Evaluation oxygen deficiency in the Chesapeake Bay

Abstract. The objectives of the Dead Zone Zooplankton research project (NSF OCE-0961942) were to study the effects of the onset, development, and dissipation of hypoxia (DO < 2 mg L⁻¹) on the plankton food web in the Chesapeake Bay. Here, we present the hydrologic and meteorology data from CTD, Scanfish, and the Safety Measurement System (SMS) of the research vessel to describe the environmental conditions of the bay during the project. We collected data from the mesohaline portion of Chesapeake Bay from 37.5–38.5° N and from 76–76.5° W during six research cruises in 2010 and 2011. We analyzed the temperature, salinity, and dissolved oxygen from hourly CTD casts using principal component analysis (PCA) to understand variations in hydrography among depths, stations, seasons, and years. In addition to using the commonly accepted standard of hypoxia (DO < 2 mg L⁻¹), we also estimated the oxygen supply and demand of the copepod Acartia tonsa according to the surrounding temperature and salinity. The hypoxia in the bay began in the late spring, developed from the bottom layer upstream, progressed toward the sea, became fully established in summer, and gradually dissipated in autumn beginning in the downstream regions. However, we observed that extreme weather could interrupt this succession and reignite hypoxia events after summer. Our PCA results indicated that temperature was the major driver of environmental conditions, and dissolved oxygen in the bottom layer was the second most important driver. Within each temperature group, we found that samples from 2011 and the north station were less oxygenated than samples from 2010 and the south station. Comparing the two metrics of oxygen deficiency, we found that the duration, distribution, and severity of environmental oxygen deficiency could be underestimated using the traditional metric, especially under warm and salty conditions. We recommend that temperature and species-specific metrics be considered along with dissolved oxygen concentration when setting water quality goals for management. We uploaded the CTD data to the Biological and Chemical Oceanography Data Management Office (https://doi.org/10.1575/1912/bco-dmo.687991; hyphen is part of the DOI), and we stored the Scanfish and SMS system data in the Rolling Deck to Repository (https://doi.org/10.7284/901570; https://doi.org/10.7284/901574; https://doi.org/10.7284/907638; https://doi.org/10.7284/901618; https://doi.org/10.7284/902443; https://doi.org/10.7284/902721 for the six cruises, respectively).