Smithsonian at the Poles: Contributions to International Polar

330 SMITHSONIAN AT THE POLES / KIEBER, TOOLE, AND KIENE
our low SR results would suggest that the Ross Sea samples
contained a relatively high proportion of HMW DOM
compared to what is observed in oligotrophic waters. The
relatively higher molecular weight DOM and low SR in the
Ross Sea may be related to the lower photobleaching rates
that are observed in the Antarctic, since photobleaching
is the main mechanism to increase SR and remove HMW
DOM (Helms et al., 2008).
CDOM SOURCE
The CDOM present in coastal areas near rivers and in
estuaries in subtropical-temperate and northern boreal latitudes
is predominantly of terrestrial origin (e.g., Blough et
al., 1993; Blough and Del Vecchio, 2002; Rochelle-Newall
and Fisher, 2002a), but in the open ocean, as in the Antarctic,
terrigenous CDOM is only a minor component of
the total DOM pool (Opsahl and Benner, 1997; Nelson
and Siegel, 2002).
Phytoplankton are expected to be the main source of
CDOM in the open ocean, although they are not thought
to be directly responsible for CDOM production (Bricaud
et al., 1981; Carder et al., 1989; Del Castillo et al.,
2000; Nelson et al., 1998, 2004; Rochelle-Newall and
Fisher 2002b; Ferenac, 2006). Our results are consistent
with this fi nding (Figure 7). With no substantial grazing
pressure (Rose and Caron, 2007, and as noted by our
colleagues D. Caron and R. Gast during our cruise to the
Ross Sea in early November 2005) and very low bacterial
activity (Ducklow et al., 2001; Del Valle et al., in press),
it is not surprising that CDOM a300 and a340 changed
very little during the bloom. In fact, a300 did not apparently
increase even during the latter stages of the Ross
Sea Phaeocystis antarctica bloom when the microbial activity
was higher (Kieber et al., 2007: fi g. 5). This fi nding
is rather remarkable since DOC is known to increase
by �50% during the Phaeocystis antarctica bloom from
�42 to �60 �M in the Ross Sea (Carlson et al., 1998).
We also observed a DOC increase during our cruise from
background levels (�43 �M at 130 m) to 53-�M DOC
near the surface (based on one DOC profi le obtained at
our last station, R14F, on 30 November 2005). This difference
suggests that the DOC increase was due to the
release of nonchromophoric material by Phaeocystis antarctica.
Field results support this supposition, showing
that the main DOC produced by Phaeocystis antarctica
is carbohydrates (Mathot et al., 2000).
The temporal and spatial decoupling between DOC
and CDOM cycles in the Ross Sea photic zone indicate
that CDOM was produced later in the season or elsewhere
in the water column. Since evidence from Kieber et al.
(2007) suggests that additional CDOM did not accumulate
later in the season, it was likely produced elsewhere.
Previous studies have shown that sea ice is a rich source of
CDOM (Scully and Miller, 2000; Xie and Gosselin, 2005).
It is therefore possible that the pack ice along the edges
of the Polynya was an important source of CDOM in the
photic zone, especially the bottom of the ice, which was
visibly light brown with ice algae. It is also possible that
CDOM was produced below the photic zone and reached
the photic layer via vertical mixing. Several lines of evidence
indicate that Ross Sea Phaeocystis antarctica may
be exported out of the photic zone (DiTullio et al., 2000;
Rellinger et al., in press) and perhaps reach the ocean fl oor
(�900 m) (DiTullio et al., 2000), as seen in the Arctic.
Arctic algal blooms often occur in the early spring when
the water is still too cold for signifi cant zooplankton grazing
and bacterial growth (Overland and Stabeno, 2004).
As a consequence, the algae sink out of the photic zone
and settle to the ocean fl oor rather than being consumed,
thereby providing a DOM source for long-term CDOM
production in dark bottom waters. A similar phenomenon
may occur in the Ross Sea. The possibility that CDOM
may be generated in deep waters, with no photobleaching,
might explain the high photosensitizing capacity of
this CDOM for important reactions like DMS photolysis
(Toole et al., 2004).
Deepwater production of CDOM in the Ross Sea
would explain why our a325 values are comparable to
those observed by Nelson et al. (2007: fi gs. 8– 9) in deep
water (�0.1– 0.2 m �1 ) and nearly the same as would be
predicted on the basis of values in Antarctic Bottom Water
and Antarctic Intermediate Water (average of �0.13– 0.14
m �1 ). Although speculative, it is possible that CDOM
is exported from the Southern Ocean to deep waters at
temperate- subtropical latitudes, which would be consistent
with CDOM as a tracer of oceanic circulation ( Nelson
et al., 2007).
CONCLUSIONS
Despite the necessity of understanding the spatial
and temporal distributions of CDOM for remote sensing,
photochemical, and biogeochemical applications, few
measurements have been made in the Southern Ocean.
Our results indicate that Antarctic CDOM shows spectral
properties that are intermediate between what is observed