surficial and submarine freshwater discharges in a tidal...
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1Hellenic Centre for Marine Research (HCMR)Institute of Oceanography46.7 km Athens-Sounio Avenue19013 Anavyssos, GreeceE-mail: [email protected]
1 1 2 2 1 1A.P. Karageorgis , D. Georgopoulos , V. Karathanassi , S. Ioannidou , V. Papadopoulos , C. Tsabaris1 1 2 2 1 1A.P. Karageorgis , D. Georgopoulos , V. Karathanassi , S. Ioannidou , V. Papadopoulos , C. Tsabaris
Surficial and submarine freshwater discharges in a tidal Mediterranean embayment: hydrology and seawater optical characteristics
Surficial and submarine freshwater discharges in a tidal Mediterranean embayment: hydrology and seawater optical characteristics
INTRODUCTIONFreshwater, groundwater and coastal seawater interactions have received considerable attention during the past decade, due to increasing interest regarding basin-scale inputs into the coastal zone. Submarine groundwater discharges (SGD) have been studied in the last few years only, as important pathways of freshwater, nutrients, and other inorganic and organic substances into the sea. Here we present information from the nationaly funded project ‘IRIS’ regarding the influence of freshwater inputs from coastal springs and SGD on: (1) temperature, salinity, chlorophyll-a, and particulate matter, and (2) optical seawater characteristics, as well as, the spatial and temporal distribution fields of the aforementioned parameters.
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REGIONAL SETTINGThe study area (Agios Stefanos Bay) is situated at the northern sector of south Evvoikos Gulf, in central Greece (eastern Mediterranean Sea); it is an area well-known for its strong tidal currents (Euripus Strait) and the recently identified SGD (Fig. 1).
RESULTS AND DISCUSSIONThe results revealed that the inflow of surficial and submarine freshwater (brackish, salinity 6-7 psu) discharges affected substantially the horizontal distribution of the parameters measured in terms of: (1)
-1lower temperature (min 14.7 °C); (2) lower salinity (min 28.3 psu); (3) high PMC (max 8.5 mg l ); and (4) -1high chlorophyll-a concentration (max 3 g l ) (Figs 2, 3). Freshwater flowing along the coast from small
sub-aerial springs was distributed over the area as a thin surficial film (~1 cm), with decreasing trend as the distance from the sources increased. By contrast, SGD appeared as permanent gyres of a 3-5 m diameter, which were visible clearly by bare eye (Fig. 4); increased turbulence and rapid mixing was a major characteristic observed, resulting in high PMCs. Probably this environment favors mussel growth on the seabed (Figs. 5, 6).Moreover, processing of radiance and reflectance data revealed the most appropriate wavelengths for the spectral detection of the SGD (488.20-573.33 and 552.85-769.70 nm for radiance and reflectance, respectively).
In addition, the evaluation of groundwater pathways and fluxes into the coastal zone can be made with the 222use of natural tracers. Rn is an excellent tracer due to its large enrichment in SGD, unreactive nature and
short half life (3.83 days). So, a test measurement has been performed with a submarine gamma-ray detector (K-A-TE-RINA) at the same study area. Preliminary results provide significant information about the radon
222 214 214concentration (Bq/m3) detected by the gamma ray emission of Rn daughters ( Bi and Pb), which appears to be highly enriched in SGD (Fig. 8)..
Preliminary results obtained from the airborne sensor TABI revealed clearly the plume of cold surficial freshwater inputs (Fig. 7).
2Laboratory of Remote SensingNational Technical University of Athens9 Heroon Polytechniou15780 Athens, GreeceE-mail: [email protected]
National Technical
University of Athens
Athens
Figure 1. Study area location map (Google Earth) and sampling
stations (yellow rhombs) occupied during June
2005. Submarine groundwater discharge points are illustrated in
red arrows.
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METHODSOceanographic measurements were conducted during April 2005 and June 2005, using standard field equipment (portable SeaBird CTD with transmissometer and optical backscatter sensors, and GER 1500 radiometer). Water samples were used for the determination of particulate matter (PMC) and chlorophyll-a concentrations; the
Radiance and reflectance of surface seawater was also measured.
salinity of the upper 1-cm seawater was determined by a salinometer.
Figure 2. Spatial distribution of temperature (left) and salinity (right) at the surface waters of Agios Stefanos Bay, during June 2005.
Figure 3. Spatial distribution of particulate matter concentration (left) and chlorophyll-a concentration (right) at the surface waters of Agios Stefanos Bay
during June 2005, and April 2005, respectively.
Session O5: Coastal Oceanography and Inland Waters, Abstract No. 776Session O5: Coastal Oceanography and Inland Waters, Abstract No. 776
Figure 4. Turbulent gyres generated by SGD at the eastern sector of Agios
Stefanos Bay in south Evvoikos Gulf. These features are characterized by lower temperature and salinity, and
higher particulate matter and chlorophyll-a concentrations than the
ambient seawaters.
Hellenic Centre for
Marine Research
Institute of Oceanography
Figure 5. A local fisherman digs the shallow muddy seabed to collect mussels, into the area where fresh
spring water and submarine groundwater discharge.
Figure 6. The seabed near the freshwater discharge sector was
generally muddy and mussel beds were abundant. Sediment resuspension due
to high turbulence was intense and visibility was very low.
In situ test for submarine groundwater discharge by using the K-A-TE-RINA underwater detector
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222Rn results: measured activity: 1450 Bq/m
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activity at open sea: 3-5 Bq/m33
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222Rn)
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Figure 8. In-situ test for SGD using the K-A-TE-RINA underwater
detector.
Figure 7. Image acquired with the airborne thermal sensor TABI (8-12
ìm) during June 2005. The freshwater plume is identified by its
darker color, indicating lower sea surface temperature.
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