•Alternation of two dense-water sources affects profoundly oxygen and nutrients dynamics in NE Ionian Sea.•Massive outflow of well-oxygenated dense waters from the Cretan sea generated an “oxygen ...poor-nutrient rich” intermediate layer in NE Ionian.•Evidence of deep waters of Adriatic origin in the Ionian abyssal plain in 2008.•Oxygen and nutrients concentrations in 2008 were still far from those of the pre-EMT phase.
The Eastern Mediterranean Transient (EMT) was a major climatic event, as it affected profoundly and for a long time period, the circulation of the deep and intermediate layers, with important consequences on the distribution of oxygen and nutrients in the Ionian Sea and more generally, in the whole Eastern Mediterranean basin.
In this paper, nutrient and oxygen data collected by the Hellenic Centre for Marine Research, in the Northeastern Ionian Sea, the Western Cretan Sea and the western straits of the Cretan Arc, during the period 1987–2008, were used in order to reveal the detailed pattern of oxygen and nutrient distribution in the NE Ionian Sea, their temporal and spatial variability and their relevance to the two competing sources of deep water, namely the Adriatic and the Cretan Seas.
The distributions of oxygen and nutrients before, during and after the EMT are presented and discussed in relation to the physical processes, the circulation and the dynamics of the area. In the post-EMT period there is evidence that deep waters of Adriatic origin are again present in the abyssal depths of the Ionian Sea. However, the oxygen and nutrients concentrations as well as the thermohaline properties in the study area are still far from those observed in the pre-EMT phase.
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
K
ρ
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
K
ρ
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.
Using an intercalibrated set of oxygen data for the eastern Mediterranean, 1987–1999, we study the evolution of oxygen concentrations that accompanied the recent changes in the thermohaline ...circulation of this sea (the so‐called Eastern Mediterranean Transient (EMT)). We find that, by way of massively transferring oxygen‐rich near‐surface waters into the deep layers, the EMT by 1995 had raised oxygen concentrations considerably relative to the pre‐EMT situation in 1987. Between 1995 and 1999, however, the oxygen concentrations decreased noticeably, we identify oxygen decreases of about 5 μmol/kg for the layers below 2200 m and between about 500 and 1000 m depth, for which layers previous work found vanishing or little replenishment during 1995–1999. Supporting evidence for absence of replenishment is obtained from the temporal evolution of tritium‐3He ages. The oxygen decreases convert into utilization rates of approximately 1.3 μmol/(kg yr). An even higher rate, 2.3 μmol/(kg yr), is obtained for the deep waters of the Cretan Sea below 1000 m. The utilization rate found for the deepest waters significantly exceeds the pre‐EMT value of 0.53 μmol/(kg yr). We propose that the massive invasion of near surface waters affected by the EMT made available large amounts of dissolved organic carbon with an unusually high fraction of labile material, which in turn enhanced oxygen consumption. Supporting evidence is obtained from data of dissolved organic carbon, and from mesozooplankton ecology data. The enhanced oxygen utilization represents a further example of EMT‐related disturbances in the biogeochemistry of the eastern Mediterranean.
This study presents the distribution and fluxes of dissolved inorganic carbon (CT), total alkalinity (AT) and anthropogenic carbon (Cant) along the Otranto strait, during February 1995. Based on a ...limited number of properties (temperature, dissolved oxygen, total alkalinity and dissolved inorganic carbon), the composite tracer TrOCA was used to estimate the concentration of anthropogenic CO2 in the Otranto strait.Total alkalinity exhibits high values and weak variability throughout the water column of the strait, probably associated with the dense water formation processes in the Adriatic basin that induce a rapid transport of the coastal alkalinity to the deep waters. Elevated Cant concentrations and high anthropogenic pH variations are observed in the bottom layer of the strait, associated with the presence of Adriatic Deep Water (ADW). The study shows that large amounts of Cant have penetrated the highly alkaline Eastern Mediterranean waters, thereby causing a significant pH reduction since the pre-industrial era.Estimates of the transports of CT and Cant through the strait indicate that during February 1995, the Adriatic Sea imports through the Otranto strait natural and anthropogenic carbon and acts as a net sink of carbon for the Ionian Sea. The anthropogenic carbon that is imported to the Adriatic Sea represents less than 1% of the net CT inflow. The Levantine Intermediate Water (LIW) contributes to about one-third of the total CT and Cant inflow. Although the amounts of Cant annually transported by LIW and ADW are almost equal, the contribution of Cant to the CT transported by each water mass is slightly higher in ADW (3.1%) than in LIW (2.6%), as a result of its higher mean Cant concentration. The ADW, despite its weak contribution to the total outflow of Cant, has a vital role for the sequestration and storage of the anthropogenic carbon, as this water mass is the main component of the Eastern Mediterranean Deep Waters and, thus, the anthropogenic CO2 is transferred in the deep horizons of the Eastern Mediterranean, where it remains isolated for many years.
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 HCO
3
- 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.
Une variabilité morphologique saisonnière est observée sur des spécimens d’
Emiliania huxleyi var.
huxleyi provenant d’échantillons d’eau de mer collectés en Mer Egée. Les analyses biométriques révèlent une taille systématiquement plus grande des coccolithes et coccosphères, incluant l’épaisseur des éléments du tube interne (INT), durant l’hiver et le printemps quand les eaux de surface sont froides et la productivité modérée, par rapport à l’été associé à des eaux de surface chaudes et une productivité faible. Les faibles variations de la salinité en Mer Egée et l’absence de fluctuations saisonnières de la teneur en nutriments ne peuvent expliquer l’augmentation de taille observée chez
E. huxleyi. Une augmentation de HCO
3
- est également observée durant le printemps ; cependant le paramètre du système carbonate pouvant avoir une influence sur l’augmentation de taille n’est pas clairement identifié.
