Early August 2019 Hypoxia Report

Maryland Department of Natural Resources monitoring data show that dissolved oxygen conditions in the Maryland portion of the Chesapeake Bay mainstem were larger than average in early August. The hypoxic water volume (areas with less than 2 mg/l oxygen) was 1.77 cubic miles in early August, down from the 2.01 cubic miles seen in late July, but significantly higher than the 1985-2018 early August average of 1.19 cubic miles. The hypoxic volumes ranked third-largest since 1985 for the early August time period.

Lower dissolved oxygen throughout the Maryland portion of the bay was the result of many factors. Larger than average hypoxia was predicted for this summer in part due to high flows into the Chesapeake Bay last fall and into the spring, which delivered higher nutrient loads. Those nutrients fuel algal blooms which die and are consumed by bacteria, which then deplete oxygen in bottom waters. Surface water temperatures in the open Bay were still quite warm, about 84 degrees. Warmer waters hold less oxygen. There were no sustained wind events since the late July sampling period, which prevented oxygen from mixing into deeper waters. Finally, surface salinities are still below average for Maryland’s main bay which creates a more stratified water column, essentially acting as a cap to prevent higher oxygen surface waters from mixing with deeper, oxygen depleted waters.

In the beginning of June, National Oceanic and Atmospheric Administration, United States Geological Survey, University of Maryland Center for Environmental Science and University of Michigan scientists predicted a large hypoxic volume for the bay in 2019 due to higher flows last fall and this spring, and higher nitrogen loading from the Susquehanna and Potomac Rivers.

Crabs, fish, oysters and other creatures in the Chesapeake Bay require oxygen to survive. Scientists and natural resource managers study the volume and duration of bay hypoxia to determine possible impacts to bay life. Each year from June to September the Maryland Department of Natural Resources computes these volumes from data collected by Maryland and Virginia monitoring teams during twice-monthly monitoring cruises. Data collection is funded by these states and the Environmental Protection Agency’s Chesapeake Bay Program. Bay hypoxia monitoring and reporting will continue through the summer.