The impact of terrigeneous inputs on the Bay of Ouinné (New Caledonia) phytoplankton communities: A spectrofluorometric and microscopic approach

The impact of terrigeneous inputs on the phytoplankton composition was studied during a 2.5 months daily survey in the Bay of Ouinné, a deep cove on the south-eastern coast of New Caledonia. Surface waters benefited from nutrients originating from nearby land drainage and the Ouinné river outflow during periods of heavy rain. The nutrient composition mirrored the composition of the drained soils, with concentrations reaching 3–4 μM for nitrate and 0.13 μM for phosphate at the river mouth. In addition to nutrient inputs, significant quantities of particulate matter (inorganic and organic compounds) were discharged into the lagoon during heavy rain periods, resulting in transitory decreases of the photic layer depth and enrichments of the water column through remineralization processes. Changes in contributions of the main phytoplankton groups in response to terrigeneous inputs were shown by chlorophyll and phycobiliprotein spectrofluorometric analyses. While dry periods were marked by the dominance of pico- (Prochlorococcus, and high-phycourobilin (PUB) Synechococcus) and microcyanobacteria (Trichodesmium spp.), developments of various eukaryote populations resulted from land drainage occurring during the wet periods. This was indicated by the increase of accessory chlorophyll pigments that doubled at a depth of 15 m: chlorophyll b (chlorophytes), chlorophyll c1+c2 (associated with diatoms and dinoflagellates), chlorophyll c3 (associated with c1 and/or c2 in prymnesiophytes, chrysophytes and/or pelagophytes). In addition, Synechococcus with a high phycoerythrobilin (PEB) content also appeared to be stimulated by river outputs. Finally, microscopic observations of the >35 μm net plankton confirmed the greater presence of diatoms and dinoflagellates during periods of rain compared to Trichodesmium, particularly in the surface layer.