OPTICAL ABSORPTION OF CHROMOPHORIC DISSOLVED ORGANIC MATTER IN THE ESTUARY
OF THE PO RIVER (NORTHERN ADRIATIC, ITALY)
D. Berto, V. Zangrando, F. Savelli, M. Giani*
Istituto Centrale per la Ricerca scientifica e tecnologica Applicata al Mare, Brondolo, 30015 Chioggia, Italy - *m.giani@icram.org
Abstract
Dissolved and particulate organic matter (DOC, POC), chlorophyll and salinity were analysed to evaluate the origin of chromophoric
dissolved organic matter (CDOM) in an estuary of the Po. The absorption coefficient (aCDOM) at 280 and 355 nm were not positively
correlated with temperature, salinity and DOC. The values of the slope coefficient of the exponential spectral, S, and the absorption
coefficients aCDOM
280
and aCDOM
355
showed a marine origin of CDOM. To characterise the aCDOM the waters were extracted and the
optical properties of extracts resulted comparable to those of the original waters.
Keywords: chromophoric dissolved organic matter, Po river, Northern Adriatic, optical absorption.
Rapp. Comm. int. Mer Médit., 37,2004
173
Chromophoric dissolved organic matter (CDOM) absorbs light
strongly in UV, limiting the penetration of biologically damaging
radiation into surface waters. In order to evaluate the variations and
the origin of chromophoric dissolved organic matter in the estuary of
Po river, analysis of dissolved and particulate organic matter (DOC,
POC), chromophoric dissolved organic matter (CDOM), and chloro-
phyll and salinity were carried out (1). The sampling was performed
in surface waters from February to May 2003 along an axis extending
from the Po river towards the open sea.
Coastal DOC and POC values ranged from 70 to 263
µ
mol/L and
from 66 to 580
µ
mol / L, respectively. DOC concentration increased
from winter through spring (Fig. 1) mainly as a seasonal response to
the increase of the phytoplankton production and thermohaline strati-
fication. The aCDOM at 280 and 355 nm were not positively correlat-
ed with temperature and DOC, stressing the fact that the accumulation
of DOC from winter to spring was mainly not chromophoric (Fig. 2).
Fig. 1. Temporal variation of mean surface DOC concentration along the
axis Po delta-seawards in winter–spring 2003. Error bars represent the
standard deviation.
High salinity in the coastal waters due to exceptionally low Po river
discharge could explain the lack of linear correlation between chro-
mophoric dissolved organic matter - and salinity as well as between
DOC and salinity. Absorption coefficients at 280 nm, aCDOM
280
, and
355 nm, aCDOM
355
, ranged from 0.46 to 6.45 m
-1
(average 1.79m
-
1
) and from 0.05 to 2.38 m
-1
(average 0.55 m
-1
), respectively. The
slope of absorption spectra, S, derived by exponential fitting, ranged
from values typical of coastal waters in?uenced by rivers to offshore
marine waters: 0.008 to 0.033 nm
-1
(average 0.019 nm
-1
).
High values of the exponential spectral slope coefficient, S, and low
aCDOM
280
and aCDOM
355
values showed a low aromatic content
and a marine origin of CDOM. To isolate and characterise the optical
properties of humic and fulvic acids, seawaters samples were extract-
ed through Amberlite XAD 2 resin column.
The optical properties of the extracted organic fractions were simi-
lar to those of the original water, as evidenced by the similar S values
(2). The high values of S found in the extracts together with their sol-
ubility at pH 2 point out that this fraction is mainly constituted of ful-
vic acid.
References
1-Ferrari M.G., 2000. The relationship between chromophoric dissolved
organic matter and dissolved organic carbon in the European Atlantic
coastal area and in the West Mediterranean Sea (Gulf of Lions). Mar.
Chem., 70: 339-357.
2-Green S., and Blough N.V., 1994. Optical absorption and ?uorescence
properties of chromophoric dissolved organic matter in natural waters.
Limnol. Oceanogr., 39(8): 1903-1916.
Fig. 2. Temporal variation of mean surface aCDOM
355
along the axis Po
delta-seawards in winter–spring 2003.
Error bars represent the standard deviation.
