Seawater samples (27) were collected from a flow-through system obtained from a depth of ~2 m as the Korean ice breaker and research vessel Araon transected from Christchurch, NZ to TNB in the western Ross Sea from 30 January to 6 February 2014 (Figure 1). Sub-samples were then filtered through GF/F filters and the filtrate stored in acid cleaned pre-combusted amber bottles with teflon-lined caps at 4°C in a refrigerator for later laboratory processing for CDOM absorption and fluorescence. Filtered GF/F samples were also preserved in a freezer for measurements of chlorophyll concentrations and particulate absorption. Sampling from the flow-through system was however discontinued between 68 and 74°S due to the presence of summer sea-ice. Seawater temperature was measured at the outflow of the flow-through system; however, differences in temperature between the inlet and outlet of the flow-through likely existed and the data has been mainly used to examine broad temperature trends along the transect. The main topographic features such as the Chatham Rise (CR), Campbell Plateau (CP), and the PAR system are shown (Figure 1), along with the locations of fronts such as the Subtropical (STF), Subantarctic (SAF) and its northern (NSAF) and southern (SSAF) branches, Antarctic Polar (APF), and the Southern Antarctic Circumpolar Current (SACCF) Fronts in the NZSSO (Figure 1).

Seawater samples (500 ml) were filtered through Whatman GF/F filters (25 mm diameter) for the fluorometric determination of chlorophyll (Holm-Hansen et al., 1965) using a Turner Designs Model 10AU fluorometer calibrated with pure chlorophyll a (Sigma Chemical Company).

Samples for dissolved organic carbon (DOC) were filtered through pre-rinsed Whatman GF/F filters and stored in acid cleaned, pre-combusted amber bottles with Teflon-lined caps in a freezer. Laboratory measurements of DOC were made on a Shimadzu TOC 5000A (with ASI-5000A autosampler) using a high temperature combustion method to convert carbon compounds to carbon dioxide, including using the Consensus Reference Material (CRM) for QA/QC (Benner and Strom, 1993; Hansell and Carlson, 2001).

Water samples collected from the flow-through system were filtered immediately through pre-rinsed Whatman GF/F filters under low vacuum and stored in acid cleaned, pre-combusted amber bottles at 4°C in the dark before laboratory analysis. After the filtered samples were allowed to reach ambient room temperature, absorbance measurements of CDOM (A(λ)) were obtained on a WPI Ultrapath™ system from 190 to 722 nm on either 0.5 m or 2.0 waveguide cells. To minimize differences in refractive index between sample and reference which cause offsets in absorbance measurements (D'Sa et al., 1999; D'Sa and DiMarco, 2009), a reference salt solution was prepared using granular NaCl (Mallinckrodt) and Milli-Q water to closely match the seawater samples. Absorbance data were corrected by subtracting the mean value over a 10 nm interval (695–705 nm) of the measured absorbance at 700 nm from each wavelength and the a_g(λ) (m⁻¹) was calculated using the equation: (1)ag(λ)=2.303×A(λ)l Absorption coefficient at 325 nm (a_g325) was used as a quantitative parameter of CDOM. The spectral slopes for the intervals of 275–295 nm (S or S_275−295, μm⁻¹) were calculated according to Helms et al. (2008). Due to very low CDOM absorption in the visible, absorption especially at 443 nm was below the detection limit of the instrument for some samples.

Particulate absorption measurements were obtained using the quantitative filter technique (QFT) procedure (Mitchell et al., 2003). To measure the particulate absorption, 500 ml of surface seawater samples were filtered onto 25 mm GF/F filters under low vacuum and preserved at −80°F before laboratory analysis. Optical density of particles on the filter paper were measured in the transmission mode on a Perkin-Elmer Lambda 850 with an integrating sphere and also on a fiber-optic based spectrophotometer (Naik and D'Sa, 2012) as it provided better sensitivity in comparison to the Lambda 850. Details on the laboratory analysis of particulate absorption on the two systems have been previously described (Naik and D'Sa, 2012; Naik et al., 2013). As both CDOM and particulate absorption were very low, measurements on both the spectrophotometer and waveguide were sensitive to instrument noise and baseline variability resulting in uncertainties for some samples in the visible band of the spectrum. Thus, due to the very low absorption values, the relative absorption budget is reported only at 443 nm, a commonly used wavelength.

