A SYNTHESIS OF PRELIMINARY RESULTS FROM OCEANOGRAPHIC STUDIES IN THE SOUTH AEGEAN SEA AND THE STRAITS OF THE CRETAN ARC <B>A SYNTHESIS OF PRELIMINARY RESULTS FROM OCEANOGRAPHIC STUDIES IN THE SOUTH AEGEAN SEA AND THE STRAITS OF THE CRETAN ARC</B> <HR><P> Balopoulos</B><SUP>1</SUP><B>, E., Theocharis</B><SUP>1</SUP><B>, A., Price </B><SUP>2</SUP><B>, B., Heussner</B><SUP>3</SUP><B>, S., Souvermezoglou</B><SUP>1</SUP><B>, A., Kontoyiannis</B><SUP>1</SUP><B>, H., Nakopoulou</B><SUP>1</SUP><B>, C., Varnavas</B><SUP>4</SUP><B>, S. Collins</B><SUP>5</SUP><B>, M. B., Gotsi</B><SUP>1</SUP><B> O., Apostolopoulou</B><SUP>6</SUP><B>, M., Ignatiades</B><SUP>7</SUP><B>, L., Voutsinou</B><SUP>1</SUP><B>, F. &Theodorou</B><SUP>8</SUP><B>, A.</B><I><P><P> 1. National Centre for Marine Research, 166 04 Hellinikon, Athens, Greece.<BR> 2. University of Edinburgh, Department of Geology and Geophysics, Edinburgh EH9 3JW, United Kingdom<BR> 3. University of Perpignan, Laboratory of Sedimentology and Marine Geochemistry, Perpignan, France.<BR> 4. University of Patras, Department of Geology, 26 100 Patras, Greece.<BR> 5. University of Southampton, Department of Oceanography, Southampton SO9 5NH, United Kingdom.<BR> 6. University of Athens, Department of Biology, 157 84 Zografos, Greece<BR> 7. National Research Centre "Demokritos", Institute of Biology, 153 10 Aghia Paraskevi, Greece.<BR> 8. University of Thessaly, Laboratory of Oceanography, Pedion Areos, 383 34 Volos, Greece.</I><P><P> Within the framework of the CEC/MAST supported Project PELAGOS, seasonal multisciplinary oceanographic data were collected in the South Aegean Sea, the Straits of the Cretan Arc and the adjacent open sea regions of the eastern Mediterranean Basin (northwestern Levantine Sea, southeastern Ionian Sea), during 1994-1995. Preliminary analysis of the data provide an improved understanding of the hydrodynamics and biogeochemistry of the area and reveals that important changes occur in the waters of the Eastern Mediterranean region, which may have a significant impact to the climatology of the overall Mediterranean Sea.<B><P><P> Introduction</B><P><P> The Cretan Sea and the Straits of the Cretan Arc form an important part of the control of the exchanges of water and mass (diluted, suspended or near-bed) between the Aegean Sea and the adjacent open sea regions of the Eastern Mediterranean Basin (Levantine Sea, Ionian Sea). In particular, fluxes through the Straits of the Cretan Arc are considered of great importance, since the regions on either side of the Straits are dominated by powerful physical and biogeochemical processes. Within the framework of the research Project PELAGOS (supported by CEC/MAST) intrdisciplinary oceanographic studies were carried out in the above mentioned areas. Scope of the studies was to obtain a better knowledge of the hydrodynamics of the South Aegean Sea and the Straits of the Cretan Arc, in order to improve understanding of the exports of water and associated material in the Eastern Mediterranean Basin. In this contribution, a synthesis of the preliminary results obtained through the PELAGOS project is attempted and important hydrodynamic and biogeochemical features of the study area are discussed.<B><P><P> Methodology</B><P><P> Seasonal oceanographic cruises, suplemented by a number of specialised surveys were carried out in the South Aegean Sea, the Straits of the Cretan Arc, the northwestern Levantine and the southeastern Ionian, during 1994-1995. An effort was made to combine fieldwork of PELAGOS with data collection, in the same area, by other national and international programmes, so as to cover a larger area. The PELAGOS field measurement programme involved the collection of a great diversity of oceanographic data, including: (i) measurements of CTD and currents (by self recording current meters and a ship mounted acoustic Doppler current profiler); (ii) analyses of water samples for dissolved oxygen, salinity, nutrients (phosphate, nitrite, nitrate, silicate), trace elements (Fe, Mn, Co, Ni, Cr, Cu, Zn, Pb, Cd), and dissolved phases of radioisotopes; (iii) analyses of suspended particulate matter for major and trace elements (Al, Ti, Ca, Sr, Li, Cl, Mg, K, Na, Si, Fe, Mn, Cu, Zn, Ni, Pb, Cd, P, S, Ba, I), radioisotepes (<SUP>2</SUP><SUP>1</SUP><SUP>0</SUP>Pb, <SUP>2</SUP><SUP>1</SUP><SUP>0</SUP>Po, <SUP>2</SUP><SUP>3</SUP><SUP>4</SUP>Th, <SUP>2</SUP><SUP>2</SUP><SUP>6</SUP>Ra), organic compounds, POC and PON, supplemented by morphological, textural and mineralogical description of suspended particulate matter; (iv) measurements of primary production, photosynthetic pigments, phytoplantkton cell counts along with size fractionated biomass estimates and zooplankton qualitative and quantitative determinations; and (v) analyses of settling samples (collected by time-series sediment traps) for major and trace elements (Si, Al, Fe, Mn, Ca, K, Mg, P, I, Ca, Cd, Pb, Zn, Ni, etc.), rare earths (La, Ce, Pr, Nd, Sm, Eu, Cd, Tb, Dy, Ho, Er, Tm, Yb, Lu) and radioisotopes (<SUP>2</SUP><SUP>1</SUP><SUP>0</SUP>Pb, <SUP>2</SUP><SUP>1</SUP><SUP>0</SUP>Po, <SUP>1</SUP><SUP>3</SUP><SUP>7</SUP>Cs), in conjuction with biogeochemical analyses of organic fraction. The methodology applied for data collection and analysis has been extensively described, for each data type individually, elsewhere (Balopoulos, 1994).<B><P><P> Results</B><I><P><P> Water Circulation, Structure and Chemical Composition.</I><BR> Analysis of CTD data provided a strong evidence of a new hydrological and dynamical regime, over the study area. In addition, it confirmed that, as it has been suggested by previous investigations, a multiscaled circulation pattern and a complex hydrographic structure prevails in the region. The general circulation does not show any significant seasonal signal. Permanent and recurrent cyclonic and anticyclonic gyres and eddies are interconnected by jets and currents. The exchange pattern through the Straits of the Cretan Arc is far more complicated than the traditional picture of inflow of Levantine Intermediate Water from the eastern straits and outflow of Aegean water from the western straits. It seems that synoptic scale variability, in relation to mesoscale activity, are dominant in the upper layers of the straits area. In addition, distributions of chemical components dissolved in sea water (nutrients, trace elements) confirm that the different scale hydrodynamic features dominant in the region of the South Aegean Sea and the adjacent open sea regions, have a significant chemical signature. Main results obtained within the framework of the PELAGOS Project, are presented and briefly discussed below. <P><P> Low salinity Modified Atlantic Water (MAW) coming from the western Ionian Sea and flowing within the surface and subsurface layers enters the Cretan Sea, through mainly the Antikithira and occasionally the Kassos Straits. The former inflow of MAW into the Cretan Sea is confirmed by long-term current meter data recorded in the surfrace layer of the Antikithira Strait. Low salinity surface waters of Black Sea origin are identified in the northwestern Cretan Sea of their influence being occasionally extended up to the Kitherian straits. High salinity and warm surface and subsurface waters of Levantine origin are transported, by two branches of the intense Asia Minor Current (AMC), towards the South Aegean Sea, through the Rhodos and Karpathos Straits and thus influencing greatly the surface and subsurface layers of the overall southeastern Aegean Sea. This view is in agreement with the results of long-term current meter measurements in the above mentioned straits, although upper layer water circulation in the Karpathos Strait appears to be more variable. Instantenous vertical profiles of current velocity obtained at several stations across the straits, showed that the structure of the flow field of the upper 200 m of the water column is, in most cases, complex, characterized by steep current speed gradients at 70-80 m depth.