Urban aquatic systems: Carbon source or carbon sink?

Background/Question/Methods . Urbanized environments are becoming an increasingly important component of many landscapes in populated regions of the world. The construction of stormwater management ponds (SWMP) is a common method to collect and store urban runoff and ultimately reduce the downstream movement of urban pollution. While the number of SWMPs is increasing in many urban landscapes, we know very little about the carbon biogeochemistry of these ecosystems, particularly their potential to release greenhouse gases.  The environmental conditions typical of these ecosystems (high microbial activity, elevated sediment and nutrient loads and retention, shallow and stratified with periods of oxygen depletion) would make them conducive to the production of significant source of greenhouse gases. We sampled 17 SWMP in two municipalities in Ontario, Canada, to assess their potential to produce carbon greenhouse gases. At each site, we measured water column concentrations of dissolved CO₂, CH_4, and DOC, along with dissolved oxygen and temperature.

Results/Conclusions . In August, individual ponds were supersaturated with regards to atmospheric equivalents with dissolved CO₂ concentrations ranging from 0.3 to 82 g C-CO₂ m^-3 and dissolved CH₄ concentrations ranging from ~0 to 0.46 g C-CH₄ m^-3 between sites.  In November,  a subset of ponds were found to have dissolved gas concentrations at or below atmospheric equivalents, with dissolved CO₂ ranging from 0.04 to 0.23 g C-CO₂ m^-3 and dissolved CH₄ concentrations ranging from 0.0001 to 0.0005 g C-CH₄ m^-3.  Gas concentrations were often correlated with dissolved oxygen, temperature and dissolved organic carbon in the ponds.  Our results (showing C-saturation) suggest that urban stormwater ponds have the potential to contribute to global warming by producing and releasing considerable quantities of greenhouse gases.