Methane and CO2 emissions from four Pacific NW US reservoirs with contrasting water management regimes

Dams and reservoirs are ubiquitous and widely used to improve human well-being, resulting in their rapid increase in number and influence. They are also major players in biogeochemical and ecosystem dynamics both at regional and global scales. One key unintended consequence of dam construction and operation is the increased emission of carbon-based greenhouse gases (GHGs) to the atmosphere. Recent synthesis efforts have suggested that collectively, at the global scale, reservoirs account for approximately 1.5% of all anthropogenic GHG emissions and 5% of anthropogenic methane (CH₄) emissions. These estimates are based on the best available information (i.e., in-situ flux estimates), but there is considerable uncertainty inherent in these estimates.  Reasons for this uncertainty include both substantial spatial and temporal flux variation, and a bias toward summer month measurements. Summer measurements often exclude ebullition fluxes, which can be an important pathway for CH₄ emissions.  Additionally, until recently, there has been a comparative lack of reservoir GHG flux estimates from temperate zone systems. This is because most of the effort to constrain these fluxes has occurred in tropical and boreal systems. Here we report results from a first complete year of bimonthly GHG flux measurements collected from 4 Columbia River Basin reservoirs undergoing contrasting management regimes (2 run of river reservoirs and 2 storage reservoirs).  We employed low-cost, CH₄ and CO₂ sensors fitted to floating flux chambers to collect more than 400 individual flux estimates from our study systems. There was considerable spatial and seasonal variability in GHG fluxes both within and among systems.  Despite this variability, surface CH₄ concentrations were consistently supersaturated, even in cold oligotrophic conditions. All study reservoirs were net CH₄ emitters year-round (CH₄ flux rates 1-140 mg CH₄-C m^-2 d^-1), with highest fluxes during summer and lowest fluxes during winter.  In contrast, CO₂ fluxes did not demonstrate a consistent seasonal pattern and appeared to be more strongly influenced by surface chlorophyll a (Chl a) than water temperatures, with negative CO₂ fluxes observed during times (and in systems) with high surface Chl a.

Primary Presenter: John Harrison, Washington State University (john_harrison@wsu.edu)

Authors:

David Ballenger, Washington State University, Vancouver  (david.ballenger@wsu.edu)

Skyler Flaska, Washington State University, Vancouver  (skyler.flaska@wsu.edu)