The State Of The Severn – It’s Not Good

It was a very rough year for the Severn River in 2018.

Our fisheries and oyster reefs suffered from a dangerous combination of hot water, nutrient pollutant overloads and a dramatic drop in salinity.

That’s the worrisome news detailed in this year’s State Of The Severn report presented by Dr. Andrew Muller, associate professor of oceanography at the U.S. Naval Academy and Captain Diana Muller, with Maritimas USA.

Dr. Muller runs the Severn River Association’s (SRA) water quality monitoring program in partnership with the Severn Riverkeeper.

Volunteer citizen scientist crews collect weekly data to create water quality profiles of the river and its creeks. The Mullers analyze this and other data to develop the State Of The Severn report.

A standing room only crowd was on hand to hear the Mullers review details during SRA’s Educational Series meeting Nov. 20.

Dr. Muller noted that the year started off with multiple winter rain events that flooded the river with sediment, nitrogen and phosphorus pollution.

Then when the water warmed up, Dr. Muller explained, this mix produced several “monstrous” algae blooms that gorged on these pollutants in hot water.

Then, when the algae die, their decomposition creates another harmful problem — a dead zone of oxygen-starved water at the bottom of the river — right where our oysters live.

In one area just above the Rt. 50 Bridge at a location known as the “Winchester Lump,” the river experienced 3.5 days of continuous zero-oxygen conditions on the bottom with less than 0.2 milligrams of oxygen per liter (mg/l). This condition is known as “anoxic”.

— Graphic: UMCES/EPA.

Overall, Dr. Muller noted that in many places, the river’s oxygen levels were “hypoxic,” meaning less than 2.0 mgl/l.

Neither levels are good for oysters. Oysters normally require 5 mg/l or greater of oxygen to thrive, especially for oyster spat-on-shell.

Sadly, at no time during the summer months, where oyster spat-on-shell were planted, did oxygen levels reach a sustained healthy range for oysters of 5 mg/l or greater. It’s unclear what has happened to oyster-spat-on-shell planted this summer.

However, mature oysters can survive low-oxygen conditions.


For example, Dr. Muller pointed out that in 2011, we experienced another year of heavy rains and and low salinity levels. The Department of Natural Resources estimated that back then 75% of the spat-on-shell may have died.

But, the mature oysters survived.

This was confirmed this July when a dive team from Paynter Labs inspected the oyster reefs between the Rt. 50 and USNA Bridges. The team brought up oysters that were six- to eight-years old and still thriving. See pic above.

Low Oxygen, Fresh Water Challenge Oysters

Making things worse this year, we had biblical amounts of rainy weather that brought a mixed blessing to the Severn River.

All that rain dramatically lowered salinity levels. This presents another challenge for oysters, especially spat-on-shell. Salinity, which is normally in the 10- to 15-practical salinity unit (PSU) range, fell to a 4- to 6-PSU range.

An additional challenge to the oysters was the big change in pH levels in the river. Acidic water weakens oyster shells, which makes them more vulnerable to predators like rockfish.

On the other hand, all that fresh water rain led to the explosion of the Dark False Mussels.

They attach themselves to anything below the water line, even oysters (see pic at right).

Many attribute the proliferation of mussels to increases in clarity because they, like oysters, filter and clean river water.

Dark False Mussel – Good or Bad?

While many welcomed the Dark False Mussels for their contribution to improving clarity in late Summer and early Spring, the mussels may end up creating problems for the river.

Dr. Muller explained that when salinity levels return to normal, the mussels will die, and all of their biomass will become fuel for new algae blooms that contribute to the dead zone.

Dr. Muller also emphasizes that the State of The Severn’s 80% clarity score is misleading because very little clarity data was collected during this year’s Spring algae blooms and mud flood-producing rain storms. Both severely reduced visibility.

River-wide, dissolved oxygen levels also appear to have been good 62% of the time.

