First HOV Alvin study of the pelagic environment at Hydrographer Canyon (NW Atlantic)

Abstract Continental slope canyonsoff the United States Atlantic coast remain poorly studied, and in particular, the distributions of pelagic organisms in waters overlying these unique environments are not well documented. During the Early Career Scientist Deep Submergence Training cruise, AT36-EAGER, the distribution of organisms in the water columnoverlying Hydrographer Canyon, which cuts through the northwestern Atlantic continental margin, was investigated through daytime midwater observations using HOV Alvin (AD4831) at three depths. Mixed swarms of krilland Themisto sp. amphipods were observed at all depths surveyed. Observations centered at 250 m were also dominated by chaetognaths, copepods, and Phronimasp. amphipods, while at 500 and 750 m, the assemblages were dominated by the fishes in the families Paralepididae, Nemichthyidae, and Mytophidae. Additionally, measurements of methane, nitrous oxide, optical properties (absorbance and fluorescence), dissolved organic carbon, and base-extracted particulate organic carbon were made to better characterize the hydrographyand biogeochemistryover Hydrographer Canyon. This study was aided by the use of telepresence to communicate between ship and shore-based researchers, and the expedition marks the first use of SMS messaging to communicate between the submersible and the ship. This study demonstrates the capabilities and utility of using Alvin for conducting water columnscience.