Vassilis Zervakis

The role of the North Aegean in triggering the recent Eastern Mediterranean climatic changes

Drastic changes have occurred in the vertical structure of the deep waters of the eastern Mediterranean in the early 1990s, as dense water of Aegean origin has displaced lighter waters of Adriatic origin at the bottom of the deep basins. This work suggests that the initiation of this process took place in the North Aegean in the winter of 1986/1987 and was intensified by another formation event in 1992/1993. The available observations from the North Aegean support such a scenario. Furthermore, we propose that the outflow of Black Sea waters into the Aegean through the Dardanelles could act as an insulator of the deeper layer from the atmosphere, thus absorbing a large part of the heat and salt exchange; despite this fact, the existence of the densest bottom water of the Mediterranean in the North Aegean, and the continuation of density increase for a large period of time, suggests that it is a region of formation, thus that the insulation layer may at times be penetrated. We suggest that reduced Black Sea outflow into the North Aegean could facilitate dense water formation during the passage of cold atmospheric fronts in the winter.

Phytoplankton size-based dynamics in the Aegean Sea (Eastern Mediterranean)

This study represents one component of the large MTP-II-MATER (MAST-III) multidisiplinary project in the Mediterranean supported by EU. Data were collected during three cruises performed in Spring and Autumn 1997 and Spring 1998 from six stations of the North and five stations of the South Aegean Sea. The work assessed the spatial, vertical and temporal variations of size fractionated chlorophyll a, primary production (in situ), photosynthetic parameters (in situ) and the taxonomic composition of phytoplankton. The population structure and dynamics were greatly influenced by the different hydrographic conditions prevailing in the Northern and Southern Aegean Sea due to the influence of Black Sea and Levantine Sea waters, respectively. The picoplankton fraction (0.2–1.2 Am) predominated and accounted for the 56% to 49% of total chl a and the 51% to 41% of total primary production in the N. and S. Aegean Sea, respectively. Throughout the sampling area, the levels of nano+microplankton (>3.0 Am) were next in abundance proportions of total chl a (21–31%) and primary production (20– 33%) and the levels of the ultraplankton (1.2–3.0 Am) were the lowest, contributing the 18–22% of total chl a and the 20–23% of total primary production. There was a highly significant (PV0.005–0.01) spatial, vertical and temporal influence on the biomass and productivity of all size classes in the N. Aegean and of most of them in S. Aegean. Light utilization efficiency (e%) and quantum yield (umax) exhibited a temporal trend having higher values in Spring than in Autumn as well as a trend affected by cell size, being higher for picoplankton in relation to ultraplankton and nano+microplankton. Assimilation ratios (P B ) increased with cell size. Daily primary production in the N. Aegean (81.36 mg C m � 2 day � 1 ) was higher than that in the S. Aegean (38.88 mg C m � 2 day � 1 ) but both are characterized as the most oligotrophic areas of the eastern Mediterranean.

Vertical diffusion and oxygen consumption during stagnation periods in the deep North Aegean

Ventilation of the deep basins of the North Aegean Sea takes place during relatively scarce events of massive dense water formation in that region. In the time intervals between such events, the bottom waters of each sub-basin are excluded from interaction with other water masses through advection or isopycnal mixing and the only process that changes their properties is diapycnal mixing with overlying waters. In this work we utilize a simple one-dimensional model in order to estimate the vertical eddy diffusion coefficient Kρ based on the observed rate of change of density and stratification. Vertical diffusivity is estimated for each of three sub-basins of the North Aegean, one of convex shape of the seabed and the other two of concave topography. It is noteworthy that the convex sub-basin exhibited much higher vertical diffusivity than the two concave sub-basins, a fact consistent with theoretical predictions that internal-wave-induced mixing is higher over the former shape of seabed. Furthermore, the estimates of Kρ are exploited in computing the vertical transport of dissolved oxygen through diffusion and the rate of oxygen consumption by decaying organic matter. The different levels of the estimated diffusion and oxygen consumption rates testify to the dynamical and biogeochemical characteristics of each basin.

