Figure 21. Effect of stepwise additions of humic acid on the spectral reflectance of a 16 liter lake
water sample with an Anabaena sp. bloom held in a laboratory tank with lamp illumination (see
text). Chl a concentration was 116.3 µg/l. Absorption (m
-1 ) values at 440 nm are shown for each
step. Inset shows the effect of the humic acid additions on band ratios for two chl a algorithms.
Attenuation by CDOM is through absorption, although these materials intergrade
with colloidal scale, detrital particles. The organic tripton (particulate detritus) fraction
commonly has absorption spectra resembling the shape of CDOM absorption spectra
(Dekker, 1993; Mobley, 1994; Rijkeboer, 1998; Brando and Dekker, 2003). The
magnitude of organic tripton absorption may exceed that of CDOM in some coastal
waters. In a transect of the St. Marys River in Georgia (Figure 22), CDOM dominated
lower salinity, upstream stations (about 20-30 mg/l as dissolved organic matter) and
organic tripton averaged about 2-3 mg/l in the same reach. However, organic tripton
averaged about 10 - 15 mg/l in the lower estuary and CDOM levels were diluted to
about 4-6 mg/l (Alberts et al., 2004). Typically, terrigenous organic matter is
conservatively diluted in the estuarine mixing zone. At reduced flows and greatly
increased residence times in the St. Marys estuary, CDOM dilution was nonconservative in the low to mid-salinity reach (Figure 22). Non-conservative mixing
patterns were attributed to the formation of fine particulates as freshwater mixes with
seawater (Alberts and Griffin, 1996). These particulates are commonly clay-detritus
aggregates, with bacterial colonization and enriched nitrogen to carbon ratios (Lind and
Davalos, 1990; Carlough, 1994). Many drainages have seasonal variations in discharge
and CDOM concentrations. In Georgia coastal rivers, higher flow conditions are
coincident with higher CDOM concentrations (Figure 22; also see Alberts and Filip,
1994), and the slope of the CDOM loading rate versus discharge relationship can
exceed 1.0. Thus, higher discharge rates can create larger offshore plumes with higher
CDOM concentrations. Estuarine and near shore waters may thus experience marked
spatial and temporal variations in this optical constituent, resulting in strong effects on
the water reflectance characteristics used to discriminate chlorophyll signals. Offshore
waters generally have much lower CDOM levels than estuaries, with ABS 440 values of
56
Schalles
water sample with an Anabaena sp. bloom held in a laboratory tank with lamp illumination (see
text). Chl a concentration was 116.3 µg/l. Absorption (m
-1 ) values at 440 nm are shown for each
step. Inset shows the effect of the humic acid additions on band ratios for two chl a algorithms.
Attenuation by CDOM is through absorption, although these materials intergrade
with colloidal scale, detrital particles. The organic tripton (particulate detritus) fraction
commonly has absorption spectra resembling the shape of CDOM absorption spectra
(Dekker, 1993; Mobley, 1994; Rijkeboer, 1998; Brando and Dekker, 2003). The
magnitude of organic tripton absorption may exceed that of CDOM in some coastal
waters. In a transect of the St. Marys River in Georgia (Figure 22), CDOM dominated
lower salinity, upstream stations (about 20-30 mg/l as dissolved organic matter) and
organic tripton averaged about 2-3 mg/l in the same reach. However, organic tripton
averaged about 10 - 15 mg/l in the lower estuary and CDOM levels were diluted to
about 4-6 mg/l (Alberts et al., 2004). Typically, terrigenous organic matter is
conservatively diluted in the estuarine mixing zone. At reduced flows and greatly
increased residence times in the St. Marys estuary, CDOM dilution was nonconservative in the low to mid-salinity reach (Figure 22). Non-conservative mixing
patterns were attributed to the formation of fine particulates as freshwater mixes with
seawater (Alberts and Griffin, 1996). These particulates are commonly clay-detritus
aggregates, with bacterial colonization and enriched nitrogen to carbon ratios (Lind and
Davalos, 1990; Carlough, 1994). Many drainages have seasonal variations in discharge
and CDOM concentrations. In Georgia coastal rivers, higher flow conditions are
coincident with higher CDOM concentrations (Figure 22; also see Alberts and Filip,
1994), and the slope of the CDOM loading rate versus discharge relationship can
exceed 1.0. Thus, higher discharge rates can create larger offshore plumes with higher
CDOM concentrations. Estuarine and near shore waters may thus experience marked
spatial and temporal variations in this optical constituent, resulting in strong effects on
the water reflectance characteristics used to discriminate chlorophyll signals. Offshore
waters generally have much lower CDOM levels than estuaries, with ABS 440 values of
56
Schalles