Abstract: Rivers process up to half of the terrestrial organic carbon before it reaches the oceans, playing an essential role in the global carbon cycle. However, little is known of the seasonal and regional patterns of riverine biological activity as most river ecosystem metabolism studies have been carried out at short spatial and temporal scales. We classify the annual river ecosystem metabolic regimes of 49 monitoring stations located in large Iberian rivers using a set of indices that summarize the annual variability of daily gross primary production (GPP) and ecosystem respiration (ER) estimates. Additionally, we uncover the key environmental drivers controlling these regimes using a multivariate ordination technique. We found three GPP and four ER regimes, differing in the magnitude of their values and seasonal extremes. The GPP and ER regimes were combined into four different river ecosystem metabolic regimes mainly controlled by an environmental gradient of land uses and topography. The least productive metabolic regime is associated with higher broadleaf forest coverage, while the most heterotrophic river ecosystem metabolic regime was associated with agricultural land uses, indicating that the food production system may be altering the carbon balance of river ecosystems. Our study uses both GPP and ER for river ecosystem metabolic regime classification, uncovering new metabolic rhythms and highlighting the importance of ER patterns in river metabolism characterization. This approach allows characterizing metabolic regimes across different environmental and anthropogenic gradients in different world regions, with applications in global carbon cycle estimation, river restoration, and climate change mitigation.

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