Surface Gradients in Dissolved Organic Matter Absorption and Fluorescence Properties along the New Zealand Sector of the Southern Ocean

Eurico J D'Sa,Hyun-Cheol Kim

doi:10.3389/fmars.2017.00021

Abstract

The Southern Ocean plays a critical role in global carbon cycling; dissolved organic matter (DOM), a component in the carbon cycle, can be characterized optically. Sea surface chromophoric dissolved organic matter (CDOM) absorption and fluorescence properties were examined in the New Zealand sector of the Southern Ocean (NZSSO) along a transect encompassing various hydrographic fronts associated with the Antarctic Circumpolar Current (ACC) during summer. Phytoplankton chlorophyll, dissolved organic carbon (DOC) and CDOM absorption were observed to be most elevated off the New Zealand shore and then decreased to low values (chlorophyll: 0.21 ± 0.06 mg m-3; DOC: 54.19 ± 4.02 μM; and CDOM absorption coefficient at 325 nm (ag325): 0.097 ± 0.061 m-1) between the Subtropical (STF) and Antarctic Polar (APF) Fronts. Increases in phytoplankton biomass and DOC concentrations between the fronts were associated with meanders or eddies observed in satellite sea surface salinity and chlorophyll imagery. Overall, CDOM absorption was the dominant contributor to total absorption at 443 nm with implications for ocean color. Beyond the APF in the Antarctic Zone, an elevated chlorophyll band likely associated with upwelled waters transitioned to low chlorophyll in the summer ice edge zone that influenced DOM optical properties. A latitudinal increase in ag325 and corresponding decrease in spectral slope S (µm-1) poleward from the STF could be due to the a number of factors including, decreasing CDOM photooxidation, upwelling of high-CDOM waters or bacterial CDOM production in the Antarctic Zone. Parallel factor (PARAFAC) analysis of fluorescence spectra identified two protein-like (C1 and C2) and two humic-like (C3 and C4) components common in the global ocean. ag325 and the humic-like C4 fluorescent component, were positively correlated to chlorophyll indicating biological control. Surface distribution of the protein-like C1 and C2 and the marine humic-like C3 components showed patterns that appeared to be influenced by both physical and biological processes. This study provides insights into surface CDOM optical properties and its transformation along a complex topographically influenced sector of the Southern Ocean that could be used to trace changes linked to the meridional overturning circulation. I

Highlights

The Southern Ocean, comprising a region from the Subtropical Front (STF) to the Antarctic continent connects the three main oceanic basins through the easterly flowing Antarctic Circumpolar Current (ACC) that plays a significant role in the global biogeochemical processes and carbon cycling (Sarmiento and LeQuere, 1996)
The MODIS imagery (Figure 2B) shows a broad band of elevated chlorophyll in the subtropical frontal region around New Zealand (NZ) that are thought to be related to the mixing of warm, macronutrient-poor, relatively micronutrientrich subtropical water with cold, macronutrientrich and micronutrient-poor subantarctic water (Orsi et al, 1995; Belkin and Gordon, 1996; Boyd et al, 1999)
Similar patterns of chlorophyll distribution were previously described with elevated chlorophyll occurring in the STF to the west and east of NZ throughout the year (Murphy et al, 2001) and along a narrow band c. 43◦S on the south side of the Chatham Rise and extending eastwards to the Chatham Islands (Figure 2B)

Summary

Introduction

The Southern Ocean, comprising a region from the Subtropical Front (STF) to the Antarctic continent connects the three main oceanic basins through the easterly flowing Antarctic Circumpolar Current (ACC) that plays a significant role in the global biogeochemical processes and carbon cycling (Sarmiento and LeQuere, 1996). Photosynthetic plankton in the surface ocean is the main source of oceanic DOM and is principally exported to deeper waters through mixing and downwelling of water parcels (Hansell and Carlson, 2001; Jiao et al, 2010; Hansell, 2013) Components of this DOM pool in the form of humic, fulvic, and amino acids impart characteristic absorption and fluorescence properties that can be used to characterize CDOM composition and the diagenetic state (Mopper and Schultz, 1993; Coble, 1996; Stedmon and Markager, 2005). CDOM can be produced in situ by biological production (authochthonous), primarily through microbial remineralization of organic matter or transported from terrestrial sources (Blough and Del Vecchio, 2002; D’Sa, 2008; D’Sa and DiMarco, 2009; Romera-Castillo et al, 2010) It is removed by photochemical degradation and microbial consumption and is influenced by physical processes such as circulation, upwelling, or mixing (Blough and Del Vecchio, 2002; Nelson et al, 2010). While CDOM optical properties have been studied in various oceanic regions (Nelson and Siegel, 2002, 2013; Kitidis et al, 2006; Swan et al, 2009), only limited studies have been reported for the Southern Ocean (Kieber et al, 2009; Ortega-Retuerta et al, 2009; Del Castillo and Miller, 2011; Nelson et al, 2010; Nelson and Gauglitz, 2016)

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