Chesapeake Bay dead zone smaller than in recent years

Chesapeake Bay dead zone smaller than in recent years Download
Annapolis, MD ()

Experts from the Maryland Department of Natural Resources report that the 2020 dead zone is the second smallest observed in the Maryland portion of the Chesapeake Bay since monitoring began in 1985. In their 2020 Chesapeake Bay Dead Zone Report Card, researchers from the Virginia Institute of Marine Science also reported that throughout the entire Bay this year’s dead zone was smaller than most recorded in the past 35 years (80%).

In June 2020, researchers from the Chesapeake Bay Program, the University of Maryland Center for Environmental Science, University of Michigan and U.S. Geological Survey forecasted that the Bay would see a slightly smaller than average dead zone this year, due to reduced spring rainfall and less nutrient-rich runoff flowing into the Bay from the watershed.

In the short-term, experts believe that several factors, including more average river flows and unseasonably cool temperatures in May and September contributed to the smaller dead zone. Over the long-term, the continued implementation of nutrient and sediment reduction strategies put in place by the six states in the Chesapeake Bay watershed (Delaware, Maryland, New York, Pennsylvania, Virginia and West Virginia) and the District of Columbia are continuing to help decrease pollution in the Bay and reduce the size of the dead zone.

Hypoxic and anoxic regions—areas with little to no oxygen, respectively—are caused by excess nutrient pollution entering the Bay. One way in which nutrients can enter the Bay is through its tributaries in the watershed that drain into it. Higher river flows bring increased amounts of nutrient pollution into the Bay. The previous two years have seen above-average river flows, with 2019 setting a record high. According to the U.S. Geological Survey, the current year (measured from October 1, 2019—September 30, 2020) has been normal, with flows entering the Bay at an average of 77,665 cubic feet per second, which is slightly below the long-term average of 79,000 cubic feet per second.

Only one out of the eight monitoring cruises showed larger-than-average hypoxic conditions. This occurred in late July as a result of below average winds and the hottest temperatures ever recorded in Maryland, causing hypoxia to increase considerably, resulting in a large dead zone. Strong winds from Hurricane Isaias in August helped to mix the waters of the Bay, reducing the dead zone; hypoxia returned in September but quickly dissipated due to cooler temperatures and windy conditions. This year’s dead zone started later and ended earlier than it has in the past several years. Additionally, no anoxic areas were noted in the mainstem of the Bay this year.


Throughout the year, researchers measure oxygen and nutrient levels as part of the Chesapeake Bay Monitoring Program, a Bay-wide cooperative effort involving watershed jurisdictions, several federal agencies, 10 academic institutions and over 30 scientists. Among these institutions, the Maryland Department of Natural Resources and Virginia Department of Environmental Quality conduct 8-10 cruises between May – October, depending on weather conditions, to track summer hypoxia in the Bay. Results from each monitoring cruise can be accessed through the Eyes on the Bay website for the Maryland portion of the Bay and the VECOS website for the Virginia portion. Estimates of river flow and nutrients entering the Bay can be accessed on the U.S. Geological Survey’s website. Scientists at the Virginia Institute of Marine Science, in collaboration with Anchor QEA, produce daily real-time estimates of dead zone size, along with forecasts of hypoxia in the Bay.


The dead zone is an area of little to no oxygen that forms in deep Bay waters when excess nutrients, including both nitrogen and phosphorus, enter the water through polluted runoff and feed naturally-occurring algae. This drives the growth of algae blooms, which eventually die and decompose, removing oxygen from the surrounding waters faster than it can be replenished. This creates low-oxygen—or hypoxic—conditions at the bottom of the Bay. Plant and animal life are often unable to survive in this environment, which is why the area is sometimes referred to as a “dead zone”.

Pollution reducing practices used in backyards, cities and on farms can reduce the flow of nutrients into waterways. Management actions taken to decrease loads from point sources (e.g. wastewater treatment plants) may immediately show detectable pollution changes, but best management practices for non-point sources (which refers to pollution resulting from excess fertilizers, agricultural run-off, etc.) often show a time lag between implementation and their impact toward improving water quality and the health of the Bay.

Weather conditions also play a role in the size and duration of the annual dead zone. Heavy rainfall can lead to high river flows entering the Bay, which carries along increased amounts of nutrient pollution. Above average spring freshwater flows to the Bay, along with hot temperatures and weak winds provide the ideal conditions for the dead zone to grow larger and last longer, as occurred in 2019. Average spring freshwater flows to the Bay, combined with cooler temperatures in May and September, provided ideal conditions for the dead zone to remain smaller in 2020 than it has in past years.


“Our analyses at VIMS are helping us better understand how and why the dead zone changes in size from year to year. After we account for year-to-year differences in summer weather, the general trend we’re seeing is that hypoxia has been decreasing with time, as a result of reductions in the amount of nutrients flowing into the Bay over the past several decades. The fact that we’re seeing decreased hypoxia despite significantly warmer summer temperatures is a testament to the fact that management actions to curb nutrient pollution are working”

  • Dr. Marjy Friedrichs, Research Professor, Virginia Institute of Marine Science

“Improved dissolved oxygen water is critical for crabs, oysters and finfish in the Bay. The monitoring and associated interpretation by the Chesapeake Bay Program partnership are the foundation for assessing progress in restoring water-quality conditions Bay and its watershed.”

  • Scott Phillips, Chesapeake Bay Coordinator, U.S. Geological Survey and Co-Chair, Scientific Technical Assessment and Reporting Workgroup, Chesapeake Bay Program

“The amount of hypoxia is a key indicator of Bay health. After two years of extremely high flows and greater than average hypoxia, it is encouraging to see improved oxygen conditions in our bottom waters providing suitable habitat for fish, crabs and oysters. It is Maryland’s goal, along with our Chesapeake Bay Program partners, to reduce nutrients and sediments entering the Bay to levels that support good water quality for our iconic Bay species.”

  • Bruce Michael, Director of Resource Assessment Service, Maryland Department of Natural Resources