Reevaluating the Canyon Hypothesis in a Biological Hotspot in the Western Antarctic Peninsula

In the Western Antarctic Peninsula, increased biological activity at many levels of the food web are spatially coherent with submarine canyons. One possible mechanism that links the presence of these canyons to increased biological productivity is through the local upwelling of nutrient-rich modified Upper Circumpolar Deep Water (mUCDW) to the surface, which supports high phytoplankton stocks, krill, penguins, and whales. In the austral summer of 2015, we investigated this hypothesis by deploying three autonomous Slocum gliders over Palmer Deep Canyon, near Palmer Station, Antarctica. Although we observed the shallowing of mUCDW consistent with canyon-driven isopycnal uplift, these deep waters did not penetrate the phytoplankton rich surface mixed layer. Waters below the mixed layer, however, were strongly coherent with bathymetry, suggesting the strong influence of the canyon. The decoupling of the surface mixed layer from the mUCDW suggests that local upwelling may not be the mechanism that supports the biological hotspot. New physical mechanisms that could support the biological hotspot at Palmer Deep Canyon are suggested. Plain Language Summary It has been suggested that the association of deep submarine canyons and the biological hotspot at Palmer Deep Canyon in the Western Antarctic Peninsula is due to the local upwelling of deep, nutrient-rich water to the surface, increasing phytoplankton growth and thus attracting local krill and, in turn, supporting upper trophic levels. However, we found that during the season of peak biological activity, summer stratification isolates the productive surface waters from the nutrient-rich water. Therefore, we suggest that upwelling of nutrient-rich water is not the mechanism driving this canyon associated biological hotspot. We suggest alternative mechanisms related to horizontal transport.