Howling Hurricane Isaias Churns It’s Way Up The Severn!
Published: August 7, 2020
Hurricane Isaias rolled up the Severn River on Aug. 3, bringing torrential downpours and high winds that did more than knock downed trees and create power outages on land.
In the Severn, Isaias left quite a mess:
– fish kills along our shorelines,
– stinky blue-algae blooms,
– sediment plumes, and
– no swimming advisories.
The picture above is what showed up overnight on the beaches of the Round Bay Community.
SRA was also tracking reports of fish kills higher up in Round Bay, north of Yantz Creek and on the beaches of Annapolis Roads.
So, what caused the fish kill? The short answer is that the fish died when they were overcome by a mass of “dead” water – ie., water with very low concentrations of oxygen. Water with less than 2 mg/Liter of oxygen is considered a “dead zone” and does suffocate fish and crabs.
Crabs, oysters and most fish need 5 mg/Liter of oxygen or better to thrive. They can tolerate 2- to 5-mg/Liters of oxygen, but when the levels of oxygen get below 2 mg/liter, fish and crabs either swim away or suffocate. During the storm, the dead zone water contained water with less than 0.10 mg/L of oxygen.
These fish kills were caused by Isaias’s strong winds, which churned and mixed up the entire water column in the Severn River. The winds flushed the low-oxygen water up from the bottom and sent it to the surface where fish congregate because that’s where there’s enough oxygen to breath. Unfortunately, when Isaias rolled up the river, it caused an upswelling of the low-oxygen water that overcame the fish before they could swim away and they suffocated.
The chart below is typical of the oxygen data our water quality monitoring teams have been collecting in Round Bay and in the Severn’s mid-river creeks all summer.
Note as you study the data that most fish need levels of oxygen in the water to be 5 mg/Liter or more to thrive. We mark this water as green. When oxygen is less than 2 mg/L, we mark it red for dead zone. Yellow is for the in-between with marginal levels of oxygen (2 to 5 mg/L).
These two charts are of data collected at our Round Bay North monitoring station (just offshore from Linstead Community at the top of Round Bay). The red boxes in the column marked mg/L indicates how tall the dead zone is at Round Bay North.
The week before Isaias came through (data on the left), the dead zone is 4 meters tall. Sitting on top of this red layer is a green layer of water with good oxygen levels (more than 5 mg/L oxygen).
The data in the chart to the right indicate oxygen levels at Round Bay North station just after Isaias. Note there is no layer of green, meaning no water with good oxygen levels, and the dead zone (in red) is now 6 meters tall. It is this layer of water that kills fish when a storm like Isaias churns up the water column and the fish (menhaden in this case) can’t swim away fast enough.
The only good news in these charts is the salinity level.
This column is all green because salinity has solidly returned to normal levels, 8- to 15- parts per thousand (ppt). During 2018-2019 salinity in the Severn river had collapsed to a 3- to 6-ppt range. This freshness is what encouraged the explosion of the dark false mussels. Now that the river is saltier, these mussels have retreated dramatically.
Special thanks to the Delaplaine Foundation for their generous support of SRA that helps keep our water monitoring program operating this season.
Isaias also dredged up a stinky blue-green algae known as Cyanobateria. See pic at left.
This was spotted by a pal of ours, Lindsey McCall Pence, looking upstream towards The Narrows. It’s hard to see, but it’s in the middle of the picture at right, just above the piling.
But Lindsey didn’t have any trouble noticing it because it gives off a strong sulfurous odor. DNR’s Cathy Wazniack inspected the area and confirmed it was Cyanobateria.
Cyanobateria is classified as a harmful algae bloom that can be harmful to wildlife.
It’s harmfulness depends on a mix of conditions: https://www.healthlinkbc.ca/healthlinkbc-files/blue-green-algae
And, as typical with many a downpour, some of our creeks were inundated with sediment runoff from poorly maintained stormwater management systems.
On the right, here’s what happened to Asquith Creek which was inundated with the 5.5 inches of downpour.
To report a fish kill: Contact the Maryland Department of Environment’s Fisk Kill Investigation Section during normal work hours, call 443-224-2731 or 800-285-8195.
During evenings, weekends and holidays, call The Chesapeake Bay Safety and Environmental Hotline (1-877-224-7229).