Filtered seawater samples used for CDOM absorption measurements were also used to record EEMs using a FluoroMax-4 (Jobin Yvon Horiba) fluorometer by scanning the emission spectra from 290 to 550 nm at 5 nm intervals over excitation wavelengths between 250 and 450 nm at 5 nm increments. The EEMs spectra were obtained after correction of the fluorescence spectra for instrument bias, and the water Raman normalization of the fluorescence intensity (Singh et al., 2010; D'Sa et al., 2014). Due to low absorbance of the samples, there was no requirement for inner filter correction of the fluorescence data. The resulting EEM fluorescence observations were evaluated by PARAFAC analysis using the DOM-Fluor toolbox (Stedmon and Bro, 2008), with the model constrained by non-negativity, and run with three to seven components. Model validation was carried out using split-half analysis, residual analysis and random starts (Stedmon and Bro, 2008; D'Sa et al., 2016).

MODIS Aqua chlorophyll imagery at 4 km were obtained from NASA Ocean Color Giovanni website (giovanni.gsfc.nasa.gov). Due to extensive cloud cover over the study area, imagery averaged between 26 Jan and 10 Feb, 2014 were used that included the sampling period (30 Jan–6 Feb, 2014). Sea surface salinity data (7-day average) from the Aquarius microwave instrument onboard the Aquarius/SAC-D satellite was obtained from NASA Jet Propulsion Laboratory corresponding to the sampling period. A comparison study of the 1° gridded Aquarius sea surface salinity with Argo array salinity product showed standard deviation in salinity values ranging between 0.15 and 0.6 for high-latitude oceans (Lee, 2016).

Sea surface temperature (~2 m depth) measured on board the Araon during its 8-day transect from New Zealand to Terra Nova Bay (TNB) show gradients and a decreasing meridional trend in temperature (Figure 1). Poleward decrease and gradients in sea surface salinity are also observed in the Aquarius satellite salinity imagery (Figure 2A) and at matching station locations extracted from the satellite data (Figure 3). While frontal positions are best identified from hydrographic data across the ACC, in this study we use underway data and satellite imagery combined with historical observations (Budillon and Rintoul, 2003) to infer latitudinal locations of the fronts during the study period (Figure 3).

The STF, which separates the warmer and saltier subtropical surface water from the cooler and fresher subantarctic surface water, marks the northernmost extent of the ACC and remains close to the coast south and east of NZ and then moves offshore. Large gradients in salinity and temperature observed in the convergence zone (Figures 1, 2) have been associated with considerable eddy activity southeast of NZ (Williams, 2004; Fernandez et al., 2014). South of NZ, the Sub Antarctic Front (SAF), a major front of the ACC system bifurcates into northern (NSAF) and southern (SSAF) branches with the NSAF (~53°S) closely following the steep southeastern flank of the Campbell Plateau (CP) and the SSAF (~58°S) found over the abyssal plain of the south-west Pacific Basin (Budillon and Rintoul, 2003; Fernandez et al., 2014). Large variability in sea surface salinity south of NZ (Figure 2A) indicates eddies and meanders associated with these fronts in the subantarctic zone lying between the STF and SAF (Budillon and Rintoul, 2003). The APF located at ~62°S and generally found between 60.2 and 62.8°S, is characterized by large gradients in temperature and salinity (Figure 3) and delineates the cold Antarctic Surface Water (ASW) to the south (Antarctic Zone) from the warmer subantarctic surface waters to the north. The Antarctic Zone contains the Antarctic divergence (Daly et al., 2001) and the southern boundary of the ACC (SACCF) which in this region follows closely the Pacific-Antarctic Ridge and is located at ~63.8°S (Budillon and Rintoul, 2003). The seasonal ice zone, generally located south of the SACCF, was ice-covered south of 68°S during the transect. In the western Ross Sea, the Terra Nova Bay polynia is an ice-free coastal region with cold ASW (Figures 1, 3; temperature up to +2°C, S < 34.5; Rivaro et al., 2011).

