Anacostia River Mussel Power: River Filters
We have been saying that mussels are river filters and that they help clean the water. But, what do scientists say about that? Let's find out!
Even though the science (and art?!) of mussel restoration as well as the general scientific knowledge about mussels is still developing, scientists do know a thing or two about the biofiltration capacity of freshwater mussels. Here we take a quick look at just a few scientific papers on the issue. First of all, mussels are considered omnivores, they eat phytoplankton and bacteria. Phytoplankton are microscopic, photosynthetic organisms that live suspended in water. And I'm sure most of you know, in general terms, what a bacterium is. Freshwater mussels are even known to consume the pesky E. coli, which our Anacostia River has had no shortage of in recent history...
The biofiltration abilities of a mussel community can vary significantly depending on different factors such as mussel abundance, the species present, and with environmental conditions such as water flow, temperature, and the productivity of the ecosystem (i.e. nutrients available in the water). The biofiltration capacity of a mussel bed can vary seasonally with temperature, right now in the dead of the winter, their activity is of course less than in mid-summer.
The combination of submerged aquatic vegetation and freshwater mussels is great news for the Anacostia River. And with the recent improvements in water quality we have seen, it can only get better.
According to a publication by a group of scientists from the Mid-Atlantic region1, freshwater mussels are not as well researched as invasive and tidal bivalve species, but an emerging literature suggests they possess many of the same filtration benefits as their marine counterparts. A study in the Delaware River estimated that the Eastern elliptio (Elliptio complanata), a species we also have in the Anacostia River, can filter more than 9.7 billion liters (2.5 billion gallons) of water per hour across the basin, a figure comparable to the potential volume processed by oysters in the Delaware Bay.Another study, surveyed the abundance of six species of freshwater mussels in four mussel beds within the tidal freshwater zone of the Delaware River and estimated their combined water clearance to exceed 25 million liters (6 million gallons) per day for a combined area of just under 4 hectares (9.8 acres). The eastern elliptio is considered to be one of the most common freshwater mussel species of the Atlantic slope of the U.S., something we have definitely noticed at Buzzard Point, which we consider to be a "mussel hotspot" in the Anacostia River in terms of species diversity and abundance. Back to the Delaware estuary, their research has found that in the summer eastern elliptios can filter between 2.6 to 3.1 liter of water per mussel per hour3, that's 16.4 to 20 gallons of water per mussel per day!! The authors of the study concluded that freshwater mussels are definitely not inferior to saltwater bivalves in regard to their organism-level filtration capacity. Actually, there are some indications that they might even be more effective than their marine counterparts, but further studies are needed to confirm that assertion.3
A projection of the water quality benefits of a hypothetical bed of 1000 freshwater mussels was done by the Partnership for the Delaware Estuary. According to the report,1000 adult mussels would typically contain about 1.2 kilograms (2.6 lbs) of dry tissue weight, would clear 2.8 million gallons of water per year, and would filter 173 kg (381 lbs) of (dry) total suspended solids per year. Nitrogen removal would depend on the N content of the total suspended solids, which varies widely among waterbodies4. Another study in Illinois found that two native freshwater mussel species significantly increased denitrification (nitrogen removal) rates and estimated the financial value of the ecosystem service5.
Other studies have found that the California floater (Anodonta californiensis) can remove pharmaceuticals, personal care products, herbicides, and flame retardants from the water column and either biodeposit or store them in their tissue2.
As you can see, this is just the tip of the iceberg, this was by no means an exhaustive literature review on the topic and only shows a fraction of the work being done on this relatively new research/restoration topic. One conclusion we have is that mussels may not be the silver bullets for all our water quality issues, but they are absolutely promising. Chances are they will become a Best Management Practice (BMP) for the reduction of nutrients and sediment in the Chesapeake Bay watershed very soon. Or at least we hope so.
For the River,
- Kreeger, D. A., C. M. Gatenby and P. W. Bergstrom. 2015. Comparative evaluation of native bivalve species restoration for water quality improvement in the Chesapeake Bay and other Mid-Atlantic watersheds. Partnership for the Delaware Estuary, Wilmington, DE. PDE Report No. 17-05. 96 p.
- Cited in: Vaughn, C. 2017. Ecosystem services provided by freshwater mussels. Hydrobiologia DOI 10.1007/s10750-017-3139-x. Springer International Publishing, Switzerland.
- Kreeger, D. A., C. M. Gatenby and P. W. Bergstrom. 2018. Restoration Potential of Several Native Species of Bivalve Molluscs For Water Quality Improvement in Mid-Atlantic Watersheds. Journal of Shellfish Research, Vol. 37, No. 5, 1121–1157, 2018.
- Cheng, K. M. & D. A. Kreeger, 2017. Determination of Filtration Capacity and Pollutant Removal of Freshwater Mussels in Delaware Streams. Partnership for the Delaware Estuary, Wilmington, DE. PDE Report No. 17-04.
- T. J. Hoellein; C. B. Zarnoch; D. A. Bruesewitz; J. DeMartini. 2017. Contributions of freshwater mussels (Unionidae) to nutrient cycling in an urban river: filtration, recycling, storage, and removal. Biogeochemistry. DOI 10.1007/s10533-017-0376-z. Springer International Publishing AG 2017.