Chemical oceanography in the Cretan Sea: Changes associated to the transient

The intensive research since 1985 in the framework of national and international programmes revealed important modifications in oxygen and nutrients distribution in the Cretan Sea. The significant increase of density and of formation rates of the Cretan Dense Water (CDW) during the last decade is basically responsible for the drastic change of the thermohaline circulation and the installation of a new hydrological regime in the Eastern Mediterranean. In the Cretan Sea, the most important effect of the new regime, is the installation of a well-defined "minimum salinity, temperature, oxygen and maximum nutrient" intermediate layer formed by the intrusion of the Transitional Mediterranean Water (TMW) compensating the massive CDW outflow.The nutrient enrichment of the intermediate layers of the Cretan Sea, due to the intrusion of the "nutrient rich-oxygen poor" TMW, was observed firstly in 1991 and became very important during 1994-95. During 1994-95 the TMW occupies the intermediate layers of the entire Cretan Sea and the concentrations of nutrients in this layer are often two times higher than in the past. Recently, in 1997-98 the chemical characteristics of TMW are less pronounced probably related to the weaker CDW outflow

High oxygen consumption rates in the deep layers of the North Aegean Sea (eastern Mediterranean)

Severe winter meteorological conditions promote dense water formation over the shelves of the North Aegean Sea. The newly formed dense water fills the deep basins of the North Aegean Sea, contributing to their ventilation and the downward transport of organic and inorganic material. The great bathymetric variability imposes limitations on the deep circulation and the communication between the various basins and makes the North Aegean Sea an appropriate area for the monitoring of oxygen consumption in the deep layers. Historical hydrographic data suggest that there was extensive production of dense water in the North Aegean Sea on two occasions during the last decade, the winters of 1987 and 1992-1993. Our data series from August 1986 to September 1989 and from March 1997 to February 1999, permitted us to follow, step by step, the oxygen consumption and the nutrient regeneration in the deep basins of the northern Aegean Sea during these periods of isolation. The organic matter reaching the bottom layer just after the deep water formation event is rich in labile and easily oxidizable material and its decomposition leads to a significant oxygen uptake during the first year of stagnation. The further decomposition of the remaining semi-labile and refractory material turns over on greater time scales, by consuming lesser amounts of oxygen. A more significant oxygen decrease is recorded in the eastern basin (Lemnos Basin) of the North Aegean Trough, than in the central (Athos Basin) and the western (North Sporades Basin) ones and is attributed to the irregular contribution of the Black Sea Water (BSW) to the water masses formed on the different shelves of the North Aegean Sea. Our results and the existing data on the Turkish straits showed that dissolved organic matter is the major constituent responsible for this high oxygen consumption. The slightly different particulate organic carbon fluxes to these depressions play a secondary role

Distribution of dissolved inorganic carbon and related parameters in the Thermaikos Gulf (Eastern Mediterranean)

Data on the distribution of dissolved inorganic carbon (measured as TCO2) and related parameters in the Thermaikos Gulf were obtained during May 1997. High TCO2 concentrations were recorded close to the bottom, especially in the northern part of the gulf, as a result of organic matter remineralisation. The positive relatively good correlation between TCO2 and both apparent oxygen utilisation (AOU) and phosphate at the last sampling depth confi rmed the regenerative origin of a large proportion of TCO2. The comparatively conservative behaviour of alkalinity, together with the relatively low value of the homogenous buffer factor β (β = ∂lnfCO2/∂lnTCO2) revealed that calcifi cation or carbonate dissolution takes place on a very small scale, simultaneously with the organic carbon production. The correlations between fCO2 and chlorophyll α, as well as AOU and the surface temperature, revealed that the carbon dioxide fi xation through the biological activity is the principal factor that modulates the variability of fCO2. A rough first estimate of the magnitude of the air-sea CO2 exchange and the potential role of the Thermaikos Gulf in the transfer of atmospheric CO2 was also obtained. The results showed that during May 1997, the Thermaikos Gulf acted as a weak sink for atmospheric CO2 at a rate of -0.60 - -1.43 mmol m-2 d-1, depending on which formula for the gas transfer velocity was used, and in accordance to recent reports regarding other temperate continental shelves. Extensive study of the dissolved inorganic carbon and related parameters, and continuous shipboard measurements of fCO2 a and fCO2 w during all seasons are necessary to safely quantify the role of the Thermaikos Gulf in the context of the coastal margins CO2 dynamics

