Giant kelp forests at critical light thresholds show compromised ecological resilience to environmental and biological drivers

Replacement of kelp and fucoid forests by low biomass turfing communities or urchin barrens has been identified in temperate marine ecosystems worldwide. Variation in the mechanisms of decline (e.g., urchin grazing, temperature stress) and apparent driving stressors (e.g., overfishing, sedimentation, global warming) has greatly limited the development of generalized management strategies. Thresholds of urchin density and thermal tolerance for habitat-forming seaweeds are increasingly well established, yet thresholds of turbidity are rarely considered. Given the universal requirement for light in aquatic macrophytes, estimation of photosynthetically active radiation (PAR) limits may help underpin frameworks for better management of terrestrial stressors. Here I have examined spatial and temporal variability of PAR in forests of the giant kelp Macrocystis pyrifera across a gradient of exposure to land-derived sediments. Exposure to excess suspended sediments pushed PAR below critical thresholds for net sporophyte growth (< 1.0 mol m(-2) day(-1)), affecting giant kelp densities, but also impacting a range of ecological metrics including: fucoid density; laminarian density; beta diversity metrics; subcanopy composition; and urchin density. While kelps and fucoids responded to similar light thresholds, kelps appear to be vulnerable to physical disruption by sediments whereas fucoids are tolerant of physical disruption mechanisms, but vulnerable to declining light availability. Light thresholds of 1.0 mol m(-2) day(-1) may be a useful target for management of stressors affecting water clarity (e.g., sedimentation, eutrophication), with universal applicability across diverse groups of aquatic photoautotrophs.