Testing the effects of coastal culture on particulate organic matter using absorption and fluorescence spectroscopy

Abstract

Aquaculture is a major type of human activity that disturbs the biogeochemistry and ecosystem in coastal environments, yet its influence on the optical properties of particulate organic matter remains largely unknown. This was studied for the Aojiang Estuary-cultural zone-adjacent seawater continuum in SE China in July and October 2019, and in January 2020, using absorption spectroscopy and fluorescence excitation-emission matrices-parallel factor analysis (EEMs-PARAFAC). The POM absorption, three humic-like components (C1, C2, and C5), and humification index (HIX) generally decreased from the river to the seawater and were higher in October than in January. The humic substances were mainly derived from terrestrial sources and sediment resuspension, which were dependent largely on the level of total suspended matter (r: 0.823–0.888). The intensity of shoulder peak in the absorption spectra and the tryptophan-like C4 were lower in the river than in seawater in July, which was in contrast to the spatial distribution in other months. They correlated positively with Chl a (r: 0.779–0.912), indicating they were mainly related to the algal production. Principal component analysis (PCA) revealed notable seasonal variations of POM with an elevated autochthonous contribution in July, indicating stronger algal production than the removal by shellfish. The low autochthonous POM in October and January identified with PCA suggested limited addition from the laver culture after the shellfish harvest. The mooring monitoring in July and October further demonstrated that the POM in the cultural zone and adjacent seawater was influenced by the active material exchange across the continuum, strong tides, sediment resuspension, algal production, and microbial transformation. Our results have implications for assessing the influences of aquaculture activities on the water quality and ecosystem in the coastal environments.