HF Radar Activity in European Coastal Seas: Next Steps toward a Pan-European HF Radar Network

High Frequency radar (HFR) is a land-based remote sensing instrument offering a unique insight to coastal ocean variability, by providing synoptic, high frequency and high resolution data at the ocean atmosphere interface. HFRs have become invaluable tools in the field of operational oceanography for measuring surface currents, waves and winds, with direct applications in different sectors and an unprecedented potential for the integrated management of the coastal zone. In Europe, the number of HFR networks has been showing a significant growth over the past ten years, with over 50 HFRs currently deployed and a number in the planning stage. There is also a growing literature concerning the use of this technology in research and operational oceanography. A big effort is made in Europe towards a coordinated development of coastal HFR technology and its products within the framework of different European and international initiatives. One recent initiative has been to make an up-to-date inventory of the existing HFR operational systems in Europe, describing the characteristics of the systems, their operational products and applications. This paper offers a comprehensive review on the present status of European HFR network, and discusses the next steps towards the integration of HFR platforms as operational components of the European Ocean Observing System, designed to align and integrate Europe’s ocean observing capacity for a truly integrated end-to-end observing system for the European coasts.

Ascending and Descending Passes for the Determination of the Altimeter Bias of Jason Satellites using the Gavdos Facility

This paper presents the improvements made on the calibration methodology conducted at the Gavdos calibration/validation facility along with the latest altimeter calibration results for Jason-1 and Jason-2 satellite missions. Calibration results are presented, for the first time, for both ascending and descending passes of Jason satellites. The altimeter bias for Jason-2 has been estimated to be +173 ± 4 mm for Pass No. 109 and +171 ± 5 mm for Pass No. 018 over cycles 1–79. In tandem mission, the difference between Jason-1 and Jason-2 has been determined to be 72mm (Pass No. 109) and 68 mm (pass No. 018) and over cycles 2–20.

Holocene Climatic Optimum centennial-scale paleoceanography in the NE Aegean (Mediterranean Sea)

Combined micropaleontological and geochemical analyses of the high-sedimentation gravity core M-4G provided new centennial-scale paleoceanographic data for sapropel S1 deposition in the NE Aegean Sea during the Holocene Climatic Optimum. Sapropel layer S1a (10.2–8.0 ka) was deposited in dysoxic to oxic bottom waters characterized by a high abundance of benthic foraminiferal species tolerating surface sediment and/or pore water oxygen depletion (e.g., Chilostomella mediterranensis, Globobulimina affinis), and the presence of Uvigerina mediterranea, which thrives in oxic mesotrophic-eutrophic environments. Preservation of organic matter (OM) is inferred based on high organic carbon as well as loliolide and isololiolide contents, while the biomarker record and the abundances of eutrophic planktonic foraminifera document enhanced productivity. High inputs of terrigenous OM are attributed to north Aegean borderland riverine inputs. Both alkenone-based sea surface temperatures (SSTs) and δO18G. bulloides records indicate cooling at 8.2 ka (S1a) and ~7.8 ka (S1 interruption). Sapropelic layer S1b (7.7–6.4 ka) is characterized by rather oxic conditions; abundances of foraminiferal species tolerant to oxygen depletion are very low compared with the U. mediterranea rise. Strongly fluctuating SSTs demonstrate repeated cooling and associated dense water formation, with a major event at 7.4 ka followed by cold spells at 7.0, 6.8, and 6.5 ka. The prominent rise of the carbon preference index within the S1b layer indicates the delivery of less degraded terrestrial OM. The increase of algal biomarkers, labile OM-feeding foraminifera and eutrophic planktonic species pinpoints an enhanced in situ marine productivity, promoted by more efficient vertical convection due to repeated cold events. The associated contributions of labile marine OM along with fresher terrestrial OM inputs after ~7.7 ka imply sources alternative/additional to the north Aegean riverine borderland sources for the influx of organic matter in the south Limnos Basin, plausibly related to the inflow of highly productive Marmara/Black Sea waters.

