Unraveling the metabolic balance of boreal rivers
While boreal rivers are recognized to be active sites of biogeochemical transformation and emissions throughout the aquatic continuum, estimates of in-situ organic matter production via photosynthesis (gross primary production - GPP) and consumption via ecosystem respiration (ER) are still scarce from northern rivers, which limit our understanding on the relative role of internally and externally derived CO2 in supporting river emissions. Here we present preliminary results of a large-scale study of ecosystem metabolism across a wide range of boreal rivers. We deployed optical dissolved oxygen and temperature loggers at 12 large and medium rivers draining the James Bay region of northern Quebec, over a 4 month-period (June-October 2019), and then modelled GPP and ER using the streamMetabolizer R package. A special effort was employed to install these probes at the same location as the pre-established hydrometric stations and thus generate coupled estimates of discharge and metabolic rates. Our preliminary results show much lower GPP than ER daily rates for all rivers, confirming that riverine metabolism is consistently net heterotrophic and a net source of CO2 fuelling river supersaturation. These metabolic data will be combined with ambient gas fluxes to determine the balance between external and internal sources sustaining CO2 emissions in these rivers, and the role of GPP in modulating these fluxes. This study stands out as an unprecedented opportunity to unravel the seasonal dynamics and landscape drivers of metabolism and CO2 flux in boreal rivers.
Michaela de Melo et Paul del Giorgio
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