Seasonal pattern of diel variability of CO2 efflux from a large eutrophic lake

Lake is commonly acknowledged as a contributor to atmospheric CO₂. Current manual sampling for estimations of CO₂ emissions from lakes predominantly relies on daytime CO₂ efflux (FCO₂) assessments, which tends to overlook the diel variability of FCO₂. This potentially introduces bias into CO₂ emission estimates. The magnitude of diel FCO₂ difference between seasons and the relative importance of underlying drivers in large eutrophic lakes remain inadequately explored. Here, we estimated FCO₂ based on the water quality and meteorological data from Lake Taihu, a large eutrophic lake in China, with a temporal resolution of 4 h throughout the year 2021. The lake was a CO₂ source with an efflux of 0.56 ± 0.66 mmol C/m²/h. We observed a 14.07 % increase in nocturnal FCO₂ compared to daytime levels annually. During the non-growing season, nocturnal FCO₂ exceeds daytime levels by 12.72 %, rising to 39.84 % in the algae-growing season (April to September). Piecewise structural equation models highlight gas transfer velocity as a key driver of diel FCO₂ changes, with seasonal algal growth intensifying diel CO₂ partial pressure variability by enhancing gross primary production, thereby amplifying diel FCO₂ fluctuations. We suggest that ongoing lake eutrophication, driven by global climate change and human activities, may introduce additional uncertainties in lake CO₂ emission estimates.