You can thank the wind during rain storms for helping oxygenate the river.

However, as Dr. Muller again emphasized, at critical times, dissolved oxygen levels were nevertheless extremely poor at the bottom of the river where the oysters live.

The Problem With Grades

Dr. Muller cautions that relying on grades doesn’t present an accurate portrait of river conditions.

The seemingly good grades are misleading, he explained, or only tell part of the story. For example, at times there’s been great clarity in the river – in certain locations.

Check out SRA’s WQ Crew member Evan Rockenbauch measuring clarity. He measured well over 1.0 m in Round Bay. “Wow, I can almost see my feet!” he tells the crew.

But as usual, as the team reached the upper river, clarity nearly zeroed out.

That’s Evan and fellow WQ Crew member Ted Delaplaine measuring the disappointing results at our Indian Landing Station at the top of the river on Nov. 7. (To our EPA pals, these are posed pictures!)

Reason: The outflow from Severn Run in the river’s headwaters is clogged with sediment. It resembles the muddy Mississippi instead of the fresh water stream it’s supposed to be.

It was also a horrible year for nitrogen levels. The river only gets a 17% grade in 2018 on nitrogen.

Blame this on runoff from septic tanks, pet and animal waste as well as the stormwater water runoff that washes nitrogen from impervious surfaces into the river.

Phosphorus, which has accumulated in the river from agricultural operations since colonial days, gets a 50% score.

Bacteria levels, as measured by Operation Clearwater, from septic systems, animal waste, scored acceptable 82% of the the time. It should be better, Dr. Muller said, because under the Clean Water Act, the goal is to be able to safely swim 100% of the time. Currently, health officials advise against entering the water within 48 hours of any rainstorm due to high fecal bacteria contamination.

Core Problem To Solve – Rapid Development

Dr. Andrew Muller says the key reason for the river’s pollution troubles is our history of “clear-cutting for decades,” and since 1987, we’ve suffered through “huge deforestation” and “an explosion of impervious surfaces” like roads, roofs, highways, parking lots, even ball fields.

“We can’t outpace the rate of development,” Dr. Muller noted, because our restoration efforts are “being blown away by outrageous development.” Despite the millions of dollars spent so far on various stormwwater and pollution control projects, “we do not yet see any significant improvement in the tidal portion of the Severn” that can be linked to upland restoration efforts.

Given this reality, “we can never restore the Severn River.” Instead, Dr. Muller said, the best that can be done is to “rehabilitate the river … because we’ve shocked the system too much” with development.

How do we turn this around?

The Mullers emphasize that the key is to focus on pollution control efforts inside the creeks and tributaries, and this includes Round Bay itself.

As Captain Muller pointed out, there’s little to no flushing of the high nutrient pollutant levels in the creeks or from the bottom of Round Bay. Although a large, open area, Round Bay doesn’t flush well due to the geography on the river’s bottom. Round Bay is essentially a deep bowl and the water at the bottom of the bowl does not mix well.

Without natural flushing capabilities, areas like Asquith Creek and Round Bay suffer through the dead zone conditions where the low-oxygen water takes up 50%, and even 70%, of the water column for most of the summer.

This phenomenon is known as “hypoxic squeezing” because fish are squeezed into a narrow layer of the river where they can breathe.

The Hypoxic Squeeze Index at left, created by Diana Muller at Maritimas USA, reveals the thick cluster of hypoxic data points where oxygen is less than 2.0 mg/l.

When things get this bad, fish and crabs swim away.  But oysters can’t leave, so they have to tough it out during these stressful conditions, which limits their growth and severely limits reproduction opportunities.

“The creeks are the source of what’s going on in the river.” The creeks are where the algae blooms are born. They grow out from the creeks and into the main stem of the Severn River.

To stop the cycle, focus on the creeks, stop the flood of stormwater runoff.

To view the State Of The Severn powerpoint presentation, click here: 2018 State of the Severn.

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