Origin and Transformation of Dissolved Organic Matter Under Contrasting Nutrient Regimes in Productive Northern Patagonian Continental Shelf (Southwestern Atlantic Ocean)

Abstract

This study analyzed the composition and optical properties of dissolved organic matter (DOM) in the Northern Patagonian Continental Shelf (40°S–36°S, Southwestern Atlantic Ocean), which embraces highly diverse hydrographic, biological and biogeochemical regimes. The main aim was to identify autochthonous and allochthonous sources of DOM, its bulk molecular characteristics and bioavailability, in relation to contrasting regional nutrient and circulation patterns. In the coastal sector at depths ≤ 50 m, only a moderate riverine input of DOM and nutrients was detected, yet high values of the humification index (HIX) pointed to a continental origin of aromatic DOM, with sediment resuspension significantly contributing dissolved humic carbon, silicate and phosphate. In the middle shelf, high dissolved organic carbon and chlorophyll levels coincided with low bacterial carbon production (BCP) and HIX values, indicating an autotrophic production of aliphatic DOM. This was fueled at the outer shelf and shelf break by nitrate input from the Malvinas Current and in the middle shelf by regenerated ammonium. In the shelf break, humic- and protein-like fluorescent DOM maxima were likely originated from phytoplankton lysates, as suggested by high tryptophan-like fluorescence signals. The variation of the biological activity index (BIX) was only partially explained by chlorophyll, while BCP patterns did not. There is evidence of a site-dependent compositional artifact in the BIX values due to a background of high-BIX, recalcitrant DOM, possibly aromatic low-molecular weight peptides. Wind-driven shifts in surface circulation influenced the distribution of DOM, nutrients, and temperature, highlighting their role in the region’s annual spring bloom and future DOM dynamics studies.

Keywords: fluorescent dissolved organic matter, chromophoric dissolved organic matter, nutrients, circulation, biological activity