Long-term changes in the benthic communities of an urban watershed: The Elizabeth River's Southern Branch

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W. Kody Muhic, Lisa Horth and Daniel M. Dauer, Department of Biological Sciences, Old Dominion University, Norfolk, VA

Background/Question/Methods:
Benthic macroinvertebrate assemblages are often used as indicators of environmental condition in estuarine systems because of their predictable responses to many types of natural and anthropogenic stressors. These assemblages were examined in the Southern Branch of the Elizabeth River—a highly industrialized tributary of the Chesapeake Bay that has been the location of a number of remediation efforts in the past fifty-years. The results were compared to a similar study conducted twenty-years earlier concerning the same location. Changes in taxonomic richness, density, abundance, and biomass were evaluated, as well as any changes in scores assigned using the Benthic Index of Biotic Integrity (B-IBI)—an index developed specifically for use with macrobenthic communities of the Chesapeake Bay. The main objectives of this study were: (1) characterizing the benthic macroinvertebrate community of the Southern Branch using major metrics (abundance, biomass, species diversity, and species richness), (2) depicting the environmental condition of the system using the B-IBI, and (3) describing any long-term changes in these metrics after two decades.
Results/Conclusions:
No significant changes were revealed in examining major biological parameters within the Southern Branch; however, significant improvements in benthic community condition between the two sampling periods were demonstrated using the B-IBI. The average B-IBI value increased significantly from 1999 (mean=2.0, SEM=0.1) to 2019 (mean=2.5, SEM=0.1) (t=2.6, df=48, p=0.01). The total estimated area of degraded benthic community condition also declined from 96% of the Southern Branch’s bottom in 1999 down to 64% in 2019. This decline is particularly suggestive when examining the percentage of severely degraded bottom condition, which was reduced from 64% down to 36%. The Southern Branch was the only section of the Elizabeth River to demonstrate these improvements between the two sampling periods. Patterns of increased degradation seen within the rest of the Elizabeth River are also seen within surrounding areas of the Chesapeake Bay, indicating potential greater drivers negatively affecting ecosystem condition within the Elizabeth River. Yet, conversely, the Southern Branch—one of the most industrialized waterways in America—has shown significant improvement within its benthic community. One possible explanation for this branch’s enigmatic progress could be its many completed and ongoing cleanup efforts. These include the ongoing EPA cleanup of the Atlantic Wood Superfund site and the removal of vast amounts of PAH contaminated sediments, as well as various improvements in industrial business practices, and ongoing oyster bed and wetland restoration projects.