sites in Nova Scotia. We also monitored kelp recruitment, abundance, size structure, and macroalgal composition for 5.5 years. Recruit densities were an order of magnitude lower compared to previous studies in the region. Large decreases in blade area of juveniles were related to cover by the invasive bryozoan Membranipora membranacea, grazing by small snails Lacuna vincta, and warm seawater temperatures. Cohort survival was low (time to 50% mortality 2.5–5.5 months) and increased risk of death was directly related to bryozoan encrustation. Modest seasonal or interannual gains in kelp abundance were lost during periods of peak temperature, which showed a warming trend during the study, favouring persistence of widespread turf-forming, opportunistic and invasive algae. We conclude that low recruitment success, high rate of tissue loss relative to growth, and high mortality, inhibited kelp recovery. Impacts of epiphytic overgrowth, grazing, and warm temperatures on these processes highlight the need to protect intact kelp populations, growing in favourable conditions, to maintain positive interactions that increase resilience to undesirable regime shifts.