137Cs vertical distribution at the deep basins of the North and Central Aegean Sea, Greece

Large volume seawater samples were collected for the determination of (137)Cs concentration along with depth in the deep basins of North and Central Aegean Sea. The vertical (137)Cs distribution showed maximum concentration at the bottom of the basins, while the minimum values corresponded to the intermediate layer, where Levantine water exists. The surface (137)Cs activity is found to lie between the two limits and is originated from the Black Sea waters. The typical oceanographic advection-diffusion balance model is modified to a diffusion-settling-decay balance model to better understand the vertical distribution and variation of the (137)Cs concentration in the deep basins. In addition, the diffusivity of each basin, as well as the settling speed of particulate (137)Cs is also estimated. The results are compared with theoretical approach as well as with previous data.

In situ calibration of biofouling-prone oceanographic sensors in the framework of the POSEIDON project

This work presents the in situ calibration procedures developed for the sensors of conductivity, dissolved oxygen and chlorophyll-α, attached to eleven Oceanor Seawatch-type buoys deployed since 1999 in the Aegean Sea. The sensors are deployed in the upper 50 m of ocean, where bio-fouling is a significant cause of measurement error. The limited resources in instrumentation and the cost of ship-time have dictated the development of parallel procedures allowing the calibration of 4–5 CT sensors, one chlorophyll-α fluorometer and one dissolved oxygen sensor within two hours. Through our calibration we have achieved significant improvement of the chlorophyll and conductivity measurements.

Absolute calibration of Jason satellite radar altimeters at Gavdos Cal/Val facility using independent techniques

The Gavdos calibration facility for satellite radar altimeters has been operational as of 2004. The island is located along repeating ground tracks of Jason-1 and Jason-2 satellites (crossover point for passes No.109 ascending and No.018 descending and adjacent to Envisat), and because of its small size, both altimeter and radiometer measurements are not significantly contaminated by land. This makes Gavdos an ideal place for the calibration of satellite altimeters. In this work, three different techniques have been applied for calibrating the Jason altimeter measurements at Gavdos Cal/Val facility. These are: (i) The conventional: In-situ observations made by tide gauges, GNSS receivers, meteorological and other sensors in conjunction with precise geoid models are applied for determining the altimeter bias; (ii) The MSS: instead of the geoid, the mean sea level, provided by the CLS10_MSS model, is used as a reference surface for estimating the bias; and (iii) Microwave transponder measurements are implemented and examined over the cross over point on land to produce the altimeter bias as well. This paper presents the results regarding these calibration techniques.

Estimating chlorophyll concentrations in the optically complex waters of the North Aegean Sea from field and satellite ocean colour measurements

In the Aegean Sea and Eastern Mediterranean there are large discrepancies between in situ and satellite ocean colour derived chlorophyll concentrations. The quantity that is monitored by ocean colour satellites and that can be used in the estimation of chlorophyll concentration is the remote sensing reflectance, defined as the ratio of the water leaving spectral radiance to the downwelling spectral irradiance. It can be determined in the field, with either above or in-water radiance and irradiance measurements. The complex optical properties of the North-East Aegean Sea, including radiance and irradiance, were studied during the AegeanMarTech project. Chlorophyll concentration estimates were derived from simultaneous above and in-water radiometric measurements. These were validated against chlorophyll concentration field data and compared against concurrent MODIS data from which chlorophyll was derived using two simple empirical algorithms. It was found that the MedOC3 algorithm outperforms the operational OC3M-547 algorithm and produces the least bias when compared against HPLC derived in situ chlorophyll. It is concluded that the greatest uncertainty in the inversion arises due to CDOM absorption below the 488 nm band. The reflectance ratios indicated that there is always an excess of yellow matter present in the study area and the water type could not be characterized optically as ‘’typical open ocean” Case 1.