About this Research Topic
Understanding the processes and mechanisms controlling carbon biogeochemistry in aquatic ecosystems under the influence of human activities and climate change is essential in global change studies and Earth System Models. New boundaries with steep gradients in aquatic systems, created in the Anthropocene, are providing corridors for rapid biogeochemical and organismal change. The boundaries along the source to sea aquatic continuum that have been altered most dramatically in the Anthropocene are aquatic critical zones (ACZs). ACZs are defined here as the interfaces where crucial geological, chemical, biological and physical processes operate together to sustain systematic functionalities of aquatic systems. Natural ACZs have always existed, but anthropogenic hydrological changes have resulted in alterations of their spatio-temporal patterns. ACZs are characterized by steep concentration gradients in abiotic and biotic-derived dissolved and/or particulate constituents that significantly alter elemental cycling from micro to mesoscales. Here, we posit that the global impact of human activities on carbon cycling along the aquatic continuum will be best understood by focusing on ACZs (e.g., fjords, lakes, reservoirs, river reaches below dams, river confluences, river plumes, riverine floodplains, irrigation ditches, wetlands, mobile mud belts, hypoxic and harmful algal bloom (HAB) regions, estuarine turbidity maximum zones).
Continental weathering and erosion, which involves the land to ocean fluvial transport of inorganic carbon and other elements, subsequent carbonate deposition and silicate diagenesis (reverse weathering) in marine environments, have been shown to play a key role in climate regulation. Meanwhile, extensive sediment dynamics in coastal ACZs can enhance remineralization of organic carbon (OC) in sediments and thus increase reverse weathering. While carbon cycling from weathering to reverse weathering in ACZs has been acknowledged as an important component of the global carbon cycle, it remains poorly understood.
Most importantly, carbon composition and transport behaviour in major rivers have changed greatly due to anthropogenic and climate effects over the past decades. For example, global estimates on the erosion and transport of higher plant detritus and soil carbon from land to rivers via surface water runoff, are highly variable due in part, to spatial heterogeneity of land-use change, deforestation, water and soil conservation, droughts, and storms. Similarly, due to anthropogenic drivers (e.g., eutrophication), the production of autochthonous OC has dramatically increased in rivers, lakes, reservoirs, estuaries, and marginal seas, where OC processing rates are unusually high. We posit that understanding the control of water residence time and decay kinetics on carbon cycling in these ACZs is key to improving predictions by Earth System Models.
The objective of this Research Topic is to integrate our understanding of the production, consumption, transport and transformation of different carbon species from weathering to reverse-weathering in ACZs along the aquatic continuum. Emphasis will be placed on studies that cover a broad range of spatial-temporal data, from molecular level to regional and global scale, and from modern processes to historical records. While manuscripts focused on the spatial-temporal evolution of carbon cycling in ACZs is particularly encouraged, we also welcome studies that attempt to understand the importance of steep elemental (inorganic and organic) concentration gradients and processes in diverse habitats along the aquatic continuum.
Keywords: aquatic critical zones, chemical weathering, reverse-weathering, carbon cycling, climate change and human activity
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