Diurnal and seasonal variations of pCO2 and fluorescent dissolved organic matter (FDOM) in different polluted lakes

Abstract

This study aimed to assess pollution and daily-to-seasonal dynamics of the partial pressure of CO₂ (pCO₂) and CO₂ degassing flux concerning the fluorescent dissolved organic matter (FDOM) from tropical lakes. A membrane-enclosed pCO₂ sensor and water quality multimeter analyzer was deployed to continuously record daily and seasonal variations in pCO₂ and CO₂ degassing flux in three lakes in Savar, Dhaka. During both wet and dry seasons, all lake water was supersaturated with CO₂ in contrast to the atmospheric equilibrium (~400 μatm). The pCO₂ values in the lake water during the dry season were relatively low in comparison, and the pCO₂ levels in the wet season were much higher due to external inputs of organic matter from watersheds and direct inputs of CO₂ from soils or wetlands. The estimated water-to-air CO₂ degassing flux in the different levels of polluted lakes varies with the pollution context. Study areas calculated the carbon flux and three lakes released respectively 86.75×10⁷g CO₂ year^-1, 13.8×10⁷g CO₂ year^-1, and 9.17×10⁷g CO₂ year^-1. Three-dimensional excitation-emission matrix (3D-EEM) fluorescence spectroscopy combined with parallel factor (PARAFAC) analysis was used to investigate the distributions of fluorescent components in DOM. EEM-PARAFAC analysis identified humic-like, fulvic-like, protein-like, and more tyrosine-like FDOM components and their environmental dynamics. Terrestrial DOM may provide inputs to the terrestrial humic-like component in the lake water. In contrast, the biological activity of plankton-derived FDOM is the most likely source for the autochthonous humic-like component. FDOM and DO concentrations have negative correlations with pCO₂, indicating that when the FDOM and DO level is decreased, the amount of pCO₂ values increases.

Keywords: Carbon flux; Degassing flux; Dissolved organic matter (DOM); PARAFAC; Pollution; Water quality; pCO2.