<P><P> The sub-surface and upper intermediate layers of the Cretan Sea are occupied by saline intermediate water masses, the characteristics of which vary locally, and, thus represented at different areas in the Q/S diagram. Consequently, are distinguished the known Levantine Intermediate Water (LIW), that enters the Aegean Sea through the Straits from both the Levantine and the Ionian Sea, as well as, the more saline Cretan Intermediate Water (CIW) and the colder Mirtoan Intermediate Water (MIW). An important new structural feature of the Cretan Sea is the presence of a well-defined intermediate layer of minimum salinity and temperature [the so-called Transition Mediterranean Water (TMW)], the appearance of which might be directly related to the Cretan Dense Water (CDW) outflow towards the deep and intermediate layers of the Eastern Mediterranean (see below). The aforementioned water mass is also characterized by remarkably higher nutrient and lower oxygen concentrations than those normally measured, at the same depths of the Cretan Sea, during the last ten years, within the framework of other research Projects. Furtheremore, it seems to have a small seasonal variability.<BR> The most striking and important feature of the deep layers, is the observed CDW outflow towards the open sea regions of the Eastern Mediterranean Basin. Vertical distributions of the water property, along various sections of stations, indicate that this water outflow takes place mainly through the Kassos and Antikithira Straits. This is clearly reflected in current meter data records collected in the above mentioned straits, for a period of one year. These current data indicate that the water flow throughout the year was notably towards a southeastward direction at the Kassos Strait and in southwestward direction at the Antikithira Strait. They further reveal that, although maximum current speeds at both the above mentioned straits are of the same order of magnitude (~45 cm/sec), outflow through the Kassos Strait is more intensive, since mean current speeds at this strait are significantly higher than the mean current speeds at the Antikithira Strait. Water property distributions further suggest that the CDW outflowing through the above mentioned straits sinks and tends to displace the Eastern Mediterranean Deep Water (EMDW) of Adriatic origin, from the adjacent open sea regions, outside the Aegean Sea, and thus resulting to the formation of a denser EMDW. As the newly formed CDW was found to be rich in oxygen and poor in nutrients, outflowing through the straits, substantially contributes to the renewal of the deep waters of the eastern Mediterranean Sea.<P><P> Concentrations of trace elements in sea water are generally small, ranging between 0.010-0.605 g/l for Cu, 0.008-0.068 g/l for Cd, 0.040-0.753 g/l for Ni, 0.004-0.310 g/l for Co, 0.010-0.903 g/l for Mn and 0.008-0.309 g/l for Cr. Vertical and spatial distributions of metals seem to be directly affected by the general circulation and hydrological structure and, for all metals, are characterized by very small seasonal variability (Fig. 1).<I><P><P> Seasonal Variability of Pelagic Biomass.</I><BR> Biological studies confirmed the extremely oligotrophic character of the study area. Levels of primary production were very low, ranging from 0.036 to 0.290 mg C m<SUP>-</SUP><SUP>3</SUP>.h<SUP>-</SUP><SUP>1</SUP> and having an average of 0.121 mg C m<SUP>-</SUP><SUP>3</SUP>.h<SUP>-</SUP><SUP>1</SUP>. There was no consistent pattern in the vertical distribution of primary production, although, in certain cases, maxima were noted between 1m and 10 m depth and minima at the surface microlayer. Seasonal fluctuations of primary production were significant, with maxima in March and December and minima in June and September. Chlorophyll-á concentrations were also very low (only rarely exceeding 0.5 g/l) and presented similar patterns of vertical distribution throughout the study area. Chlorophyll-á maxima were observed at surface in winter and sub-surface (75-100 m) in summer, while during the other seasons an almost homogeneous structure, extending from the surface down to 100 m, prevailed, in most cases. Very low concentrations were also observed for phytoplankton, which did not indicate any distinct seasonal or spatial pattern. The same lack of pattern was also detected in the vertical distribution of phytoplankton and only a tedency was noted for higher concentrations at the subsurface layer (50-75m), especially during summer. Coccolithophores were the most dominant groups in March and September, dinoflagellates and diatoms in June, whereas diatoms and