Rapp. Comm. int. Mer Médit., 37,2004
125
OPTICAL AND THERMOHALINE CHARACTERISTICS OF THE SOUTH ADRIATIC WATER IN JULY 2003
Morovic Mira*, Matic Frano, Branka Grbec
Institute of Oceanography and Fisheries; Šetalište Ivana Meštrovica63, P.O. Box 500, 21000, Split, Croatia - * morovic@izor.hr
Abstract
Vertical irradiance attenuation coefficients were determined for 14 wavelengths from recent spectral irradiance measurements in the South
Adriatic, during Meduza experiment. The first results enabled optical and thermohaline characterization of the water masses.
Key words: spectral irradiance attenuation; ?uorescence; thermohaline conditions.
Introduction
Thermohaline, chemical and biological conditions in the
intermediate layer of the South Adriatic are in?uenced by in?ows of
more saline (1), warmer and nutrient richer LIW from the
Mediterranean. These intrusions fluctuate, causing increase of
temperature, salinity and productivity in the waters of the South
Adriatic, in?uencing also optical properties (2). Summer season, like
this measuring period is generally characterized by weaker in?uence
of LIW, entering the Adriatic in the intermediate layer only.
Measurements were performed in the frame of international
experiment Meduza (see www.izor.hr/meduza/index.htm) onboard the
35m long research vessel
, from the Croatian Ministry of
Science.
Materials and methods
The spectral irradiance and radiance were measured with
Biospherical profiling radiometer at 14 wavelengths in the range 340-
710nm, together with the PAR attenuation, and natural ?uorescence.
The CTD measurements were taken with the Idronaut probe. The
cruise from 22-29 July 2003 was confined to the area above 1200m
isobath, within 5miles around the position 42.17
°
N 17.82
°
E. The
daylight attenuation measurements were performed three times, while
CTD casts and other parameters (not presented here) were taken four
times a day. Since measurements were repeated daily in the same
time, aparent optical data with the same solar altitudes were acquired.
The measurements were interupted from the noon time 25th till the
morning 27th July, due to unconvenient weather conditions.
Results
Vertical attenuation coefficients for downwelling irradiance were
about two times higher than the coefficients for upwelling radiance,
but their spectral dependence is very similar. Considering the different
solar elevation, attenuation was stronger for the lower solar elevation
(3). In the investigated water column, between 0-80m, distinguished
are spectral characteristics by layers (Fig. 1).
Fig 1. Vertical attenuation coefficients for downwelling irradiance and
upwelling radiance for 24.07.2003, measured at the noon time.
Both attenuation coefficients for downwelling irradiance and
upwelling radiance have in the first 10m maximum in the red
spectrum, due to strong apsorption in water. Attenuation coeficients
have minimum at 490nm, showing that South Adriatic is most
transparent for this nuance. Attenuation coefficients are increasing
from the blue light toward the UV wavelengths. Especially in the deep
layers, coefficients for UV are high, while the minimum is reached in
the first few meters.
Since the area of measurements is far from the coast in the
oligotrophic sea, it is assumed that principal factor in?uencing optical
properties is phytoplankton, and eventualy its decomposing products.
Therefore, part of attenuation in UV and blue spectral region come
from chlorophyll.
From the natural ?uorescence channel Lu(chl), vertical profile was
obtained (Fig. 2). In addition to exponential decrease of Lu(chl), in
deep layers, increased upwelling signal from ?uorescence appears
from the first deep chlorophyll maximum. Depth of more intense
?uorescence signal moved around 60 to 80m during the measuring
period. These data, together with the rate of PAR attenuation enabled
calculating ?uorescent ?ux (4).
Fig 2. Natural ?uorescence signal determined from upwelling radiance
Lu (Chl) during 22-28.07. period at 18h.
The thermohaline conditions during the cruise were characterized
with very warm and stratified surface layer (around 27
°
C). Relatively
shallow thermocline for the mid-summer at only 10m is caused by the
lack of stronger wind action this summer. Salinities were high
(>38.7), but within the usual range for the recent period and for the
summer season.
Conclusions
The surface 80 meters of the South Adriatic are the most
transparent for 490nm, as expected for the oligotrophic waters. Depth
dependence of attenuation coefficients and natural ?uorescence
indicate the presence of higher chlorophyll concentrations in the deep
layers around 60-80m.
Optical measurements performed for the first time in the Eastern
part of Adriatic waters with sofisticated instrumentation, opened new
possibilities for obtaining high resolution vertical profiles of
chlorophyll concentration, and primary production estimates.
References
1-Zore-Armanda, M., 1969. Water exchange between the Adriatic and
the Eastern Mediterranean. Deep-Sea Research, 16: 171178.
2-Marasovic I., Grbec, B., Morovic, M.,1995. Long-Term Production
Changes in the Adriatic. Netherlands Journal of Sea Research,34(4): 267-
273.
3-Lundgren, B. and N. Hřjerslev, 1971. Daylight measurements in the
Sargasso sea; Results from the «Dana» expedition January-April, 1966,
Copenhagen University, Institut od Physical Oceanograph, Report No14:
1-33.
4-Chamberlin, W.S., C.R. Booth, D.A. Kiefer, J.H. Morrow, and R.
C.Murphy, 1990. Evidence for a simple relationship between natural
?uorescence, photosynthesis and chlorophyll in the sea. Deep-Sea
Research, 37(6): 951-973.
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