Evaluating greenhouse gas efflux across a rural-urban gradient in the Hudson River Estuary
The tidal Hudson River Estuary (HRE) receives significant inputs of readily dissolvable carbon (C) and nitrogen (N) from incomplete wastewater treatment and sewer overflow associated with New York City (NYC) and other urban centers. These sewage inputs intensify during storm events, delivering large concentrated pulses of C and N to the estuary, which may alter the natural C cycle and significantly enhance greenhouse gas (GHG) emissions. In low oxygen estuarine ecosystems, microbial activity and subsequent GHG production is limited by the availability of degradable C. We hypothesize that increased sewage inputs will enhance GHG production in close proximity to urban centers resulting in increased GHG efflux from the rural to urban gradient found in the HRE. To test this hypothesis, carbon dioxide (CO2) and methane (CH4) efflux rates were quantified over ten research cruises. In addition, GHG concentrations were measured at Flushing Bay (NYC) following wet and dry weather from both shore locations and direct sewage input. GHG were extracted from water samples via syringe agitation, as well as directly measured by an infrared probe. Biogeochemical (organic C and inorganic N) and molecular (454 sequencing) analyses were also conducted to place GHG efflux values into the context of background nutrient concentrations and microbial community composition.
The Hudson River was found to be both a CO2 and CH4 source under all conditions. The greatest GHG effluxes (37 - 289 mg C m-2 day-1) were quantified at mid-channel sites in close proximity to NYC and other urban centers. Conversely, the lowest GHG effluxes (14.3 - 140 mg C m-2 day) were quantified at mid-channel sites in relatively undeveloped regions. Over ten cruises the average efflux rates established an urban-rural gradient. If climate warning potential is considered, the ratio of GHG efflux between urban: rural regions is driven primarily (77%) by CH4 emissions. Further, following precipitation events CH4 (but not CO2) efflux increased 2X in urban areas with no differences observed in rural regions. Increased mid-channel CH4 efflux was observed, associated with significant increases in CH4 efflux from urban tributaries that receive wastewater input and greater (6.6X) CH4 concentrations measured directly from sewage input compared to baseline waters. Overall, these data indicate that C additions associated with sewage input has the potential to significantly increase GHG emissions from the HRE. The magnitude of increased GHG emissions seen here can be used to determine the climate impact incomplete wastewater treatment may have in other urban estuarine systems.