Abstract
Streams and rivers form dense networks that drain the terrestrial landscape and are relevant for biodiversity dynamics, ecosystem functioning, and transport and transformation of carbon. Yet, resolving in both space and time gross primary production (GPP), ecosystem respiration (ER) and net ecosystem production (NEP) at the scale of entire stream networks has been elusive so far. Here, combining Random Forest (RF) with time series of sensor data in 12 reach sites, we predicted annual regimes of GPP, ER, and NEP in 292 individual stream reaches and disclosed properties emerging from the network they form. We further predicted available light and thermal regimes for the entire network and expanded the library of stream metabolism predictors. We found that the annual network-scale metabolism was heterotrophic yet with a clear peak of autotrophy in spring. In agreement with the River Continuum Concept, small headwaters and larger downstream reaches contributed 16% and 60%, respectively, to the annual network-scale GPP. Our results suggest that ER rather than GPP drives the metabolic stability at the network scale, which is likely attributable to the buffering function of the streambed for ER, while GPP is more susceptible to flow-induced disturbance and fluctuations in light availability. Furthermore, we found large terrestrial subsidies fueling ER, pointing to an unexpectedly high network-scale level of heterotrophy, otherwise masked by simply considering reach-scale NEP estimations. Our machine learning approach sheds new light on the spatiotemporal dynamics of ecosystem metabolism at the network scale, which is a prerequisite to integrate aquatic and terrestrial carbon cycling at relevant scales.
Highlights
Primary producers fix carbon dioxide (CO2) as organic carbon through photosynthesis, a flux known at the ecosystem level as gross primary production (GPP)
The River Continuum Concept has fundamentally shaped our conceptual understanding of how ecosystem metabolism may change along the longitudinal continuum from small streams to larger rivers downstream (Vannote and others 1980; Fisher and others 2004)
We have not been able to reliably predict stream ecosystem metabolism at the scale that is relevant to carbon cycling
Summary
Primary producers fix carbon dioxide (CO2) as organic carbon through photosynthesis, a flux known at the ecosystem level as gross primary production (GPP). Today we understand that the CO2 evasion fluxes from these ecosystems, including the smallest headwaters within the networks, are within the same range as ocean uptake fluxes of CO2, in the opposite direction (Raymond and others 2013; Drake and others 2018; Horgby and others 2019b; Rocher-Ros and others 2019). These fluxes are becoming increasingly better constrained, the relative importance of CO2 sources within the catchment versus those from instream metabolism to the emitted CO2 is still often unclear (Hotchkiss and others 2015; Duvert and others 2018; Horgby and others 2019a). The proper quantification of metabolic fluxes in streams and rivers networks, and this across spatial and temporal scales, is critical to pinpoint the role of these ecosystems in the global carbon cycle
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