Watershed DOC uptake occurs mostly in lakes in the summer and in rivers in the winter: The CUPS-OF-DOC model

River networks transport dissolved organic carbon (DOC) from terrestrial
uplands to the coastal ocean. The extent to which a reach or lake within a
river network uptakes DOC depends on the stream order, the seasonal
conditions, and the flow. At the watershed scale, it remains unclear
whether DOC uptake is dominated by biological processes such as
respiration, or abiotic processes like photomineralization. The
partitioning of DOC uptake in lakes versus rivers is also unclear. In this
study, we present a new model that unifies year-round controls on DOC
cycling for an entire river network, including river-lake connectivity, to
elucidate the importance of biotic vs. abiotic controls on DOC uptake. We
present the Catchment Uptake and Sinks by Season, Order, and Flow for DOC
(CUPS-OF-DOC) model, which quantifies terrestrial DOC loading, gross
primary productivity (GPP), and uptake via microbes and
photomineralization. The model is applied to the Connecticut River
Watershed and accounts for cascading reach- and lake-scale DOC cycling
across ninety-eight scenarios spanning combinations of flows, seasons, and
stream orders. We show that riverine DOC uptake is nearly constant with
stream order, but the proportion of DOC uptake from photomineralization
varies. Photomineralization dominates in rivers in most flow conditions
and stream orders, especially in winter, accounting for at least half of
whole-watershed DOC uptake in February across all flows. Whole-watershed
summer DOC uptake occurs mostly via biomineralization in lakes, accounting
for 80% of DOC uptake during the growing season, despite accounting for
less than 6% of watershed open water surface area.