Effects of nitrate and phosphate supply on chromophoric and fluorescent dissolved organic matter in the Eastern Tropical North Atlantic: a mesocosm study

[摘要] In open-ocean regions, as is the Eastern Tropical North Atlantic (ETNA),pelagic production is the main source of dissolved organic matter (DOM) andis affected by dissolved inorganic nitrogen (DIN) and phosphorus (DIP)concentrations. Changes in pelagic production under nutrient amendments wereshown to also modify DOM quantity and quality. However, little informationis available about the effects of nutrient variability on chromophoric(CDOM) and fluorescent (FDOM) DOM dynamics. Here we present results from twomesocosm experiments ("Varied P" and "Varied N") conducted with anatural plankton community from the ETNA, where the effects of DIP and DINsupply on DOM optical properties were studied. CDOM accumulatedproportionally to phytoplankton biomass during the experiments. Spectralslope (S) decreased over time indicating accumulation of high molecularweight DOM. In Varied N, an additional CDOM portion, as a result ofbacterial DOM reworking, was determined. It increased the CDOM fraction inDOC proportionally to the supplied DIN. The humic-like FDOM component(Comp.1) was produced by bacteria proportionally to DIN supply. Theprotein-like FDOM component (Comp.2) was released irrespectively tophytoplankton or bacterial biomass, but depended on DIP and DINconcentrations. Under high DIN supply, Comp.2 was removed by bacterialreworking, leading to an accumulation of humic-like Comp.1. No influence ofnutrient availability on amino acid-like FDOM component in peptide form(Comp.3) was observed. Comp.3 potentially acted as an intermediate productduring formation or degradation of Comp.2. Our findings suggest that changes innutrient concentrations may lead to substantial responses in the quantityand quality of optically active DOM and, therefore, might bias results ofthe applied in situ optical techniques for an estimation of DOCconcentrations in open-ocean regions.

[学科分类] 地球化学与岩石