others prevailed in winter. Highly diversified phytoplankton communities thrive, throughout the study area. These are characterized by low dominance indices, high diversities and high species number per sample. Zooplankton showed a clear seasonal pattern of highest abudance in September and December (when the superficial layer proved more rich), declining to minimal abudance in March and June (when the more rich layer was between 100-0 m). The 300-100 m layer proved at all cases the most poor. Due to the abudance of large zooplanktonic forms at the 100-0 m layer, this exhibited, at almost all cases, higher biomass values than the surface layer. No important spatial differences and clear quantitative differentiation was seen for the region. Copepods dominated absolutely to mesozooplankton asseblages, of their contribution to the total zooplanktonic community, ranging from 71% to 76%. <I><P><P> Biogeochemistry of Suspended Particulate Matter and Settling Particles.</I><BR> Suspended particulate matter (SPM) analyses indicated that Al concentrations are very low, rarely exceeding 1.5 g/l and much lower than in oceanic waters at continental margins. Al highs are noted at 200 m and ~700 m suggesting major contributions from sediment resuspensions at the tope of the slope and basin sides. Horizontal supply of Al is probably much more important than direct input to the basin. Biogenic silicon (Sibio) estimates show a much higher concentration in winter and its distribution conforms to that of chlorophyll-á highs in photic waters, and to the presence of cyclonic eddies that allows some water to upwell. Distribution of particulate Ca in the waters shows an extremely close association with Al and indicates that most of this element in the waters has been introduced to the basin by sediment resuspension rather than from direct fallout of biological remains. Semi quantitative Corg and Ntot analysis indicates higher particulate organic carbon and nitrogen in surface waters (~25 g/l) compared with intermediate and deep waters (10-15 g/l). C/Ntot ratios are very variable but suggest some increase with water depth. Patterns of Mg, K and Ti to Al ratios show well defined increases in the particulate matter of the photic zone, and indicate that Mg, K and possibly Ti are taken up in biological systems. Distribution of Fe shows that for much of the water column iron oxyhydroxide probably resides as coatings on detrital aluminosilicates. Distribution of particulate Mn is extremely low within the photic zone and increase dramatically below it, and is similar to that seen in many oceanic profiles. Marked enrichements of Sr found in the upper part of the water column, combined with increased concentrations of SPM and primary production, at this depth level, support strong contribution of biogenic carbonates. Higher concentrations of Sr found in winter than in summer suggest greater biogenic contribution with carbonate affiliation, in winter than in summer. Copper concentrations in SPM are low and show a tedency to increase in the surface/subsurface layer, along with Sr and SPM, which may suggest that its vertical variations are at least partly controlled by biological processes.<BR> Sediment traps, deployed in Antikithira Strait, measured the lowest mass fluxes ever registered in the world ocean, confirming the oligitrophy of the area. Very low mass fluxes, on either sides of the Strait [less than 1 mg m<SUP>-</SUP><SUP>2</SUP>d<SUP>-</SUP><SUP>1</SUP> in the 500-800 m water layer and 2-3 mg m<SUP>-</SUP><SUP>2</SUP>d<SUP>-</SUP><SUP>1</SUP> in the deeper layer (1000-1300 m)] were seen during the maximum summer stratification (June-September) of the water column, while maximum mass fluxes at all sites were noticed from September to mid-November (Fig. 2). The first sequence showed a pronounced marine character, whereas the second presented a more terrigeneous signal. During the trap experiment it was indicated that the Aegean deep waters exported matter and energy to the Ionian Sea through the Antikithira Strait. The amount of total exported material that settles on the Ionian side represents 40 mg m<SUP>-</SUP><SUP>2</SUP> d<SUP>-</SUP><SUP>1</SUP> with about 2 mg