Variability of chromophoric dissolved organic matter in three freshwater-influenced systems along central-southern Chile

Chromophoric dissolved organic matter (CDOM) is a biologically active component of dissolved organic matter that influences the optical properties of aquatic environments, thereby playing an important role in the photochemical and photobiology processes occurring in the euphotic layer. Three cruises were carried out at three contrasting oceanographic areas off central and southern Chile (36.8°S, 44.6°S, 54.9°S) between November 2013 and July 2017 in order to assess the variability of the CDOM pool's optical properties (a325, SCDOM, SR). Marked differences, including vertical variability, were found for a325, SCDOM, and SR within and among each of the three zones, although surface DOC concentrations remained constant at all sites (87.59 ± 26.04 µmol L−1). At the coastal area close to the Biobio River (36.8°S), a325 values varied between 0.26–1.93 m−1 with maximum measured in surface waters. For the Puyuhuapi fjord in northern Patagonia (44.6°S), a325 ranged between 0.69 and 3.86 m−1, while in the Beagle Channel (54.9°S) it oscillated between 0.59 and 1.73 m−1. We identified the terrestrial organic matter input to surface waters as the main factor influencing surface CDOM concentrations in central Chile. In the Puyuhuapi fjord, terrestrial influence was heterogeneous because of its seasonal variability of the Channel but it was overall less significant than in situ produced CDOM. We also observed the lowest contribution of terrestrial CDOM inputs compared to the in situ production in southern Patagonia (Beagle Channel). We observed that the distribution of CDOM is potentially dominated by biological in situ production below the surface freshwater-influenced layer in the southern area while highly influenced by terrestrial input in the coastal upwelling area at 36°S. Overall, this study shows that the optical properties of dissolved organic matter (mainly SR) can be suitable for identifying different CDOM pools in coastal areas of contrasting oceanographic conditions. In stratified systems, freshwater inputs determine the distribution of surface CDOM while in well-mixed conditions, the in situ production determine the general CDOM concentration.