Production–consumption mismatches across depths suggest that kelp forests at the Arctic-boreal interface may be vulnerable to coastal darkening

Abstract Climate change is predicted to enhance the abundance and productivity of Arctic and sub-Arctic kelp forests. These gains may be constrained, however, by coastal darkening — the increased turbidity and light attenuation in nearshore waters, and sea urchin overgrazing. Here, we undertook a snapshot survey to examine how kelp productivity and sea urchin consumptive demand vary along a gradient of light availability (depth) across kelp forests in southwest Iceland. To do so, we quantified depth-dependent patterns in kelp ( Laminaria hyperborea ) canopy structure and per capita growth, and sea urchin ( Echinus esculentus and Strongylocentrotus droebachiensis ) abundance, biomass and consumptive demand. We found that kelp per capita growth and longevity declined sharply with depth, with canopies below 10 m accumulating up to 90% less biomass than their shallow (5 m) counterparts. In contrast, sea urchin populations at depth were dominated by large individuals with high metabolic demands, likely resulting in elevated consumptive demands despite lower overall densities. This mismatch between kelp production and consumption led to a greater “consumption debt” in deeper canopies, where urchin consumptive demands exceeded local kelp productivity. Our findings suggest that predicted poleward expansions of L. hyperborea could be depth-limited, with sea urchin grazing at depth likely to counteract productivity gains.