Geoscience Reference
In-Depth Information
seagrass detritus, but not from chambers placed over bare sand. The similarity of temper-
ature dependence of CDOM production to temperature dependence of microbial decom-
position led to the conclusion that production was microbially mediated. The new material
was very readily photodegraded. Part of the refractory CDOM found in tropical estuaries
can be attributed to production in mangrove porewaters (Trembly et al., 2007) based on
molecular as well as optical methods. Shank et al. (
2010b
) found that production of CDOM
from red mangrove leaf litter was highest from mid-senescent orange leaf litter during the
wet season. They also found CDOM production for floating Sargassum mats at potentially
significant rates for oligotrophic ocean waters. The CDOM exported from tidal marshes
in Chesapeake Bay has a similar refractory nature, with red-shifted emission maxima and
lower fluorescence per absorbance (Tzortziou et al.,
2008
). Release of CDOM has been
observed from shallow coral reef environments in the Bahamas (Boss and Zaneveld,
2003
;
Otis et al.,
2004
), either from high productivity on the reef or from remineralization in the
sediments (Burdige et al.,
2004
). Maie et al. (
2006
) found that the CDOM from Florida
Bay was likely to be bacterial in origin, due to blue-shifted fluorescence maxima, whereas
CDOM from nearby regions in the study area was more terrestrial in nature, reflecting a
tide marsh and mangrove source.
Destruction of CDOM in the ocean is due primarily to photodegradation by sunlight.
Several recent studies have shown differential photodegradation of pools of CDOM. In a
study of photodegradation off Ria Vigo, Spain, it was found that peak T fluorescence was
degraded in proportion to decrease in DOC concentration and resulted in the formation
of peak M fluorescence. Humic-like peaks C and A were found to be tracers of refractory
DOC (Lonborg et al.,
2010
). Another study comparing rates of photobleaching found that
freshly produced CDOM from mangrove leaves and Sargassum bleached faster than did
terrestrial or ambient marine CDOM (Shank et al.,
2010a
). Application of CDOM photo-
chemistry has been extended to provide estimates of CDOM degradation rates (Belanger
et al., 2006) CO production (Fichot et al.,
2010
) and surface seawater concentrations of the
partial pressure of carbon dioxide (pCO
2
) (Lohrenz and Cai, 2006).
3.2.3 CDOM in Open Ocean Waters
CDOM concentrations are at minimal levels in the surface of the ocean gyres due to long
exposure to sunlight and low biological activity. Thus, both production of new fluorescent
materials and the consequences of photobleaching are more readily observed in the open
ocean away from the influence of rivers. In the Equatorial Pacific Ocean, diel variability in
the composition and concentration of CDOM was observed in the surface waters (Coble,
unpublished data). Samples collected at dawn showed the presence of both humic-like
(peaks C and A
C
) and protein-like fluorescence (peak B), which was greatly diminished or
absent from at the same station at noon (
Figure 3.6
). The mean concentration observed in
these surfaces waters was 0.3 ppb QSE, but lower values of less than 0.1 ppb QSE were
observed at the Hawaii Open Time Series (HOTS) station (Coble,
1996
).
