Compositional differences of chromophoric dissolved organic matter derived from phytoplankton and macrophytes

Abstract

Chromophoric dissolved organic matter (CDOM) is an important component in the aquatic environment and plays a key role in light attenuation and in carbon biogeochemical cycles. We examined CDOM production in each of two laboratory experiments in which phytoplankton and macrophyte degradation were monitored using absorption and excitation–emission matrix fluorescence spectroscopy (EEMs). During the incubation period, CDOM was produced from phytoplankton and macrophytes, and partly decomposed by microorganisms. The absorption spectra of the phytoplankton derived and the macrophyte derived CDOM were distinct and characterized by peaks and shoulders in the UV bands. Production of CDOM absorption at 350nm, a(350), was 0.0125m2/g per unit of chlorophyll a from phytoplankton CDOM from 0–3d. Meanwhile a(350) production was 2.708×10−4m2/g per unit of wet biomass from macrophytes CDOM from 1–7d. Despite the high production of CDOM by phytoplankton and macrophytes, extrapolation of these values to the field indicated that about 15% of total CDOM was produced from phytoplankton during algal blooms in Meiliang Bay in summer and about 8% of total CDOM was produced from macrophytes in the macrophyte dominated littorals. The mean value of the spectral slope (S) describing the exponential decrease of the absorption spectrum, which was strongly correlated to an optical index of molecular size, for the phytoplankton derived CDOM was 10.26±2.05μm−1, which was significantly lower than the mean S of 14.47±2.88μm−1 for the macrophyte derived CDOM (t-test, p<0.001). The mean value of the spectral slope ratio (SR) for the phytoplankton derived CDOM was 1.79±0.52, which was significantly higher than that of 0.35±0.58 for the macrophyte derived CDOM (t-test, p<0.001). Three fluorescent components were validated in parallel factor analysis (PARAFAC) models calculated separately for phytoplankton derived and macrophyte derived CDOM, each CDOM source resulting in distinct excitation and emission maxima for each component. The significant differences in CDOM absorption spectra, S, SR and PARAFAC fluorescence component characteristics, all showed that phytoplankton derived CDOM was compositionally distinct from macrophyte derived CDOM. Overall both sources were important to the CDOM pool in the shallow temperate lake.