Surface chlorophyll estimates from MODIS reveal elevated concentrations southeast of NZ and Chatham Rise and the STF region (Figure 2B). Along the rest of the transect from STF to APF, chlorophyll values were low with few elevated bands that appear related to eddies and meanders associated with the interaction of the STF and the SAF (Figures 2A,B). An elevated chlorophyll band south of the APF is observed in the Antarctic Zone while high chlorophyll values were observed further south in the TNB region of the western Ross Sea. A similar pattern was observed in surface chlorophyll from field observations along the transect (Figure 3) with low chlorophyll values in the core of the ACC between STF and APF (0.21 ± 0.06 mg m⁻³) that varied in the range 0.12–0.38 mg m⁻³, and the more elevated values associated with peaks observed due to meanders and eddies (Figure 2A). Higher chlorophyll values were however observed off NZ shore and STF region (0.66 ± 0.43 mg m⁻³), the elevated band south of the APF (0.64 ± 0.15 mg m⁻³), with highest values in TNB (2.42 mg m⁻³).

Surface DOC concentrations were high off the NZ shore (66.71±21.88 μM) and TNB (62.11 μM), with slightly elevated values observed coincident with chlorophyll peaks at 50°S, 55°S, and around the APF. In the ACC core (STF to APF) values ranged between 48.07 and 60.01 μM with a mean of 54.19 ± 4.02 μM. a_g325 between the STF and SACCF ranged between 0.024–0.145 m⁻¹ (mean of 0.097 ± 0.061 m⁻¹) and showed an increasing trend poleward (Figure 4). a_g325 was however elevated in the NZ shelf waters (0.179 ± 0.110 m⁻¹) and in TNB (0.188 m⁻¹). The spectral slope S values (29.45 ± 3.71 μm⁻¹) generally showed an inverse relationship to a_g325 along the transect (Figure 4). S was low closest to the NZ shore (station 1) but increased toward STF and generally decreased poleward into the Antarctic Zone (Figure 4). a_g325 was positively correlated with chlorophyll (Figure 5B; r² = 0.48) but was uncorrelated to DOC concentrations (Figure 5A).

The relative absorption coefficients in the visible (443 nm) of CDOM, particulate matter (phytoplankton a_phy and nonalgal a_nap) varied across the transect (Figure 6) ranging for a_g443 from 0.0043 to 0.14 m⁻¹, a_phy(443) from 0.0043 to 0.14 m⁻¹, and a_nap(443) from detection limit to 0.0033 m⁻¹, respectively. Although a_nap(443) contributed the least to the total absorption (<10%), its relative contribution increased closer to the NZ coast and south of the APF, coinciding with the band of elevated chlorophyll (Figure 6A). Variability in phytoplankton absorption a_phy(443) was similar to chlorophyll along the transect with elevated values off the NZ shore and south of APF (Figures 3, 6A). However, its contribution relative to CDOM at 443 nm was variable across the transect being generally greater in regions of high chlorophyll while CDOM dominated at other locations as also indicated by the ternary plot (Figure 6B).

Typical EEMs spectra obtained from surface samples in the STF, APF, and TNB reveal the presence of prominent peaks in the spectra (Figure 7A). These peaks have traditionally been used to characterize CDOM composition using terminology such as A, C, M, B, and T peaks (Coble, 1996). The presence of the marine humic-like M peak (ex/em, ~300/380–420 nm) is clearly observed, while the fluorescence peaks associated with the protein-like material, namely, the tyrosine-like and tryptophan-like aromatic amino acids (in the UV spectral range) appear combined visually. In contrast, the typical humic-like A and C peaks though not visually observable, were present at low background levels. PARAFAC analysis identified four major fluorescent components in the surface waters between NZ and TNB (Figure 7B; Table 1). The excitation and emission loadings of each of the four components (Figure 7B, bottom right; Table 1) identified components 1 (C1) and C2 with excitation/emission (ex/em) wavelengths of 260/300 nm and 270/330 nm associated with tyrosine- and tryptophan-like material that is microbially produced (Yamashita and Tanoue, 2003; Murphy et al., 2008). Component 3 with ex/em wavelength of 295/400 nm has been previously identified as the marine or “M” humic-like component of microbial origin (Coble, 1996; Yamashita and Tanoue, 2003; Stedmon and Markager, 2005). The broad fluorescence peak of component 4 (C4) with primary (and secondary) ex/em wavelength peaks at <260(360)/460 nm is similar to previously identified terrestrial humic-like A and C like material and generally ubiquitous in the marine environment (Coble, 1996; Murphy et al., 2008, Table 1).

