Urban streams are degraded by a suite of factors, including burial beneath urban infrastructure, such as roads or parking lots, which eliminates light and reduces direct organic matter inputs to streams from riparian zones. These changes to stream metabolism and terrestrial carbon contribution will likely have consequences for organic matter metabolism by microbes and dissolved organic matter (DOM) use patterns in streams. Respiration by heterotrophic biofilms drives the nitrogen and phosphorus cycles, but we lack a clear understanding of how stream burial and seasonality affect microbial carbon use. We studied seasonal changes (autumn, spring, and summer) in organic matter metabolism by microbial communities in open and buried reaches of three urban streams in Cincinnati, OH. We characterised DOM quality using fluorescence spectroscopy and extracellular enzyme profiles, and we measured the respiration response to carbon supplements in nutrient diffusing substrata (NDS). We hypothesised: (1) that algal production would lead to higher quality DOM in spring compared to other seasons and in open compared to buried reaches, (2) lower reliance of microbial respiration on recalcitrant carbon sources in spring and in open reaches, and (3) that microbial respiration would increase in response to added carbon in autumn and in buried reaches. Several fluorescence metrics showed higher quality DOM in spring than autumn, but only the metric of recalcitrant humic compounds varied by reach, with more humic DOM in open compared to buried reaches. This likely reflected open reaches as an avenue for direct terrestrial inputs from the riparian zone. Extracellular enzyme assays showed that microbes in buried reaches allocated more effort to degrade recalcitrant carbon sources, consistent with a lack of labile carbon compounds due to limited photosynthesis. Nitrogen acquisition enzymes were highest in autumn coincident with riparian leaf inputs to the streams. Buried and open reaches both responded more strongly to added carbon in autumn when terrestrial leaf inputs dominated compared to the spring when vernal algal blooms were pronounced. Our data show that stream burial affects the quality of the DOM pool with consequences for how microbes use those carbon sources, and that heterotrophic respiration increased on carbon-supplemented NDS in buried and open stream reaches in both seasons. Different carbon quality and use patterns suggest that urban stream infrastructure affects spatiotemporal patterns of bacterial respiration, with likely consequences for nitrogen and/or phosphorus cycling given that carbon use drives other biogeochemical cycles. Management actions that increase light to buried streams could shift the balance between allochthonous and autochthonous DOM in urban streams with consequences for spatiotemporal patterns in bacterial metabolism.
Bibliographical noteFunding Information:
We thank Kendall Jo Stanavich for assistance in the laboratory and Mike Bosko for assistance in the laboratory and with data compilation. This manuscript was improved by comments contributed by two anonymous reviewers. This research would not have been possible without the permission of numerous private property owners who allowed site access and the assistance of site selection by Cincinnati Metropolitan Sewer District. Field and laboratory support was provided by Pegasus Technical Services under contract #EP-C-006 and by Dynamac Corporation under contract #EP-D-11-073. This research was supported by EPA NNEMS Award 2010-309, the NSF Graduate Research Fellowship Program under Grant No. DGE1144243, NSF Awards DBI 0640300, and CBET 1058502, NSF DEB 102788 Baltimore LTER Site. The U.S. Environmental Protection Agency, through its Office of Research and Development, participated in the research described herein. It has been subjected to the Agency’s administrative review and has been approved for external publication. Any opinions expressed in this article are those of the authors and do not necessarily reflect the views of the Agency, therefore, no official endorsement should be inferred. Any mention of trade names or commercial products does not constitute endorsement or recommendation for use.
Pegasus Technical Services; Dynamac Corporation; EPA NNEMS, Grant/Award Number: 2010-309; NSF Graduate Research Fellowship, Grant/Award Number: DGE1144243; NSF, Grant/Award Number: DBI 0640300, CBET 1058502, DEB 102788
- buried streams
- dissolved organic matter fluorescence
- extracellular enzyme activity
- nutrient diffusing substrata