Comparaison de la distribution et du bilan d'echanges des sels nutritifs en Mediterranee et en Mer Rouge

The Mediterranean and the Red Sea are two concentration basins with very similar hydrological characteristics. The water of the adjoining ocean, entering over a sill, holds the water and the nutrient budgets of the basins in balance. Changes in this inflowing oceanic water are analogous in the two basins: reduction in nutrient content, due to mixing with the nutrient-poor water of the basin, biological activity and the existence of successive sills (in the Mediterranean). The Mediterranean receives from the Atlantic Ocean the main part of its nutrient supply (of the order of 70%, the rest being provided by the rivers). This supply is practically constant over the year. The inflowing water from the Indian Ocean provides the entire nutrient supply of the Red Sea. This oceanic supply follows the two circulation regimes at Bab-el-Mandeb straits, the gain in summer (over 3-4 months) compensating the loss in winter (over 8-9 months)

Organic matter stoichiometry based on oxygen consumption - Nutrients regeneration during a stagnation period in Jabuka Pit (middle Adriatic Sea)

The Jabuka Pit, a continental shelf depression in the middle Adriatic Sea, acts as a collection basin for the dense water formed during winter in the northern Adriatic. Its deep waters are usually renewed at least every second year. The deep waters of Jabuka Pit remained practically isolated after the strong flushing occurred in early spring 1993. Our data from May 1993 to November 1994, permitted to follow, step by step, the oxygen consumption and the nutrient and carbon regeneration in the deep layer of Jabuka Pit during this period of isolation. Extremely high oxygen consumption and nutrients and carbon regeneration rates were estimated; the oxygen reduction rate is about 66.5 μmol/kg/yr, while the accompanying increasing rate of nitrate is 2.64 μmol/kg/yr, of silicate 4.2 μmol/kg/yr, of phosphate 0.24 μmol/kg/yr and of ΣCO 2 81.0 μmol/kg/yr. The temporal evolution of the regeneration ratios of nitrogen and carbon, as well as of the N inorg/PO 4 ratio, imply that possibly other processes, besides aerobic respiration, like denitrification, take place and modulate the values of the ratios. Furthermore, the changes of the concentrations of oxygen, nitrate, phosphate and ΣCO 2 in the deep layers of Jabuka Pit were exploited in order to estimate the elemental composition of the organic matter that is remineralised. Our results suggest that the most probable empirical formula that corresponds to the composition of the organic matter in the study system is (C 4H 6ON) 8; (C 6H 10O 5) 3; (CH 2) 108 being 11.4% carbohydrates, 20.3% proteins and 68.3% lipids. Furthermore, the investigation of our results show that there is evidence that the observed changes of the concentrations of the chemical parameters are better interpreted if additionally to the remineralisation of organic matter, small-scale CaCO 3 dissolution with simultaneous denitrification processes are considered. © 2004 Elsevier Ltd. All rights reserved

Seasonal variation in Emiliania huxleyi coccolith morphology and calcification in the Aegean Sea (Eastern Mediterranean)

A seasonal morphological variability is observed in Emiliania huxleyi var. huxleyi specimens, collected from discrete water samples in the Aegean Sea. Biometric analyses reveal a consistent pattern of increase in the size of coccoliths and coccospheres, including the thickness of the inner tube elements (INT), in winter/spring time low sea surface temperature and moderate productivity samples when compared with summer time high temperature-low productivity samples. The small range of salinity change in the Aegean Sea and the absence of seasonal pattern in nutrient content do not support any association with the observed increase in E. huxleyi coccolith size. A relatively increased [HCO3-] content is observed during spring-time interval related with the increase in the coccolith size, however it remains unclear which parameter of the carbonate system causes the observed effects. © 2009 Elsevier Masson SAS. All rights reserved