Population Assessment and Potential Functional Roles of Native Mussels in the Upper Mississippi River

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Impact of UMESC Science

The results of this study suggest that native mussels play an integral role in this ecosystem by sequestering large volumes of suspended materials that can be used by other benthic organisms.
 

Managers now have critical data on population size, distribution, and relative health—these data are being used to guide habitat restoration activities to benefit native mussel populations in large rivers.

How many mussels are there?

  • Large scale systematic surveys for native mussels in three reaches of the Upper Mississippi River documented mussel communities composed of 16-23 species with densities ranging from 2.9-4.5 live mussels m-2.  Pool wide population estimates ranged from 60-212 million mussels.
Functional Role of Mussels within the Ecosystem

(Public domain.)

How healthy are mussel populations?

  • Mussel populations were actively recruiting new cohorts into their populations. About 40-62% of the pool wide populations were juveniles ≤5 years old.
     
  • Further, we found juveniles for most of the species present. Between 87-100% of the species detected were found as juveniles.

Where are mussels found?

  • The spatial distribution of mussels varied among pools. In Pools 5 and 18, highest densities of mussels were generally found in the lower impounded section, with a few high density areas in the border of the main navigation channel or larger side channels in the upper one-third of these reaches.
     
  • In contrast, the spatial distribution of mussels in Pool 6 was relatively similar throughout the pool, with just a few high density areas in the larger side channels and along the border of the main navigation channel. This suggests the presence of expansive impounded areas may create conditions favorable to mussels, although the mechanisms for this are unclear at this time.

What is their potential functional role?

  • As relatively immobile filter-feeders, mussels can remove large amounts of particles from the water column and transfer these nutrients and energy into other organisms that reside near the river bottom. In addition, large concentrations of mussels can stabilize river substrates.
  • Based on their filtration rate and density, we have evidence of the importance of native mussels in particle dynamics in the Upper Mississippi River.
     
  • During high river flows the mussel populations in these reaches can filter about 1% of the water volume and during low flows mussels can filter about 12%.
     
  • Across the 480 km reach (Pool 5 to Pool 18) the total water filtration by mussels is about 75 times that of the water filtration performed at the Minneapolis-St Paul, Minnesota metropolitan wastewater treatment plant, one of the largest in the United States.
Did You Know? Mussel fact

Did You Know?  Across the 480 km reach from Upper Mississippi River Pool 5 to Pool 18 the total water filtration by mussels is about 75 times that of a large metropolitan wastewater treatment plant in the Midwest.(Public domain.)