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River hydrology shapes the sources, concentration, and stoichiometry of organic matter within drainage basins. However, our understanding of how the microbes process dissolved organic matter (DOM) and recycle nutrients in tropical rivers needs to be improved. This study explores the relationships between elemental DOM composition (carbon/nitrogen/phosphorus: C/N/P), C and N uptake, and C mineralization by autochthonous bacterioplankton in the Usumacinta River, one of the most important fluvial systems in Mexico. Our study investigated changes in the composition and concentration of DOM and evaluated carbon dioxide (CO2)production rates (C-CO2) through laboratory experiments. We compared three sites representing the middle and lower river basins, including their transitional zones, during the rainy and dry seasons. After incubation (120 h at 25°C), the DOM decreased between 25% and 89% of C content. Notably, the initial high proportion of C in DOM in samples from the middle-forested zone and the transition led to elevated C-CO2 rates (>10 mg l-1 day-1), in contrast to the lower initial C proportion and subsequent C-CO2 rates (<7 mg l-1 day-1) in the lower river basin. We also found that dissolved organic carbon uptake and NO3- and NH4+ production were higher during the dry season than in the rainy season. The low water flow in the river during the dry season accentuated the differences in elemental composition and microbial processing of DOM among the sites, while the high water flow of the rainy season homogenized these factors. Our findings indicate that microbial metabolism operates with reduced efficiency in C-rich environments like forests, particularly when faced with high C/N and C/P ratios in DOM. This study highlights the influence of the tropical hydrological regime (rainy and dry seasons) and the longitudinal changes in the river basin (middle and lower) topography and land cover on microbial metabolism by constraining DOM characteristics, emphasizing the crucial role of elemental ratios in river DOM processing., Competing Interests: The authors have declared that no competing interests exist., (Copyright: © 2024 Cuevas-Lara et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.)

Ecological systems may occur in alternative states that differ in ecological structures, functions and processes. Resilience is the measure of disturbance an ecological system can absorb before changing states. However, how the intrinsic structures and processes of systems that characterize their states affects their resilience remains unclear. We analyzed time series of phytoplankton communities at three sites in a floodplain in central Spain to assess the dominant frequencies or "temporal scales" in community dynamics and compared the patterns between a wet and a dry alternative state. The identified frequencies and cross-scale structures are expected to arise from positive feedbacks that are thought to reinforce processes in alternative states of ecological systems and regulate emergent phenomena such as resilience. Our analyses show a higher species richness and diversity but lower evenness in the dry state. Time series modeling revealed a decrease in the importance of short-term variability in the communities, suggesting that community dynamics slowed down in the dry relative to the wet state. The number of temporal scales at which community dynamics manifested, and the explanatory power of time series models, was lower in the dry state. The higher diversity, reduced number of temporal scales and the lower explanatory power of time series models suggest that species dynamics tended to be more stochastic in the dry state. From a resilience perspective our results highlight a paradox: increasing species richness may not necessarily enhance resilience. The loss of cross-scale structure (i.e. the lower number of temporal scales) in community dynamics across sites suggests that resilience erodes during drought. Phytoplankton communities in the dry state are therefore likely less resilient than in the wet state. Our case study demonstrates the potential of time series modeling to assess attributes that mediate resilience. The approach is useful for assessing resilience of alternative states across ecological and other complex systems.