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

A N Loginova,A Engel,C Borchard,J Meyer,R Kiko,H Hauss

doi:10.5194/bg-12-6897-2015

Abstract

Abstract. In open-ocean regions, as is the Eastern Tropical North Atlantic (ETNA), pelagic production is the main source of dissolved organic matter (DOM) and is affected by dissolved inorganic nitrogen (DIN) and phosphorus (DIP) concentrations. Changes in pelagic production under nutrient amendments were shown to also modify DOM quantity and quality. However, little information is available about the effects of nutrient variability on chromophoric (CDOM) and fluorescent (FDOM) DOM dynamics. Here we present results from two mesocosm experiments ("Varied P" and "Varied N") conducted with a natural plankton community from the ETNA, where the effects of DIP and DIN supply on DOM optical properties were studied. CDOM accumulated proportionally to phytoplankton biomass during the experiments. Spectral slope (S) decreased over time indicating accumulation of high molecular weight DOM. In Varied N, an additional CDOM portion, as a result of bacterial DOM reworking, was determined. It increased the CDOM fraction in DOC proportionally to the supplied DIN. The humic-like FDOM component (Comp.1) was produced by bacteria proportionally to DIN supply. The protein-like FDOM component (Comp.2) was released irrespectively to phytoplankton or bacterial biomass, but depended on DIP and DIN concentrations. Under high DIN supply, Comp.2 was removed by bacterial reworking, leading to an accumulation of humic-like Comp.1. No influence of nutrient availability on amino acid-like FDOM component in peptide form (Comp.3) was observed. Comp.3 potentially acted as an intermediate product during formation or degradation of Comp.2. Our findings suggest that changes in nutrient concentrations may lead to substantial responses in the quantity and quality of optically active DOM and, therefore, might bias results of the applied in situ optical techniques for an estimation of DOC concentrations in open-ocean regions.

Highlights

Dissolved organic matter (DOM) is the largest dynamic pool of organic carbon in the ocean
Our findings suggest that changes in nutrient concentrations may lead to substantial responses in the quantity and quality of optically active dissolved organic matter (DOM) and, might bias results of the applied in situ optical techniques for an estimation of dissolved organic carbon (DOC) concentrations in open-ocean regions
Our study shows that during phytoplankton blooms DOM is largely derived from phytoplankton, while its optical properties undergo considerable changes due to bacterial reworking

Summary

Introduction

Dissolved organic matter (DOM) is the largest dynamic pool of organic carbon in the ocean. Labile and semi-labile high molecular weight (HMW) DOM is released primarily by phytoplankton (Carlson and Hansell, 2015). It is used as substrate by heterotrophic communities, which, in turn, release less bioavailable semi-refractory or even refractory DOM, thereby modifying the quantity and quality of the DOM pool (Azam et al, 1983; Ogawa et al, 2001; Jiao et al, 2010). The presence of conjugated double bonds (polyenes) results in the absorption of light in the UV and visible wavelengths (Stedmon and Álvarez-Salgado, 2011). The light absorbing DOM fraction is referred to as “chromophoric” or “colored” DOM (CDOM; Coble, 2007).

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Conclusion