Impact of Contrasted Weather Conditions on CDOM Absorption/Fluorescence and Biogeochemistry in the Eastern Lagoon of New Caledonia

Abstract

New Caledonia (Southwest Pacific), like all tropical Pacific Island countries, is impacted by weather events, climate change, and local anthropogenic forcing. Strong erosion of particles and dissolved organic matter (DOM) from ultramafic rocks, associated with trace metals dissemination (i.e., nickel, manganese and cobalt), potentially affects lagoon waters and coral reefs surrounding the main island. The CALIOPE (CALedonian Inherent Optical PropErties) cruises were performed along the Eastern Lagoon of New Caledonia (ELNC) (400 km, 13 transects from Bay to open ocean, 51 stations) during contrasted meteorological conditions: a dry period (October 2011), a windy situation (March 2014), and a strong rainy event (March 2016). CDOM absorption and fluorescence (FDOM), particulate absorption, backscattering, suspended particulate matter (SPM), total chlorophyll a (TChla), nutrients (NOx), pigment and phytoplankton composition were measured. Among the four CDOM fluorophores, the humic-like component (λ Ex /λ Em : 235/460 nm), assimilated to a photoproduct of terrestrial organic matter, had relatively low fluorescence compared to protein-like fluorophores. As CDOM absorption, particulate absorption, backscattering, SPM, total chlorophyll a (TChla) and nutrient (NOx) concentrations, this humic-like material generally showed the highest values during rainfall events, the latter inducing an increase in riverine terrigeneous inputs and change toward higher phytoplankton size classes. The tyrosine 1-like (λ Ex /λ Em : 220, 275/304 nm) and tryptophan-like fluorophores (λ Ex /λ Em : 230, 300/352 nm) were strongly influenced by wind displaying a 7-fold and 3-fold increase, respectively in windy situation compared to calm conditions. These increases could be related to enhancements of autochthonous biological activities (highest mean concentrations of Synechococcus spp., phycoerythrin, pico-and nano-eukaryotes, heterotrophic bacteria and nanoplankton observed in wind condition) through the inputs of organic and mineral materials issued from the wind-induced sediment resuspension, atmospheric deposition and water mass mixing. By contrast, the tyrosine 2-like fluorophore (λ Ex /λ Em : 245, 275/304 nm) substantially increased during rain events and presented the lowest values

In-Situ Variability of DOM in Relation with Biogeochemical and Physical Parameters in December 2017 in Laucala Bay (Fiji Islands) after a Strong Rain Event

“…Brownish-colored water is likely to have a high carbon load from the humic substances found in plants and soil. The organic carbon load from soil sediment runoff explains the acidic properties of the waters and the resulting yellow-brown coloration [14,15]. DOM occurs naturally in many forms such as fatty acids, carbohydrates, amino acids, hydrocarbons, fulvic acids, humic acids, viruses, and clay-humic metal complexes [10] and has both allochthonous and autochthonous origins [15,16].…”

“…Allochthonous DOM is linked to degraded organic materials of terrestrial origin while autochthonous DOM arises from in-situ biological activities [13,14] such as phytoplankton exudation and lysis, animal excretion, and fragments or cadavers derived from biogenic sources [10,[17][18][19][20]. A fraction of DOM contains chromophores that strongly absorb short-wavelength light ranging from ultraviolet (UV) to blue of the electromagnetic spectrum [14][15][16], hence the term chromophoric dissolved organic matter (CDOM). CDOM can be characterized by its absorbance in the UV-visible domain.…”

“…CDOM plays a significant role not only in the transport of DOC from the terrestrial to coastal and oceanic environments but also by the provision of numerous ecosystem services. CDOM provides sources of energy to heterotrophs [26,27], protection of microorganisms from harmful UV radiation [15,16,28], regulation of photosynthetically available radiation (PAR), for marine algae and benthic organisms [13,14], and affects the availability of complex metal or organic pollutants [16,29,30].…”

Editorial: Biogeochemical Responses of Tropical Ecosystems to Environmental Changes

“…The study by Dupouy et al (2020) deals with the identification of the origin of DOC compounds in the coastal waters in and around the Eastern Lagoon of New Caledonia and examines its relationships to environmental forcing. Different biologicallyderived DOC in the water column of the lagoon showed that the humic-like components are particularly influenced by rainfall and terrigenous inputs, both variables that are strongly affected by global warming.…”

“…Inter-related and spatially variable pressures including sea level rise, increased storminess, altered precipitation regime, varying continental runoff and increasing temperature are impacting these ecosystems at regional scales (Ward et al, 2016); but the intensity of the response seems site-specific (Godoy and Lacerda, 2015) and species-specific (McKee and Rooth, 2008;Ramachandran et al, 2020). Interactions with local anthropogenic drives, in particular damming, basin diversion, and eutrophication, also modify the intensity of the ecosystems' response, either favoring or hampering adaptability (Dupouy et al, 2020;Lacerda et al, 2020). The biogeochemistry of tropical coastal ecosystems is affected by the interaction between multiples stressors of local, regional, and global scales.…”