This study presents the distribution and fluxes of dissolved inorganic carbon (C...), total alkalinity (A...) and anthropogenic carbon (C...) along the Otranto strait, during February 1995. Based on ...a limited number of properties (temperature, dissolved oxygen, total alkalinity and dissolved inorganic carbon), the composite tracer TrOCA was used to estimate the concentration of anthropogenic CO2 in the Otranto strait. Total alkalinity exhibits high values and weak variability throughout the water column of the strait, probably associated with the dense water formation processes in the Adriatic basin that induce a rapid transport of the coastal alkalinity to the deep waters. Elevated C... concentrations and high anthropogenic pH variations are observed in the bottom layer of the strait, associated with the presence of Adriatic Deep Water (ADW). The study shows that large amounts of C... have penetrated the highly alkaline Eastern Mediterranean waters, thereby causing a significant pH reduction since the pre-industrial era. Estimates of the transports of CT and C... through the strait indicate that during February 1995, the Adriatic Sea imports through the Otranto strait natural and anthropogenic carbon and acts as a net sink of carbon for the Ionian Sea. The anthropogenic carbon that is imported to the Adriatic Sea represents less than 1% of the net C... inflow. The Levantine Intermediate Water (LIW) contributes to about one-third of the total C... and C... inflow. Although the amounts of C... annually transported by LIW and ADW are almost equal, the contribution of C... to the CT transported by each water mass is slightly higher in ADW (3.1%) than in LIW (2.6%), as a result of its higher mean C... concentration. The ADW, despite its weak contribution to the total outflow of C..., has a vital role for the sequestration and storage of the anthropogenic carbon, as this water mass is the main component of the Eastern Mediterranean Deep Waters and, thus, the anthropogenic CO2 is transferred in the deep horizons of the Eastern Mediterranean, where it remains isolated for many years. (ProQuest: ... denotes formulae/symbols omitted.)
This study presents the distribution and fluxes of dissolved inorganic carbon (C
T), total alkalinity (
A
T) and anthropogenic carbon (C
ant) along the Otranto strait, during February 1995. Based on ...a limited number of properties (temperature, dissolved oxygen, total alkalinity and dissolved inorganic carbon), the composite tracer TrOCA was used to estimate the concentration of anthropogenic CO
2 in the Otranto strait.
Total alkalinity exhibits high values and weak variability throughout the water column of the strait, probably associated with the dense water formation processes in the Adriatic basin that induce a rapid transport of the coastal alkalinity to the deep waters. Elevated C
ant concentrations and high anthropogenic pH variations are observed in the bottom layer of the strait, associated with the presence of Adriatic Deep Water (ADW). The study shows that large amounts of C
ant have penetrated the highly alkaline Eastern Mediterranean waters, thereby causing a significant pH reduction since the pre-industrial era.
Estimates of the transports of C
T and C
ant through the strait indicate that during February 1995, the Adriatic Sea imports through the Otranto strait natural and anthropogenic carbon and acts as a net sink of carbon for the Ionian Sea. The anthropogenic carbon that is imported to the Adriatic Sea represents less than 1% of the net C
T inflow. The Levantine Intermediate Water (LIW) contributes to about one-third of the total C
T and C
ant inflow. Although the amounts of C
ant annually transported by LIW and ADW are almost equal, the contribution of C
ant to the C
T transported by each water mass is slightly higher in ADW (3.1%) than in LIW (2.6%), as a result of its higher mean C
ant concentration. The ADW, despite its weak contribution to the total outflow of C
ant, has a vital role for the sequestration and storage of the anthropogenic carbon, as this water mass is the main component of the Eastern Mediterranean Deep Waters and, thus, the anthropogenic CO
2 is transferred in the deep horizons of the Eastern Mediterranean, where it remains isolated for many years.
► Study of the inorganic carbon system parameters in the Otranto Strait. ► Assessment of anthropogenic CO
2 fluxes through the Otranto Strait using TrOCA method. ► Elevated C
ant content and high anthropogenic pH variations observed close to bottom. ► Adriatic Sea imports natural and anthropogenic carbon acting as net sink for the Ionian. ► Large C
ant amounts penetrated E. Mediterranean waters, causing important pH decline.
The Mediterranean Sea has been sampled irregularly by research vessels in the past, mostly by national expeditions in regional waters. To monitor the hydrographic, biogeochemical and circulation ...changes in the Mediterranean Sea, a systematic repeat oceanographic survey programme called Med-SHIP was recommended by the Mediterranean Science Commission (CIESM) in 2011, as part of the Global Ocean Ship-based Hydrographic Investigations Program (GO-SHIP). Med-SHIP consists of zonal and meridional surveys with different frequencies, where comprehensive physical and biogeochemical properties are measured with the highest international standards. The first zonal survey was done in 2011 and repeated in 2018. In addition, a network of meridional (and other key) hydrographic sections were designed: the first cycle of these sections was completed in 2016, with three cruises funded by the EU project EUROFLEETS2. This paper presents the physical and chemical data of the meridional and key transects in the Western and Eastern Mediterranean Sea collected during those cruises.
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.
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