m<SUP>-</SUP><SUP>2</SUP>d<SUP>-</SUP><SUP>1</SUP> of organic carbon. Close examination of the results obtained, some become puzzling if not paradoxal: (i) the organic carbon content in particles settling in the deep water layers is relatively high (3.5%-12%) indicating a significant organic input to the benthic system; during the period of highest stratification (June-September), near-bottom mean organic carbon fluxes amounted to 1.4 mg m<SUP>-</SUP><SUP>2</SUP> d<SUP>-</SUP><SUP>1</SUP> on the Aegean side and 8.7 mg m<SUP>-</SUP><SUP>2</SUP> d<SUP>-</SUP><SUP>1</SUP> on the Ionian side (ii) the presence of numerous fresh diatoms and, in time, silicoflagellates mixed with the refractory material, imply a rapid export from surface waters (iii) the coccolithophorids, which are most often the dominant group in local phytoplankton, are continuously recorded but not well-preserved, except in the fecal pellets which also contribute to the biogenic character of the settling particles in the area. <B><P><P> References</B><P><P> Balopoulos E.Th. (1994). PELAGOS - Hydrodynamics and Biogeochemical Fluxes in the Straits of the Cretan Arc (Aegean Sea, Eastern Mediterranean Basin), First Annual Scientific Progress Report (September 1993-August 1994) to CEC/MAST, 483p.<BR> Varnavas S.P. And Balopoulos E.TH. (1994). Compositional and Morphological Variability of Suspended Particulate Matter in Major Aegean Sea Straits, Ist Workshop of the Mediterranean Targeted Project, Barcelona, Spain, 21-23 November 1994, pp 119-125.<BR> Theodorou A.J., Theocharis A. And Balopoulos E. (1994). Aspects of Hydrology and Circulation of the South Aegean Sea and Adjacent Regions during the late winter-early spring PELAGOS-I Cruise, Ist Workshop of the Mediterranean Targeted Project, Barcelona, Spain, 21-23 November 1994, pp 21-28.<BR> Balopoulos E.TH. (1995). Hydrodynamics and Biogeochemical Fluxes in the Straits of the Cretan Arc (Aegean Sea, Eastern Mediterranean Basin), European Commission, IInd MAST Days and EUROMAR Market, Sorrento, Italy, 7-10 November 1995, Project Reports, Vol. I, 453-468.<BR> Balopoulos E.Th., Theocharis A. And Theodorou A.J. (1995). Seasonal variability of the water structure and circulation in the South Aegean Sea and the Straits of the Cretan Arc, IInd MAST Days and EUROMAR Market, Sorrento, Italy, 7-10 November 1995.<BR> Theodorou A.J., Theocharis A. And Balopoulos E. (1995). Hydrological structure of the Cretan Sea and adjacent regions in late winter 1994, Boletino di Oceanografia Teorica et Applicata (in press).<BR> Theodorou A.J., Theocharis A. And Balopoulos E. (1995). Circulation of the Cretan Sea and adjacent regions in late winter 1994.<BR> Gotsis-Skretas O., Balopoulos E., Psyllidou-Giouranovits, Georgakopoulou-Gregoriadou E. & Voutsinou-Taliadouri F. (1995). Geochemistry and water circulation features of the Kerkyra passage (N.E. Ionian Sea, Hellas). Proceedings of VIIIth International Symposium on Environmental Pollution and its Impact on Life in the Mediterranean Region, (MESAEP), October 1995, Rhodos, Greece.<BR> Voutsinou-Taliadouri F., Balopoulos E., Georgakopoulou-Gregoriadou E., Psyllidou-Giouranovits R. & Gotsis-Skretas O. (1995). Aspects on the biology and the distributions of physical and biological parameters of the water masses of the Kerkyra Sea (N.E. Ionian Sea, Hellas). Proceedings of VIIIth International Symposium on Environmental Pollution and its Impact on Life in the Mediterranean Region, (MESAEP), October 1995, Rhodos, Greece.<BR> Psyllidou-Giouranovits R., Balopoulos E., Gotsis-Skretas O., Voutsinou-Taliadouri F. & Georgakopoulou-Gregoriadou E. (1995). Eutrophication Assessment of the Kerkyra Sea (N.E. Ionian, Hellas) based on seasonal chemical, physical and biological surveys. Proceedings of VIIIth International Symposium on Environmental Pollution and its Impact on Life in the Mediterranean Region, (MESAEP), October 1995, Rhodos, Greece.<BR> Friligos N. Ch., Balopoulos E.Th. & Psyllidou-Giouranovits R. (1955). Eutrophication and Hydrography in the Amvrakikos Gulf, Ionian Sea. Proceedings of VIIIth International Symposium on Environmental Pollution and its Impact on Life in the Mediterranean Region, (MESAEP), October 1995, Rhodos, Greece.<P><P>