Rapp. Comm. int. Mer Médit., 37,2004
110
SEASONAL DYNAMICS OF PARTICULATE MATTER IN THE NE MEDITERRANEAN SEA:
ANALYSIS OF A HISTORICAL (1991-2001) DATA SET OF LIGHT TRANSMISSION MEASUREMENTS
Karageorgis A.P.
1*
, Gardner W.D.
2
, Mishonov A.V.
2
, Krasakopoulou E.
1
, Anagnostou Ch.
1
1
Hellenic Centre for Marine Research, P.O. Box 712, 19013 Anavyssos, Greece - * ak@ncmr.gr, ekras@ncmr.gr, chanag@ncmr.gr
2
Department of Oceanography, Texas A&M University, College Station, TX 77843, USA. wgardner@ocean.tamu.edu,
avm@ocean.tamu.edu
Abstract
Light transmission measurements in the NE Mediterranean, during the period 1991-2001 are homogenized and interpreted. The period of
high precipitation and river freshwater discharge (winter-spring) is characterized by high beam cp values in the Northern Aegean Sea and
coastal areas of Greece at the surface (0-50 m) waters. The Rhodes cyclonic gyre is related to relatively higher beam cp due to an increase
in productivity. Summer-autumn conditions exhibit more relaxed features and generally less turbid waters. Beam cp is markedly correlated
with particulate matter concentration and particulate organic carbon, enabling the definition of empirical equations to be used in future
projects, budget estimates and modeling.
Keywords : light transmission, particulate matter concentration, particulate organic carbon, NE Mediterranean Sea
Introduction
Light attenuation is an important parameter providing information
on the quantity and dispersal patterns of particulate matter (PM)
throughout the water column. PM dynamics studies have attracted the
scientific community’s interest in large multidisciplinary projects, i.e.
the JGOFS North Atlantic Bloom Experiment [1, 2]. However, within
the NE Mediterranean Sea, similar investigations have focused only in
specific geographical regions, i.e. the NW Aegean Sea [3], and the NE
Aegean Sea [4]. A major objective of the present communication is to
study, for the first time, particulate matter dynamics over the entire NE
Mediterranean Sea. For this purpose, the historical data set of light
transmission measurements, obtained by the Hellenic Centre for
Marine Research (HCMR) during 1991-2001, was analyzed and
interpreted.
Methods
The initial data set comprised light transmission measurements
conducted in parallel with routine CTD casts in 3136 stations (Fig. 1).
Data have been obtained from 40 cruises of 14 research projects on
board the R/V Aegaeo. Metadata, light transmission readings, and
beam c
p
(attenuation due to particles, m
-1
) were stored in the ‘Ocean
Data View’format [5] for archiving and visualization.
Particulate matter concentration (PMC, 1689 samples) and
particulate organic carbon (POC, 638 samples) data obtained over the
period 1997-2001 were utilized to calibrate optical measurements
against those parameters.
Fig. 1. Light transmission stations obtained during 1991-2001 by HCMR
in the NE Mediterranean.
Results and discussion
Transmissometry
A preliminary assessment of beam c
p
distribution in the NE
Mediterranean Sea shows the following:
1.The surface nepheloid layer (at 5-m depth, SNL) exhibits the
highest signal in c
p
during the winter-spring (wet) period. The
continental shelf of the Northern Aegean Sea appears to be the most
turbid water area. This feature is attributed to the particulates
introduced from a number of rivers (Axios, Aliakmon, Pineios,
Strymon, Nestos, and Evros), which discharge into the area.
2.Relatively high c
p
values appear in other coastal areas, such as
the gulfs of Patras, Corinth, Kyparissiakos, and Saronikos.
3.Low c
p
values generally characterize the deep sector of the N.
Aegean Sea, the central Aegean Sea, the Cretan Sea and the Ionian
Sea, indicating minimal terrigenous supply and oligotrophic
conditions.
4.The Rhodes gyre (cyclone) is an area of elevated beam c
p
signal,
attributed to increased productivity due to the presence of nutrient-
rich upwelling waters [6].
5.The aforementioned features can be observed at 20 m and 50 m
depths. From 100 m depth up to 500 m depth beam c
p
distribution is
more homogeneous all over the NE Mediterranean Sea and reaches
minimum values.
6.The dry period (summer-autumn) of low precipitation and low
river discharge is characterized by more relaxed c
p
signal. Relatively
high beam c
p
values appear only at the surface waters of the Northern
Aegean Sea, whilst deeper waters maintain decreased c
p
values.
Beam c
p
vs. PMC and POC
Linear regression between c
p
and PMC revealed a marked
correlation (r=0.829, n=1689). Likewise, c
p
was strongly correlated to
POC (r=0.819, n=638). The overall positive and marked correlation of
the aforementioned parameters may enable the definition of empirical
functions relating optical measurements to PMC and POC. This could
be useful for the estimate of PM and POC budgets and/or their use in
models.
Conclusions
The analysis of historical light transmission measurements from the
NE Mediterranean Sea was successful. Overall, we were able to diffe-
rentiate distribution patterns between wet and dry seasons, in order to
estimate the seasonal variability of PM in the NE Mediterranean. Ele-
vated and/or relaxed features of beam c
p
were identified and directly
related to physical processes affecting PM distribution in the water
column.
The correlation of optical measurements vs. PMC and POC is
noticeable and may result to the definition of global (for the NE
Mediterranean) equations, to be used in future research projects,
estimations of budgets and in modeling exercises.
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