The distribution pattern of the four FDOM components differed along the transect (Figure 8). The humic-like C4 component was elevated off NZ shore and TNB and uniformly low between the STF and APF (Figure 8A) and was positively correlated to chlorophyll (Figure 9A). The C3 marine M-like humic component was found to be elevated off NZ and in regions with elevated phytoplankton chlorophyll (Figure 8A) but showed no clear trends with chlorophyll (not shown). The two protein-like components C1 and C2 were elevated off NZ and TNB, as well as at/near the fronts (Figure 8B), with C1 showing a weak negative correlation to salinity (Figure 9B).

The STF located south and east off NZ forms the boundary between the warmer/salty, nutrient-poor subtropical waters and the colder/fresher, nutrient rich subantarctic waters. Off the NZ shore, the subtropical surface water, generally located 15–40 km offshore, is modified by mixing with freshwater runoff from land and with the subantarctic water further offshore (Sander et al., 2015; Figure 2A). The MODIS imagery (Figure 2B) shows a broad band of elevated chlorophyll in the subtropical frontal region around NZ that are thought to be related to the mixing of warm, macronutrient-poor, relatively micronutrient-rich (especially iron) subtropical water with cold, macronutrient-rich 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). South of New Zealand, the complex marine topography associated with the Macquarie Ridge and the Campbell Plateau strongly influences the circulation pattern with the ACC undergoing intense wave-like migrations and energetic eddies (Gordon, 1975; Bryden and Heath, 1985). Surface expression of such circulation patterns and associated meanders and eddies observed in the satellite sea surface salinity imagery south of New Zealand (Figure 2A) also appear associated with more elevated chlorophyll concentrations observed in the MODIS imagery and field measurements along the transect (~50 and 55°S; Figures 2B, 3). Similar trends were previously observed linking meanders and eddies to the interaction of the two SAF branches and the complex topography (Campanelli et al., 2011). Nonetheless, chlorophyll concentrations were low in the subantarctic waters (0.2–0.3 mg m⁻³) and comparable to levels reported in other subantarctic regions (Boyd et al., 1999; Han and Takahashi, 2001; Murphy et al., 2001; Hunt and Hosie, 2006). South of the APF, an elevated chlorophyll band (0.64 ± 0.15 mg m⁻³; Figures 2B, 3) showed similarity to earlier observations that appear persistent in the region (Moore and Abbott, 2000; Campanelli et al., 2011). This significant increase in phytoplankton biomass has been attributed to iron replenishment in the mixed layer of the Antarctic zone by the Upper Circumpolar Deep Water (UCDW; Hiscock et al., 2003) or to the wind-induced increase of the mixed layer depth (Carranza and Gille, 2015). As in previous observations (Daly et al., 2001; Han and Takahashi, 2001), elevated surface chlorophyll were not observed at fronts, but near fronts or in eddies and meanders (Figures 2, 3). Further south in the summer ice edge zone, relatively low chlorophyll values (Figure 3) were similar to values reported in the Australian sector of the Southern Ocean in early austral summer that were attributed to the short time for phytoplankton growth following ice melt or to reduced water column stability due to strong prevailing winds (Han and Takahashi, 2001). In the TNB region of the western Ross Sea, chlorophyll biomass was relatively high (Figures 2, 3) and likely associated with a late summer bloom with values in the range previously reported for surface waters in the area (Rivaro et al., 2011). Overall, surface summer chlorophyll distribution along the NZ transect supported the Southern Ocean as a HNLC region with productive sites linked to meanders and eddies and other local processes.

