Urban Stream Burial Increases Watershed-Scale Nitrate Export

Nitrogen (N) uptake in streams is an important ecosystem service that reduces nutrient loading to downstream ecosystems. Here we synthesize studies that investigated the effects of urban stream burial on N-uptake in two metropolitan areas and use simulation modeling to scale our measurements to the broader watershed scale. We report that nitrate travels on average 18 times farther downstream in buried than in open streams before being removed from the water column, indicating that burial substantially reduces N uptake in streams. Simulation modeling suggests that as burial expands throughout a river network, N uptake rates increase in the remaining open reaches which somewhat offsets reduced N uptake in buried reaches. This is particularly true at low levels of stream burial. At higher levels of stream burial, however, open reaches become rare and cumulative N uptake across all open reaches in the watershed rapidly declines. As a result, watershed-scale N export increases slowly at low levels of stream burial, after which increases in export become more pronounced. Stream burial in the lower, more urbanized portions of the watershed had a greater effect on N export than an equivalent amount of stream burial in the upper watershed. We suggest that stream daylighting (i.e., uncovering buried streams) can increase watershed-scale N retention

Effects of urban stream burial on organic matter dynamics and reach scale nitrate retention

Nitrogen (N) retention in streams is an important ecosystem service that may be affected by the widespread burial of streams in stormwater pipes in urban watersheds. We predicted that stream burial suppresses the capacity of streams to retain nitrate (NO3 −) by eliminating primary production, reducing respiration rates and organic matter availability, and increasing specific discharge. We tested these predictions by measuring whole-stream NO3 − removal rates using 15NO3 − isotope tracer releases in paired buried and open reaches in three streams in Cincinnati, Ohio (USA) during four seasons. Nitrate uptake lengths were 29 times greater in buried than open reaches, indicating that buried reaches were less effective at retaining NO3 − than open reaches. Burial suppressed NO3 − retention through a combination of hydrological and biological processes. The channel shape of two of the buried reaches increased specific discharge which enhanced NO3 − transport from the channel, highlighting the relationship between urban infrastructure and ecosystem function. Uptake lengths in the buried reaches were further lengthened by low stream biological NO3 − demand, as indicated by NO3 − uptake velocities 17-fold lower than that of the open reaches. We also observed differences in the periphyton enzyme activity between reaches, indicating that the effects of burial cascade from the microbial to the ecosystem scale. Our results suggest that stream restoration practices involving “daylighting” buried streams have the potential to increase N retention. Further work is needed to elucidate the impacts of stream burial on ecosystem functions at the larger stream network scale

Urban infrastructure influences dissolved organic matter quality and bacterial metabolism in an urban stream network

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

Box and whisker plots of nitrate uptake velocity (ʋ_f) in the buried and open reaches in Cincinnati, Ohio and Baltimore, Maryland, as reported in Beaulieu et al. [20] and Pennino et al. [21].

<p>Literature data were derived from a recent survey of 72 streams spanning several biomes and land-use conditions [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0132256#pone.0132256.ref016" target="_blank">16</a>]. Plots display 10^th, 25^th, 50^th, 75^th, and 90^th percentiles and individual data points outside the 10^th and 90^th percentiles. Nitrate uptake velocity was 13 times greater in open than buried reaches (p<0.001, paired <i>t</i>-test).</p

Results of simulation scenarios involving an even distribution of burial across the watershed with incremental increases of 5%.

<p>The primary y-axis and solid line represent the average volumetric NO₃^- uptake rate among in the open reaches. The secondary y-axis and dashed line represent total NO₃^- uptake in the open reaches.</p

Stream burial is an extreme, but ubiquitous, consequence of urbanization in stream ecosystems.

<p>The buried stream channels in the cited studies were constructed from various materials including (a) a cement-lined corrugated metal pipe in Baltimore, Maryland (USA), (b) a concrete tunnel in Cincinnati, Ohio (USA), and (c) a corrugated metal pipe in Cincinnati.</p

Percent change in nitrate export in response to stream burial simulation scenarios.

<p>The simulation scenarios involve an even distribution of burial across the watershed with incremental increases of 5% and include: 1) Allowing both uptake rate constants and water velocities to change in response to burial (Combined response); 2) Allowing water velocity to change following burial, but holding uptake rate constants at open reach values; and 3) Allowing uptake rate constants to change following burial, but holding water velocities at open reach values.</p