Spatio-temporal variability in suspended particulate matter concentration and the role of aggregation on size distribution in a coral reef lagoon

Abstract This paper presents the concentration and size distribution of suspended particles in the South-West Coral reef lagoon of the island of New Caledonia. Data are provided by filtration techniques, Optical BackScattering (OBS) measurements and in situ laser diffraction particle sizing. The concentration of suspended particles increased from reef to land. A bottom nepheloid layer occurred over the entire lagoon, and was more distinct on the nearshore area. Small particles were more abundant in the bottom nepheloid layer than in the overlying water column. The concentration of suspended particles showed more variability over space than over time. Conversely, the particle size distribution of suspended particles showed more variability at a given location over a month (time variability) than at a given moment over the lagoon (space variability). Analysis showed that aggregates represented a large fraction of suspended particles. Microscope visualization and chemical analysis of a sample suggest the inclusion of a transparent exopolymeric matrix. The relative abundance of aggregates over suspended particle volume concentration was found to increase as the quantity of suspended particle decreased. The spatial distribution of the relative abundance of aggregates suggests more aggregates proximal to coral reefs. The high concentration of aggregates at low turbidity and the spatial distribution of the relative abundance, infer that aggregation is induced by the presence of organic ligands. Unlike optical backscattering and light attenuation measurements that are size sensitive measurements of suspended particle concentration, in situ laser diffraction particle sizing provides a relevant optical measurement of suspended particulate matter in such an aggregate-dominated system.