Environmental Engineering Reference
In-Depth Information
In coastal regions not influenced by large riverine inputs, lower values of the
CDOM absorption coefficient are observed (~0.1 m
-1
at 355 nm) that do not correlate
with salinity
30,33
, suggesting that sources other than the terrestrial ones are involved in
the CDOM production.
CDOM may also be produced by the release from phytoplankton or by diffusion
from sediment porewater
34-35
as well as through atmospheric deposition of CDOM, a
possibility not yet investigated. The lack of correlation between CDOM and
phytoplankton chlorophyll in both laboratory experiments
36
and in field measurements
in coastal regions
19,37
suggests that other sources beside direct phytoplankton input may
be important for the CDOM budget.
On the contrary, in the open ocean, low values of CDOM absorption coefficient
are observed (~0.05 m
-1
at 355 nm). At these locations, a correlation between CDOM
and chlorophyll-a is sometimes, but not always, observed suggesting that CDOM could
be produced
in situ
32,37
, although the clear mechanism of this production is not yet
known.
Although rivers are large sources of terrestrial DOM, this material does not seem
to contribute significantly to the DOM in seawater and in marine sediments
38
. In fact, a
recent field study on the lignin distribution in open oceans has shown that lignin
represents only a few percent of the total DOM in the ocean and has a shorter ocean
residence time than the marine DOM
39
. Similar conclusions have been reached by
Blough and Del Vecchio
20
for CDOM, based on the differences of CDOM absorption
between freshwater and central gyre end-members. Both of these results indicate the
presence of a quite effective sink of terrestrial material during its transit from inshore to
offshore waters. Losses of terrestrial CDOM due to flocculation appear to be
inconsequential, based on the conservative or quasi-conservative mixing that has been
observed for many estuaries. Further, bacterial consumption alone does not appear to be
a substantial sink of CDOM
40
. Other possible sinks are photochemical
degradation
19,28,41-43
and/or photodegradation coupled to bacterial uptake
40,44-45
.
5. Field evidence of CDOM photobleaching
As described above, for regions influenced by high riverine input, CDOM
absorption coefficients often decline linearly with salinity from inshore to off-shore
locations, indicating conservative behavior
14,20,28-32
. However, departures from this
behavior have been also reported
14,46
, due to input or depletion of CDOM or simply to
mixing of water masses with different CDOM end-members.
At some locations (as it occurs in the MAB during the summertime) the vertical
mixing depth decreases, concomitant with the development of a seasonal stratification,
while the CDOM exposure to sunlight increases. Under these conditions, a net decrease
of CDOM absorption is observed in surface waters over a narrow salinity range; in
waters below the thermocline the CDOM absorption is higher and maintains an inverse
linear relationship with salinity (Figure 4). The seasonal surface sink of CDOM has
been attributed to photodegradation
19,28,41
. Studies of CDOM absorbance at high
temporal and spatial resolution have also provided evidence for a surface sink of
CDOM due to photobleaching
43
.
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