Substantial increase of organic carbon storage in Chinese lakes

Lakes play a crucial role in the global carbon cycle, acting as recipients, regulators, reactors, and storages of carbon. Globally, inland waters receive a substantial amount of carbon from terrestrial ecosystems, with a portion buried in sediments, emitted as CO2 to the atmosphere, and exported to oceans. The remaining carbon is stored in lake waters, either organically or inorganically. Total organic carbon (OC) storage, the product of OC concentration and water volume, is particularly important, influencing processes such as phytoplankton photosynthesis, dissolved oxygen consumption, and lake carbon burial and outgassing. Despite its significance, large-scale research on changes in lake OC storage has been limited, with previous studies often assuming constant OC storage. However, OC storage is dynamic, influenced by factors such as phytoplankton growth, water volume, and sewage discharge. In recent decades, global lakes have undergone significant changes due to climate warming and intensified human activities, leading to increased algal blooms and changes in water volume. China, with its diverse limnetic zones and varying climatic conditions and human activities, provides an ideal setting to explore spatiotemporal variations in lake OC storage. This study uses extensive field data and satellite monitoring to investigate these changes across 24,366 Chinese lakes during 1984-2023, aiming to provide valuable insights into the global carbon cycle and the role of lakes in carbon sequestration.

Existing literature highlights the multifaceted role of lakes in the global carbon cycle. Studies have focused on OC production and mineralization, spatiotemporal changes in OC concentration and composition, and carbon balance estimations. However, a significant gap exists in understanding the large-scale changes in lake OC storage, with previous research often simplifying the process by assuming constant storage. While some research has examined OC dynamics in specific lakes or regions, a comprehensive, large-scale assessment of spatiotemporal variations in OC storage across diverse lake types is lacking. Studies on algal blooms and water volume changes in global lakes provide context, but a direct link to OC storage dynamics across a wide range of lake characteristics remains largely unexplored. The current study addresses this gap by providing a comprehensive analysis of OC storage changes in Chinese lakes, a region characterized by diverse climatic and anthropogenic influences.

This study utilized a combination of in-situ data and remote sensing techniques to estimate DOC and POC concentrations and subsequently calculate OC storage in 24,366 Chinese lakes larger than 0.01 km² during 1984-2023. In-situ data collected between 2004 and 2023 included DOC, DIC, POC, chlorophyll-a (Chl-a), total suspended matter (TSM), pH, temperature, and conductivity measurements from 4,201 stations in 348 lakes. Publicly available datasets were also used, including daily surface reflectance from Landsat 5/7/8/9 satellites, daily water storage from GloLakes, lake polygon/area/depth and river network/basin data from HydroSHEDS, and monthly averaged basin property data (population density, DEM, evaporation, LAI, runoff, temperature, precipitation, and wind speed) from ECMWF. The study developed algorithms to estimate DOC and POC concentrations from Landsat reflectance data. For DOC, a multilayer back propagation neural network (MBPNN) was trained to simulate lake-based annual mean DOC concentration using nine basin properties. Two Random Forest models were then built to retrieve DOC concentrations in freshwater and saline lakes, respectively. The POC retrieval algorithm involved constructing four empirical equations to roughly estimate POC concentration using DEM, longitude, red band reflectance (Rred), and normalized difference carbon index (NDCI), followed by training a Random Forest model for precise retrieval using these estimates and other relevant parameters. The accuracy of the models was assessed using MAPD values. Annual mean DOC and POC concentrations were calculated using satellite-derived daily data, and OC storage was calculated by multiplying concentrations by water stocks from HydroLAKES and GloLakes. Lake CO2 concentration was calculated using in-situ pH, temperature, and DIC data and the carbonate equilibrium method. Statistical analyses, including Random Forest analyses and regression analyses, were performed to determine the relationships between variables and the temporal trends.

The study revealed substantial spatiotemporal variations in OC concentration and storage across Chinese lakes. DOC concentrations were high in northwest saline lakes and low in southeast regions, showing general increasing trends. POC concentrations showed the opposite pattern, with high values in southeast eutrophic lakes and low values in the northwest, also exhibiting increasing trends. Overall, mean DOC concentration increased from 14.79 ± 12.49 mg L⁻¹ in 1984 to 18.14 ± 21.75 mg L⁻¹ in 2023, while mean POC concentration also increased significantly. Dissolved OC storage increased by 44.6%, and particulate OC storage increased by 33.5% during 1984-2023. Intensified human activities, particularly population density increase, were major drivers of POC changes, while water input (precipitation, runoff, evaporation) and wind speed were crucial factors influencing DOC changes. Increased OC storage led to an 11.0% increase in nationwide OC burial and a decrease in carbon emissions from 71.1% of northwest lakes. The relationship between in-situ CO2 concentration and the DOC/POC ratio was also investigated.

The findings highlight the significant role of Chinese lakes as carbon sinks, with substantial increases in OC storage driven by a complex interplay of anthropogenic and natural factors. The increased OC storage, particularly POC, directly contributed to higher OC burial rates. The decrease in CO2 emissions from many northwest lakes suggests that lake expansion, coupled with reduced salinity and increased nutrient inputs, enhanced CO2 absorption through algal proliferation. The study emphasizes the importance of considering the dynamic nature of OC storage in carbon cycle assessments and suggests that similar changes may be occurring globally, with implications for carbon sequestration. The positive correlation between POC concentration and OC accumulation rate indicates that increased algal biomass, largely due to eutrophication, enhances carbon sequestration. However, the impact on CO2 emissions is complex and varies depending on factors such as lake location and eutrophication status. This variability introduces uncertainty in global carbon emission estimates.

This study demonstrates a substantial increase in organic carbon storage in Chinese lakes over the past four decades, driven by both anthropogenic and natural factors. The findings highlight the significant role of lakes in carbon sequestration and the importance of considering the dynamic nature of OC storage in regional and global carbon cycle assessments. Further research should focus on the global extent of these changes and improve the ability to estimate OC storage and carbon fluxes using global revisit satellite data. Future studies could explore the long-term effects of climate change and human activities on lake carbon dynamics, and refine models for predicting future carbon sequestration potential in lakes.

The study's reliance on satellite data introduces potential limitations related to cloud cover and the accuracy of remote sensing estimations. In-situ data were collected across a subset of lakes, potentially leading to some sampling bias. The models used for DOC and POC retrieval were developed based on Chinese lake characteristics; their applicability to lakes in other regions may need further validation. The long-term impacts of climate change and human activities on lake carbon dynamics require further investigation. Finally, uncertainties remain in extrapolating the findings from Chinese lakes to a global scale.