The fate of freshwater mussels in a suburbanizing landscape

Dromedary pearly mussel (Dromus dromas), a highly endangered mussel of the southeastern US. Image credit, Jeffry D. Grabarkiewicz, US EPA.The fate of threatened mussels in the southeastern United States illustrates the potential problems that suburbanization of rural landscapes can cause to freshwater ecosystems. Of the roughly 250 species of mussels historically found in the Southeast, scientists believe that 75% are either extinct or highly threatened.

A new study in the journal Hydrobiologia finds evidence that suburban development may be at least partly to be blame. The findings suggest that increased sedimentation and a deterioration in water quality may be leading to mussel decline.

From 1999-2007, the researchers surveyed mussel populations and measured substrate and water chemistry variables in Chewacla Creek near Auburn, Alabama, a rapidly developing area (at least pre-financial crash).

They collected data at sites both downstream and upstream from urbanized tributaries including an area that receives discharge from a wastewater treatment plant. They also measured survivorship of adult mussels (Villosa lienosa) trapped in cages at three areas downstream of the treatment plant and one control site upstream.

They found that mussel abundance and diversity was greatest at the sites farthest upstream and then dropped off downstream at the urban impacted areas. Similarly, nutrient concentrations and sedimentation increased at the sites downstream, notably after the confluences with urbanized catchments.

They also found that mussels in the downstream cages had a much lower survivorshsip (as low as 22%) versus the mussels in the control site upstream (100%).

One of the challenges of this study is disentangling what exactly from the urbanized setting is causing the mussel decline. The researchers suspect that sedimentation is the driving factor.

They observed that mortality in the cages tended to coincide with sediment movement associated with high flow events suggesting that these occurrences cause stress to mussels. This makes sense given that many species of mussels prefer the stable areas of the stream bed.

The results suggest that increased nutrient exposure is also causing mussel decline, but the researchers doubt that this is the primary cause of increased mortality in the cages. Most deaths occurred after six weeks, and acute water quality degradation likely would have caused mortality sooner. On the other hand, the researchers only studied adult mussels in cages - juveniles may be more sensitive to nutrient loading, and they did not measure the full suite of water quality parameters that could cause toxicity.

They conclude that suburbanization in the area is likely leading to a decline of endangered freshwater mussels and actions should be taken to reduce stream sedimentation and nutrient inputs. They state:

"Ensuring the survival of highly fragmented mussel populations in urban or developing watersheds like the Uphapee subbasin will require significant reductions of both point and non-point source nutrient loading. Reductions in sediment inputs and stabilization of both riparian and instream habitat within tributary sub-catchments are critical to limiting migration of fines into mainstem reaches and will likely reduce non-point nutrient loading of tributaries. Meaningful conservation of imperiled aquatic biota in Piedmont streams will likely require substantial changes to ongoing development practices in these sensitive watersheds."

--Reviewed by Rob Goldstein

Gangloff, M., Siefferman, L., Seesock, W., & Cliff Webber, E. (2009). Influence of urban tributaries on freshwater mussel populations in a biologically diverse piedmont (USA) stream Hydrobiologia DOI: 10.1007/s10750-009-9948-9