Both natural and human-made inland waters are frequently affected by drying, leading to partial or complete desiccation, or even permanent disappearance. This drying can stem from natural hydrological factors (like seasonal fluctuations or desiccation of intermittent streams) or anthropogenic factors (such as agricultural diversions or reservoir fluctuations). Climate change and increased water abstraction are expected to worsen this issue. A significant portion of the world's streams and rivers are only temporarily flowing, and seasonal desiccation exposes large areas of previously submerged sediments to the atmosphere. These hydrologically dynamic environments are often excluded from inland aquatic carbon budgets, creating a potential blind spot in global carbon cycle estimates. The study defines dry inland waters as areas where surface water is absent and sediments are exposed to the atmosphere. Gaseous carbon emissions from inland waters are important in the global carbon cycle, and exposed sediments following desiccation can emit CO2 at higher rates than inundated periods. Initial estimates suggested that these emissions could be globally relevant, but these were based on limited localized studies. The urgent need to quantify global CO2 emissions from dry inland waters and understand their environmental regulators motivated this research. The study hypothesized that CO2 emissions from dry inland waters exceed reported mean aquatic rates, making them globally relevant, and that these emissions vary based on factors controlling CO2 production (organic matter, temperature, moisture) and gas transport (sediment texture) along with geographical properties influencing biogeochemical conditions. The aim was to determine if CO2 emissions from dry inland waters are closer to aquatic or terrestrial ecosystem values, to improve the accuracy of global CO2 emission upscaling models.

Previous research has highlighted the role of inland waters in the global carbon cycle, but the contribution of dry inland waters has been largely overlooked. Studies have shown that exposed sediments following desiccation emit CO2 at rates exceeding those observed from water surfaces during inundated periods. However, these initial findings were limited by the small number of localized studies. Existing global estimates of CO2 emissions from inland waters often excluded desiccated areas, leading to potentially significant underestimations. The lack of a comprehensive global dataset on CO2 emissions from dry inland waters hampered accurate upscaling to global estimates and a robust understanding of underlying drivers. This study aimed to address this gap by providing the first global survey of CO2 fluxes from these frequently overlooked ecosystems.

A global survey was conducted to quantify CO2 fluxes from 196 dry inland waters across all continents except Antarctica. Sites represented diverse inland water ecosystem types (rivers, lakes, reservoirs, and ponds) and climate zones (tropical, arid, temperate, continental, and polar). CO2 fluxes were measured using closed chamber methods, monitoring the CO2 concentration within the chamber over time. The CO2 flux was calculated using a linear regression of the change in CO2 partial pressure over time, accounting for chamber volume, surface area, temperature, and the ideal gas constant. In addition to CO2 flux, fourteen environmental variables were measured at each site, including air and sediment temperature, sediment texture, pH, conductivity, water content, and organic matter content. Climate zones were assigned to sites based on their location using the Köppen-Geiger climate classification system. Annual mean temperature and precipitation data were obtained from the WorldClim database. A linear mixed-effects model (LMM) was used to analyze the relationships between CO2 fluxes and environmental variables, accounting for both fixed and random effects. Data were log-transformed where necessary to meet the assumptions of normality and homogeneity of variance. The analysis was conducted using R software.

Sediment CO2 fluxes ranged from -27 to 2968 mmol m-2 d-1 (mean ± SD = 186 ± 326, median = 93, n = 196). Emissions from dry inland waters were an order of magnitude higher than average water surface emissions previously reported for lentic waters but lower than average emissions reported for lotic waters. Higher CO2 emissions from exposed sediments compared to lentic waters are likely due to a closer coupling of CO2 production and gas flux in dry sediments and increased CO2 production rates due to increased oxygen availability. CO2 emissions from dry inland waters were slightly lower but not significantly different than those from adjacent uphill soils. The study did not find significant differences in CO2 fluxes between climate zones, though this needs careful interpretation due to unbalanced sampling. CO2 emissions from all climate zones fell within the same range. However, CO2 emissions from temperate sites with dry winters were significantly lower than those from dry-summer or sites lacking dry seasons, indicating an interaction between temperature and moisture. All lentic ecosystem types showed higher CO2 emissions from dry sediments than globally estimated for their inundated stages. Ponds exhibited significantly higher CO2 emissions than streams and reservoirs, possibly due to high temperatures and a large perimeter-to-area ratio, leading to organic matter accumulation. Variation in CO2 fluxes was higher between sites than between climate zones or ecosystem types, indicating that local conditions outweigh geographical patterns. LMM modeling explained 39% of the total variance by fixed effects and 52% by the entire model. Organic matter content, moisture, temperature, and the interaction between organic matter and moisture were the strongest predictors of CO2 fluxes. Under low-moisture conditions, neither organic matter content nor temperature affected CO2 emissions. High moisture facilitates microbial activity but requires sufficient organic matter. Elevation, latitude, and conductivity also influenced CO2 fluxes, likely reflecting local geographical conditions not captured in the sampling design. A global upscaling of the measured CO2 emissions resulted in an estimate of 0.12 ± 0.13 Pg C y-1 from dry inland waters, equivalent to 6 ± 6% of currently estimated global C emissions from inland waters. This highlights the potential for significant underestimation of global inland water CO2 emissions when excluding desiccated areas.

This study provides compelling evidence that dry inland waters are significant and globally prevalent sources of CO2 to the atmosphere. Their contribution has been largely missing from global carbon budgets of inland waters. The consistent drivers of CO2 emissions across diverse ecosystems and climate zones suggest a universal control mechanism. The strong influence of organic matter, moisture, and temperature highlights the importance of biogeochemical processes in these systems. The higher CO2 emissions from dry inland waters compared to inundated lentic systems underscore the need to incorporate these areas into global carbon cycle assessments. The substantial variation between sites emphasizes the role of local conditions, highlighting the need for further research to understand the underlying mechanisms driving CO2 emissions and improve the accuracy of global upscaling models.

This global survey demonstrates that CO2 emissions from dry inland waters are substantial and consistently driven by factors related to organic matter availability, moisture, and temperature. Including these emissions significantly increases global estimates of carbon emissions from inland waters. The findings highlight the critical need to incorporate dry inland waters into future global carbon cycle assessments and emphasizes the importance of continued research to refine our understanding of these dynamic ecosystems and their contributions to the global carbon budget. Future research should investigate temporal and seasonal variability, link emissions to sediment organic matter loss, and assess the role of vegetation.

The study acknowledges limitations related to unbalanced sampling sizes across climate zones and the underrepresentation of polar sites. The observed relationships between environmental variables and CO2 fluxes might be influenced by unmeasured factors like organic matter quality, groundwater discharge, microbial community composition, and antecedent conditions such as time since desiccation. The upscaling of CO2 emissions to global estimates involves inherent uncertainties due to the variability between sites and potential underestimation of the global surface area of desiccated inland waters. Rewetting events, which can contribute significantly to CO2 fluxes, were not specifically included in the estimates.