ALBERT

Description: The biochemical composition and fluorescence properties of DOM were assessed in relation to phytoplankton and major aquatic bacterial clades in a regenerative area of the Argentine Shelf. DOM was mainly of autochthonous biological origin, containing humic- and protein-like substances of medium degree of unsaturation and diagenesis. Biochemical-DOM accounted for 25% of total DOC, being dissolved combined amino acids (DCAA) the dominant fraction followed by free carbohydrates. Phytoplankton was the main source of serine, alanine, and valine, and particulate carbohydrates. Gammaproteobacteria abundance correlated negatively with ammonium and positively with DCAA, suggesting a coupling between ammonium consumption and refractory amino acid production. A preferential utilization of alanine, leucine and threonine as nitrogen source was inferred from the distribution of Cytophaga-Flavobacteria-Bacteroidete in relation with dissolved free amino acids (DFAA). Notably, Alpha- and Betaproteobacteria correlated with the large pool (75%) of chemically unidentified DOC and not with DCAA or dissolved combined carbohydrates. Particularly, Alphaproteobacteria (∼40% of EUB total heterotrophic bacteria) either significantly contribute to the production of the “humic”, refractory fraction of marine DOM, or the latter impairs resource control on their abundance. Spatial heterogeneity inherent to coastal-shelf areas drives important regional variability in the biochemical properties of DOM.

Description: Dissolved organic matter (DOM) plays a crucial role in the biogeochemistry of coastal ecosystems, particularly nutrient cycling and distribution. Little is known about these processes in the highly productive Southern Patagonian shelf. This study was conducted to better understand the sources, composition, and behavior of DOM and inorganic nutrients in the sector between 51 and 56°S and 64–69°W with particular emphasis on inorganic nitrogen and DOM fractions. Surface water samples taken during late austral summer from the Beagle Channel (BCW), Subantarctic (SAW), Subantarctic Shelf (SASW), Grande Bay (GBW) and Tierra del Fuego Waters (TFW) and were analyzed for properties of fluorescent DOM (FDOM), dissolved organic carbon (DOC) and inorganic nutrients. Data were related to hydrographic and plankton conditions. Highest values of ammonium, DOC, humic-like FDOM (FDOMC and FDOMM peaks) and humification index (HIX) were found in BCW, and the lowest in SAW, suggesting that terrigenous input is a main source of ammonium and refractory carbon in this region, which is supported by a highly significant inverse correlation of these parameters with salinity. In contrast, nitrate, phosphate, silicate and the fluorescence index (FI) were positively correlated with salinity, pointing to the contribution of autochthonous FDOM from the saltier and nutrient-rich Antarctic Circumpolar Current to the Southern Patagonian shelf. In TFW and GBW, high nitrite concentrations, accompanied by elevated values of BIX (biological activity index of DOM), circulation patterns and high particle residence times computed from model results suggest the occurrence of regeneration processes that deserve further investigation of the poorly known dynamics of the nitrogen-rich water in this region.