Surface DOC concentrations in the NZSSO between the STF and SACCF (54.19 ± 4.02 μM) were comparable to those reported in the Australian sector of the Southern Ocean (45–55 μM; Ogawa et al., 1999) and summer surface concentrations in the Ross Sea (55 ± 5μM; Carlson et al., 2000). In comparison to the refractory DOC pool (~42 μM) observed in deeper waters of the Ross Sea, and the Southern Ocean (Ogawa et al., 1999; Carlson et al., 2000), surface DOC in excess of that in deep water is likely associated with a labile or semi-labile fraction of DOM with turnover on varying time scales (hours, days, months to years) that supports heterotrophic microbial production and is potentially important for export (Carlson, 2002; Hansell, 2013).

a_g325 was highest in the NZ shore station and decreased to its lowest values near the STF indicating terrestrial influences on CDOM in the northern sector of the transect. Surface patterns and range of a_g325 distribution in the ACC were generally consistent with earlier reported values in the Pacific region of the Southern Ocean (Nelson et al., 2010). A general poleward increase in a_g325 (0.024–0.145 m⁻¹) and a corresponding decreasing trend in spectral slope S between the STF and APF were likely due to a combination of factors such as (i) decreasing rates of CDOM loss poleward due to solar photodegradation, (ii) upwelling of the CDOM-rich subsurface waters in the Antarctic Zone and its convective transport associated with the wind-driven overturning circulation (Nelson et al., 2010; Marshall and Speer, 2012) or, (iii) to biogeneration of CDOM by bacteria (Ortega-Retuerta et al., 2009). Elevated values of the spectral slope S observed at a meander south of the STF and at fronts (e.g., NSAF, APF, SACCF; Figures 2, 4) suggest that this CDOM optical property could potentially be used to identify such features in the Southern Ocean. CDOM was uncorrelated with DOC (Figure 5A) but positively correlated to chlorophyll (Figure 5B) suggesting strong biological control on CDOM in the surface waters of the study region.

The light absorption properties of CDOM and particulate matter in the visible (e.g., 443 nm) influences the light field and ocean color. a_g(443) was similar to those reported in other regions of the Southern Ocean (Clementson et al., 2001; Kieber et al., 2009; Del Castillo and Miller, 2011). a_phy(443) and a_nap(443) between the STF and APF were comparable to those reported in the same latitudinal bands in the Australia sector of the Southern Ocean (Clementson et al., 2001), and other oceanic waters (Bricaud et al., 1998; Naik et al., 2013). a_nap(443) was generally <10% of the total absorption but its contribution increased in the elevated chlorophyll band south of the APF (Figures 6A,B) likely due to the persistence of this band and the multiple pathways for decay and regeneration of the phytoplankton biomass. Overall, CDOM contribution was greater (50–60%) than phytoplankton absorption (30–40%). However, in the high chlorophyll band south of the APF and in the northern stations off NZ, phytoplankton absorption was dominant. Terrestrial influences off NZ (station 1) and at TNB stations likely contributed to the dominant CDOM contribution (~70 and 80%, respectively; Figure 6B). Thus, while the general surface characteristics of the absorption field within the ACC region appear consistent with other regions of the Southern Ocean, effects of deviations in regions such as off NZ on ocean color should be considered in its interpretation.

