Severn River Oysters – Are Fertilizers A Threat?

— by Joan Treichel, SRA Science Reporter

Oyster restoration in the Severn River is a major SRA program, enlisting some 400 volunteers each year.

Volunteer oyster growers planting oysters in Severn River (Lisa Borre).

In 2018, volunteers and donors planted 47 million oysters. This past July, the number was 17 million. And more will be coming.

But there is a threat to this master plan — fertilizers.

Although fertilizers are remarkably valuable in feeding the world’s population, when they end up in the Severn River or other rivers that feed into the Chesapeake Bay, they can inflict grave damage on animals living in those rivers, including oysters.

So reported Jeffrey Cornwell, PhD, a research professor at the University of Maryland Center for Environmental Science, during a recent virtual presentation at Cafe Scientifique in Annapolis, Maryland.

His presentation was based on science conducted not only by himself, but by numerous other environmental scientists.

Here is how fertilizers exert their damage if they end up in water, Cornwell explained. Nitrogen and phosphorus in fertilizers feed algae blooms. The algae blooms, in turn, suck oxygen from the water when the algae die and are decomposed by bacteria. And without oxygen, animals in the water, such as oysters, cannot live.

So how do fertilizers end up in the Severn River?

Algae blooms turn your river red

It flows into the river and creeks with stormwater runoff, Cornwell said. For instance, farmland fertilizer runoff is a big problem on the Eastern Shore.

But in the Severn River, lawn fertilizers are the major challenge.

In 2020, the Severn was afflicted by three major algae blooms, and the cause appeared to be primarily lawn fertilizers.

This thick algae bloom last May turned the Severn River into an angry orange-red color and it blocked the sunlight from reaching underwater grasses right as the summer grasses most needed the sun (see pic at right).

Now, oysters can help temper the pernicious impact of fertilizers on life in the Severn and other rivers by sucking up nitrogen and phosphorus in the fertilizers, Cornwell conceded.

But “they are no magic bullet” for solving the fertilizer problem because the amount of nitrogen and phosphorus they dispose of pales to the amount being poured into the water.

Wetlands, too, can help sop up nitrogen and phosphorus from the water, Cornwell noted. “Wetlands are doing a lot of work for us for free.” But again, their success cannot begin to offset the amount of fertilizer running into the Severn and other rivers.

So what’s the ultimate solution?

Are you fertilizing your river?

Preventing fertilizer runoff into the Severn and other rivers in the first place, Cornwell emphasized.

If you live along the Severn, for example, you could ask yourself:

“Am I letting my lawn fertilizer run into the Severn?”

And if so, “What can I do to keep that from happening?”

Here are several suggestions from the University of Maryland, Cornwell pointed out:

  • Since the Maryland Lawn Fertilizer Law was passed in 2011, which also applies to commercial lawn businesses, we are reminded not to over-fertilize.
  • Use a lawn spreader to apply fertilizer, never by hand. Most band-name fertilizers will list the proper setting to apply the correct amount to your lawn.
  • Sweep or blow fertilizer and lawn clippings that land on hard surfaces back into grassy areas.

“Maryland, on average, has done an excellent job with wastewater, and modern practices have decreased nitrogen and phosphorus inputs dramatically,” Cornwell told the SRA.

“However, the key to Chesapeake Bay restoration is decreasing inputs of nitrogen and phosphorus everywhere possible. A single lawn may not have a noticeable impact, but a million do!”