Baltimore Ecosystem Study Institute of Ecosystem Studies

2010 BES Annual Meeting Presentation and Poster Abstracts

Patchiness in Microbial Biomass and Activity in Geomorphic Features in Forested and Urban Restored and Degraded Streams
Harrison, Melanie
Co-Authors: Peter M. Groffman, Paul M. Mayer, Sujay S. Kaushal

Abstract: Instream geomorphic features such as pools, riffles and organic debris dams can be "hotspots" for biogeochemical processes. In urban degraded and restored streams, little is known about how fluctuating sediment conditions and geomorphic spatial heterogeneity affect sediment denitrification, an anaerobic microbial process that results in the permanent loss of nitrogen (N) as well as other anaerobic microbial processes such as methanogensis. We measured sediment denitrification potential (DEA), methanogensis, and a suite of microbial variables known to influence denitrification in four geomorphic stream features (organic debris dams, pools, riffles, and sloughs) located in forested, urban degraded and urban restored streams in the Baltimore, Maryland, U.S.A metropolitan area, between June 2005 and November 2006. Among features, DEA was significantly higher in organic debris dams compared with pools, riffles and sloughs ranging from 271 to 2790 ng N g-1 hr-1. We found strong positive relationships between DEA, microbial biomass N (MBN) (p < 0.0001) and percent sediment organic matter (SOM) (p < 0.0063), explaining much of the spatial variation in sediment denitrifcation across geomorphic features. There was no significant difference in DEA among forest, urban degraded and restored sites. Denitrification potential and MBN were significantly higher in June 2005 and August 2006 than in November 2006 and this temporal variation appeared to be driven by changes in microbial biomass. Methanogenesis was active in all stream geomorphic features across all study sites, with the highest rates observed in organic debris dams. These results suggest that in-stream geomorphic features in urban restored and degraded sites have the potential to function as N sinks by maintaining anaerobic conditions and microbial biomass and activity that stimulate denitrification.