Effects of Low Oxygen Levels on Copepod Size Distribution with Depth in Hood Canal

Deoxygenation and hypoxia are affecting marine trophic webs throughout the world’s oceans. Hood Canal, Puget Sound is a basin that experiences seasonal hypoxia due to its restricted circulation and high primary production. The region supports a large secondary trophic level dominated by mesozooplankton, and particularly copepods. Studies suggest that zooplankton exhibit changes in their vertical distribution when faced with low oxygen concentrations and even face mortality when concentrations fall below ~1 mg/l. Because zooplankton are an important food resource for many aquatic animals, a shift in their distribution could have major implications for the food chain. This study examines how low oxygen levels affect the distribution of copepods relative to the oxygen minimum to explore how predator/prey interactions may be impacted by changing water conditions. Samples were collected in summer and fall 2017 and 2018 across normoxic and hypoxic conditions in Hood Canal. Zooplankton pumps and depth-stratified net tows were deployed during the day and night to observe diel vertical migration (DVM), and a camera equipped with a telecentric lens was mounted on a profiling mooring, which collected a continuous stream of photos during multiple profiles of the water column each day. A convolutional neural network, ResNet-18, was used to process the thousands of images collected in situ. Preliminary results suggest oxygen levels significantly impact species composition and distribution in the water column. In-situ imaging data are currently being analyzed to substantiate these findings and expand them across a broad range of oxygen conditions