Air-water CO₂ exchange in transformed saltmarshes for different uses and under various management models

The rise of atmospheric carbon dioxide (CO₂) caused by anthropogenic activities is accelerating global warming. Assessing the capacity of natural ecosystems to act as sources or sinks for atmospheric CO₂ is fundamental to elaborate CO₂ global budgets and to establish climate change mitigation strategies. This work analyses the air-water exchange of CO₂ in saltmarshes of the Bay of Cadiz (SW Spain), which have been transformed for different uses and commercial practises and are managed distinctly. Specifically, the study examines four kind of saltmarsh systems (an artisanal saltpan, two industrial saltpans, and two aquaculture farms), with different management strategies based on their exploitation. These systems were sampled during five seasonal field campaigns conducted between 2020 and 2021. Untransformed tidal inundated marshes were also considered as control points for comparative purposes. The study sought to quantify the biogeochemical parameters that influence CO₂ exchange, such as water temperature, total alkalinity, pH, salinity, chlorophyll-a, dissolved organic carbon, dissolved oxygen and inorganic nutrients, were performed. In addition, unmanned aerial vehicle (UAV) flights and satellite imagery were used to determine the surface area of the transformed marshes and estimate the contribution of each system to the net CO₂ fluxes between the aquatic ecosystems and the atmosphere. The study's objectives were to evaluate the seasonal variations in CO₂ fluxes, assess the role of different saltmarsh systems as sources or sinks of CO₂, and determine the impact of management practices on carbon exchange. According to the results, aquaculture farms and water storage ponds in saltpans for salt extraction, acted as sources of CO₂ to the atmosphere during spring and summer months, switching to mild CO₂ sinks during winter and colder fall periods. In contrast, extremely hypersaline ponds for brine production and the subsequent salt extraction in both artisanal and industrial installations acted predominantly as atmospheric CO₂ sinks. Notably, extensive industrial saltpans contribute markedly to atmospheric CO₂ withdrawal, with an average annual CO₂ influx of -437.97 tCO₂·yr^-1. Hence, this finding underscores the critical role of these practices, which are widespread in the region, in providing important ecosystem services related to climate regulation.