Grazing by sea urchins is a potential co-driver of Greenland kelp forest distribution

Abstract

Grazing by sea urchins is a main driver of kelp forest dynamics causing shifts from kelp forests to urchin barrens in the temperate region but there is limited information on such patterns from the Arctic region. Based on information from 132 underwater video transects along Greenland's West Coast at 59.9–77.8°N, we explored the distribution of kelps and sea urchins, including the occurrence of sea urchin barrens, along latitudinal- and depth-gradients. We observed more extended kelp cover and deeper-growing kelp forests toward south, although with marked variability among sites. Sea urchins were present along the entire West Coast, with no clear depth-related pattern, but their densities were generally higher toward north. Kelp cover declined toward deeper, light-limited waters and was also reduced where sea urchins were abundant, in some cases with urchin barrens within the kelp beds, suggesting a potential top-down control of kelp distribution by sea urchins. Sea urchins left, on average, 10% of the potential kelp area barren, with the largest proportion (20%) of barren ground toward north. Earlier studies have identified the length of the sea-ice free period and water temperature as main bottom-up drivers of spatial distribution of kelp in this region. We conclude that sea urchins are present all along Greenland's West Coast and occur at densities suggesting they may act as local co-drivers of kelp forest distribution and cover.