Inflow-modulated inputs of dissolved organic matter fuel carbon dioxide emissions from a large hyper-eutrophic lake

Abstract

Terrestrial dissolved organic matter (DOM) is potentially reactive and, upon entering lake ecosystems, can be readily degraded to low-molecular-weight organic matter and dissolved CO₂. However, to date, there has been limited research on the links between long-term variation in the composition of DOM and CO₂ emissions from lakes. Lake Taihu is a large, shallow, and hyper-eutrophic lake where DOM composition is strongly influenced by inputs from the rivers draining cultivated and urbanized landscapes. This study aims to investigate the impact of water chemistry and DOM characteristics, modulated by inflow-discharge dynamics, on the variability of pCO₂ in Lake Taihu. Based on long-term seasonal observations from 2000 to 2022, we estimated the annual areal average CO₂ flux (FCO₂) to be 470 ±107 gCO₂ m^-2 yr^-1 corresponding to a summed flux of 1.1 ± 0.3 TgCO₂ yr^-1 from the lake. Elevated levels of FCO₂ were found in 2004-2007, and across the dataset, the highest FCO₂ was often observed in February and the lowest in August, corresponding with higher algal uptake in summer. Northwestern inflowing riverine DOM, consisting of both soil leachate and rapidly degradable domestic wastewater along with the input of riverine CO₂ itself, explained the elevated FCO₂ in the northwestern lake regions. Ultra-high resolution mass spectrometry (FT-ICR MS) can resolve the composition of DOM at the molecular level, revealing that CO₂ concentrations vary with the relative abundance of terrestrially derived condensed aromatics. Incubation experiments further confirmed the link between the degradation of biologically labile DOM and the aerobic CO₂ production. We conclude that riverine organic matter inputs and subsequent degradation, modulated by inflow discharge, play a key role in shaping CO₂ emissions from Lake Taihu over the past two decades.