Annual dead-zone report card for the Chesapeake Bay, U.S.

An annual model-based report on low-oxygen conditions in Chesapeake Bay during 2018 indicates a total volume of hypoxic waters very similar to the previous year, but with a dramatic drop in hypoxia during late July due to mixing by strong winds. The duration of hypoxia in 2018 was greater than in recent years. Dead zones are one of the major water-quality concerns facing the Bay and coastal waters worldwide. They form when rivers carry in excess nitrogen from fertilizers, wastewater, and other sources, fueling short-lived blooms of algae. Bacteria then eat the dead, sinking algae, consuming from bottom waters the dissolved oxygen that fish, shellfish, crabs, and other animals need to survive. Bay dead zones peak during summer, when hot weather encourages algal growth and drives gases from the water, while calm winds typically preclude the mixing of relatively oxygen-rich surface waters into the depths.

The team's report card summarizes oxygen conditions in the Bay each year as estimated by their 3-D, real-time hypoxia forecast model, originally developed with funding from NOAA. The model is based on 30 years of water quality data collected by the Chesapeake Bay Program, and is forced daily by wind data provided by NOAA and river-input data provided by the U.S. Geological Survey. Because springtime inflows from the Susquehanna River Chesapeake Bay's largest tributary were high in 2018, scientists predicted that summer 2018 would have an above average amount of hypoxia, a forecast that held true through mid-July, when unusually strong winds reduced the Bay's hypoxic volume to near zero.