The four PARAFAC fluorescent components identified in the surface waters of the NZSSO have also been reported in other oceanic regions (Murphy et al., 2008; Jorgensen et al., 2011; D'Sa et al., 2014; Catalá et al., 2015, 2016) including the Southern Ocean (Wedborg et al., 2007; Nelson and Gauglitz, 2016). The relative concentrations and latitudinal distribution patterns of the four components however, appeared to be influenced by physical and biological processes along the NZSSO. A positive correlation of the humic-like C4 component to chlorophyll (Figure 9A) suggests a strong biological control consistent with previous results documented for the global epipelagic ocean (Catalá et al., 2016). A decreasing trend of the C4 component with increasing distance from the NZ shore as well as its high intensity in TNB suggest a strong terrestrial influence on this component in these two regions (Figure 8A). However, in the isolated region of the Antarctic Zone, elevated C4 values (Figure 8A) suggest likely contribution by upwelling associated with the UCDW and is consistent with higher intensity of this humic-like component in deeper waters of the ocean (Catalá et al., 2016; Nelson and Gauglitz, 2016). Furthermore, uniformly elevated C4 intensity in contrast to highly variable chlorophyll distribution in the Antarctic Zone suggests no direct linkage of this component to local productivity. A decreasing trend of C4 to uniformly low background values north of the APF likely due to photobleaching (Catalá et al., 2016) suggests a transformation of this humic-like material in surface waters as it flows equatorward associated with the Southern Ocean's meridional overturning circulation. The transformation of this DOM fluorescent material as the upwelled waters of the UCDW continue equatorward to form the Antarctic Intermediate Water (AAIW) and the Subantarctic Mode Water (SAMW) (Abernathey et al., 2016; Armour et al., 2016) has potential to influence DOM fluorescent composition in the global ocean (Marshall and Speer, 2012). The marine humic-like C3 component in contrast, revealed large variability across the surface waters of the NZSSO. Although a previous study indicated biological control on the distribution of this component in the global epipelagic waters (Catalá et al., 2016), such direct linkage was not observed in this study; rather, trends of elevated C3 in the productive region offshore of NZ and the STF, and the productive band in the Antarctic Zone south of the APF were observed (Figure 8A). This component was, however, low in the highly productive surface waters of the TNB, the summer ice edge region, and between the fronts. This pattern likely suggests elevated production of this fraction associated with microbial activity at more productive sites or at sites such as the STF with predominance of regenerated production (Kopcsynska et al., 2001). In contrast, although the TNB site was highly productive (Figure 3), the short-term cycling of the cold and high saline waters associated with the TNB polynia may not provide the conditions for the microbial production of this fluorescent fraction of CDOM.

The tyrosine- and tryptophan-like components C1 and C2 revealed broadly similar patterns of distribution with C1 having greater intensity and variability across the transect (Figure 8B). Off NZ, C1 showed a steep decline in intensity up to the STF suggesting strong terrestrial influence. It then increased significantly at the STF and thereafter declined toward the APF in concert with decreasing salinity (Figure 8B) suggesting some physical control (Catalá et al., 2016) as indicated by a weak negative relationship to coarse estimates of satellite-derived sea surface salinity (Figure 9B). However, anomalous increase of this component at or near hydrographic fronts (e.g., STF, SSAF, APF ad SACCF) were observed (Figure 8B) and are similar to those reported in the South Africa sector of the Southern Ocean, wherein, pronounced concentrations of the protein-like fluorescent fractions were detected even at great depths (Wedborg et al., 2007), likely indicating linkage to more resistant DOM fractions in the deeper ocean waters (Catalá et al., 2015). Both C1 and C2 showed increases at fronts and low values between the fronts (e.g., NSAF and APF) suggesting some photooxidative loss of these components. Large variability in these two protein-like components in the Antarctic Zone could be due to both physical and biological processes associated with upwelling, elevated phytoplankton biomass and seasonal sea ice effects.

In summary, surface field data combined with satellite (salinity and chlorophyll) and historical hydrographic observations were used to understand the physical regime (specifically, the locations of fronts) and the presence of meanders along the NZSSO, a topographically complex region of the Southern Ocean. CDOM absorption and fluorescence variability along the NZSSO were then examined in relation to frontal positions and biochemical properties (chlorophyll and DOC concentrations) in addition to assessing CDOM absorption contribution to total absorption in the visible band that influences ocean color. In the STF region south of NZ, the interaction of the subtropical and subantarctic waters generally resulted in decreasing levels of chlorophyll and DOC concentrations, and CDOM optical properties that suggested terrestrial influences. However, in the core of the ACC, between the STF and the APF, surface chlorophyll and DOC concentrations and CDOM absorption were low and consistent with other studies. Fluorescence fractions identified using PARAFAC analysis were consistent with those in other oceanic regions, but revealed patterns influenced by fronts and water properties typical of the NZSSO. In the Antarctic Zone, upwelling of CDOM-rich waters likely associated with the Southern Ocean's overturning circulation and its equatorward transport and transformation could contribute to modified CDOM and FDOM properties that could serve as a tracer of changes in biochemistry associated with the overturning circulation.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.