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

Abstract

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.