A Bay Journal article (2018-11-01) reported that for 2018 the Chesapeake Bay’s oxygen “dead zone” was average-sized but did go through several extreme swings due the heavy rains and extreme weather earlier in the year. As previously reported by Conduit Street, early predictions held that the dead zone would be larger than normal due to the severe rainfall but this turned out not to be the case. Over time, the Bay’s dead zone has been shrinking due to pollution reduction efforts.
The findings were separately confirmed by the Maryland Department of Natural Resources (DNR) and the Virginia Institute of Marine Science. From the article:
Researchers with the Virginia Institute of Marine Science determined via a computer model that the total volume of “hypoxic” water in the Bay, with oxygen levels low enough to stress fish, crabs and shellfish, was on par this year with that of 2017, taking up about 7 percent of the mainstem Chesapeake.
The Maryland Department of Natural Resources, using water-quality data gathered in sampling cruises from June through September, reported similar findings for the Maryland portion of the Bay. The DNR said that 17 percent of its waters had low dissolved oxygen levels — which matched the long-term average from 1985 through 2017.
But the DNR also found that abnormally wet weather through the first nine months of this year caused the low-oxygen zone in Maryland to gyrate in size from above-average in spring to near-record size by late June. It then plummeted to a record low in late July as a result of strong winds, followed by a rebound to higher than average in August and near-record again by late September.
The article noted that Maryland experienced double the normal amount of rainfall in May, washing more sediment and nutrients into the Bay. Excess nutrients can trigger algae blooms that consume the dissolved oxygen in Bay waters. However, some of the rainstorms also caused higher than normal in-flows from the Bay’s tributaries and brought heavy winds as well. The heavy waterflow and winds kept the Bay waters churning, which allowed more oxygen to be mixed into the deeper parts of the Bay. This action offset some of the additional nutrients and also led to the record low hypoxia zone in late July.
The article stated that scientists remain concerned that climate change could make the dead zone onsets happen earlier and last longer.