Groundwater-surface water interaction and its mediated biogeochemical processes in the large shallow eutrophic Lake Taihu

Abstract

Eutrophication and harmful algal blooms (HABs) are global threats to lakes, especially for shallow lakes (with a depth < 5m). Their occurrences are governed by various biogeochemical, climatological and anthropogenic factors. Groundwater may play a key role in controlling HABs, but the underlying mechanisms remain unclear. Basin-wide field data of Lake Taihu—China’s largest eutrophic lake—and global archives were leveraged, and the dominance of evaporation on lacustrine groundwater discharge (LGD) in shallow lakes was demonstrated. By extrapolating decadal LGD and the derived nutrients, HABs were found to promptly consume ubiquitous groundwater-borne nutrients, leading lake water N:P ratios to be 2 - 3 months lagged behind LGD N:P ratios. It is concluded that evaporation dominated LGD is an unraveled but crucial regulator of nutrient states and HABs in shallow lakes, advocating the need for synergistical studies from both climatological and hydrogeological perspectives when restoring lake ecosystems. Eutrophic shallow lakes have received growing attention due to the emissions of greenhouse gases (GHGs: CO2, CH4 and N2O) globally. To delineate the synergy of LGD and trophic level on CH4 and CO2 pathways and emissions, the continuous monitoring of radon-222, nutrients, CH4, CO2 and δ13C was conducted at two locations in Lake Taihu with distinctive trophic levels. LGD significantly regulates N:P ratios and delivers a substantial amount of CH4 and CO2 into the lake. CH4 and CO2 productions in the lake are dominated by organic matter degradation, which is more intensive under the highly eutrophic state. As the trophic level increases, CH4 ebullition and CO2 diffusion are amplified, and the processes of CH4 oxidation and atmospheric CO2 exchange within the lake are attenuated. LGD contributions to gaseous carbon inventories are also dampened by the enhanced productions from organic matter degradation. To further study the LGD-regulated biogeochemical processes related to GHG effluent in the eutrophic shallow lake, the method of multiple stable isotopes (δ2H, δ18O, δ13C, and δ15N) was applied. Results reveal that LGD is the predominant source of additional carbon into the lake. Denitrification controls nitrogen transformation in the lake, and the process can be intensified with the increasing of LGD derived nitrogen. LGD serves as not only a source of carbon and nitrogen but also a regulator of related biogeochemical processes, which may accelerate GHG productions and emissions in the eutrophic lake. The study highlights LGD regulation on the carbon and nitrogen cycles and suggests a comprehensive understanding from both ecological and hydrogeological viewpoints in global warming. Radium-224 and thorium-228 disequilibrium in sediments in the lakebed was used to quantify benthic fluxes and nutrient transport across the sediment–water interface in Lake Taihu. Benthic fluxes are found to be dominated by bioturbation, while these internal nutrient cyclings are largely controlled by the nutrient states of overlying water, which are predominantly regulated by external nutrient loadings in the eutrophic lake. This study expands radium application in fresh aquatic systems, and is instructive for water remediations and ecosystem restorations in Lake Taihu and other large eutrophic lakes elsewhere.