Characterization of dissolved organic matter in a coral reef ecosystem subjected to anthropogenic pressures (La Réunion Island, Indian Ocean) using multi-dimensional fluorescence spectroscopy

Abstract

La Saline fringing reef is the most important coral reef complex of La Réunion Island (southwestern Indian Ocean; 21°07′S, 55°32′E). This ecosystem is subjected to anthropogenic pressures through river inputs and submarine groundwater discharge (SGD). The goal of this study was to characterize the pool of fluorescent dissolved organic matter (FDOM) in different water bodies of La Saline fringing reef ecosystem using excitation–emission matrix (EEM) spectrofluorometry. From EEMs, we identified the different fluorophores by the peak picking technique and determined two fluorescence indices issued from the literature: the humification index (HIX) and the biological index (BIX). The main known fluorophores were present within the sample set: humic-like A, humic-like C, marine humic-like M, tryptophan-like T1 and T2, and tyrosine-like B1 and B2. In some samples, unknown fluorophores (“U”) were also detected. The surface oceanic waters located beyond the reef front displayed a typical oligotrophic marine signature, with a dominance of autochthonous/biological material (presence of peaks: T1 > B1 > A > T2 > M > C; HIX: 0.9 ± 0.4; BIX: 2.3 ± 1.1). In the reef waters, the autochthonous/biological fingerprint also dominated even though the content in humic substances was higher (same relative distribution of peaks; HIX: 1.6 ± 0.6; BIX: 1.0 ± 0.1). Sedimentary and volcanic SGD showed very different patterns with a strong terrestrial source for the former (A > T1 > C > B1 and A > C > B1; HIX: 9.8 ± 2.0; BIX: 0.8 ± 0.0) and a weak terrestrial source for the latter (A > B1 > U3 > B2 > C and A > U4 > C; HIX: 2.4 ± 0.3; BIX: 0.9 ± 0.0). In the Hermitage River, both humic substances and protein-like material were abundant (T1 > A > U5 > B1 > C > B2; HIX: 2.3; BIX: 1.4). We provide evidences for the presence of anthropogenic DOM in some of these water bodies. Some oceanic samples (presence of peaks U1 and U2) were likely contaminated by oil-derived PAHs from ships navigating around the reef front, whereas the Hermitage River was highly impacted by sewage effluents, numerous in this coastal area of La Réunion Island. We conclude that multi-dimensional fluorescence spectroscopy (EEM) coupled to the determination of HIX and BIX is a good tool for assessing the origin and distribution of DOM in the coral reef ecosystems submitted to anthropogenic impacts.