Zooplankton drive diurnal changes in oxygen concentration at Tisler cold-water coral reef

Tisler Reef is a Norwegian cold-water coral reef in the Northeastern Skagerrak, which lies at an average depth of 120 m, is constructed principally of the scleractinian coral Lophelia pertusa and hosts a dynamic and diverse ecosystem. The availability of oxygen within Tisler Reef, recorded between 2006 and 2008, showed a decline during the summer months, caused by both the isolation of the reef from the atmosphere under conditions of seasonal stratification, and the enhanced respiration in the water column during the seasonal zooplankton proliferations. Concentrations of dissolved oxygen were replenished from high-current flows advecting water from off the reef. Low current flow conditions (< 0.05 m s⁻¹) coincided with a short-term reduction in oxygen, the extent of which varied seasonally and were observed to be greatest during July and August, coinciding with the summer Calanus proliferation in the Skagerrak. Normalized acoustic backscatter amplitude during the summer months showed a strong signal of zooplankton diurnal vertical migration, coinciding with the lowest oxygen concentrations at the reef observed during, and lagging slightly after, the deep phase of the zooplankton vertical migration. This effect was most obvious during low-flow conditions; highlighting the importance of zooplankton and associated activity as a consumer of oxygen at the reef.