1. G. Civitarese, D. Bregant, S. Cozzi,
2. A. Boldrin, A. De Lazzari, S. Rabitti,
3. E. Krasakopoulou, E. Souvermezoglou,
4. M. Gacic, V. Kovacevic, A. Vetrano.

Cruises and field work.

The grid of stations occupied during six seasonal cruises (from February 94 to May 95) is presented in fig.
The present analysis is restricted to the results obtained during 1994 (four seasonal cruises) and the distributions discussed are relative to transect 3, the same transect in which current meter moorings were deployed. Due to the lack of sufficient time coverage during 1994, the current meter data and the related water transport estimates used for the biogeochemical budget calculations refer to 1995. As we will discuss later, the variability of the exchange is mainly driven by the variability of the current regime, biogeochemical distribution patterns being almost constant on a seasonal scale (i. e. no appreciable interannual variability), especially in terms of average properties of the water masses dynamically involved.

Dissolved oxygen and nutrients.

Dissolved oxygen and nutrients analyses were performed by Italian (CNR-Istituto Talassografico, Trieste: OTR-1 and OTR-3 cruises) and Greek (National Center for Marine Research, Athens: OTR-2 and OTR-4 cruises) laboratories. Dissolved oxygen was determined by means of a Metrohm automatic burette according to the procedure by Winkler (Carpenter, 1965). In some cruises the final point was determinated automatically by means of a redox electrode. During OTRANTO-1 and OTRANTO-3 cruises all nutrient determinations were carried on board by means of an hybrid autoanalyzer equipped with a Chemlab flow-colorimeter, following the procedure described by Grasshoff et al. (1983) with some modifications. For the remaining cruises, samples were collected in 100 mL polyethylene bottles and kept continuosly in a deep freezer at -20 C until their analysis in the laboratory by means of a Technicon CSM6 autoanalyzer. Phosphate was measured by means of a Perkin-Elmer Lambda 2S UV/VIS Spectrometer following the procedure described by Murphy and Riley (1962).

Particulate matter.

Particulate matter studies were undertaken by Istituto di Biologia del Mare, CNR, Venice. During the six seasonal surveys vertical profiles were carried out with a CTD probe (SeaBird SBE 16) coupled with SeaTech transmissometer and fluorometer. Discrete water samples were collected for the following determinations: total suspended matter (TSM, dry weight), particulate organic carbon (POC), particulate nitrogen (PN), particle concentration and size distribution. For TSM, POC and PN determinations prewashed and precombusted (at 500 C for 4 hours) glass fiber filters Whatmann GF/F (25 mm diameter) were used. Samples were immediately stored at -20 C. Dry weight filters were subsequently measured by gravimetric method (Strickland and Parson, 1972); the organic fraction was estimated as weight loss after incineration at 480 C for 4 hours. POC and PN were determined by means of a Perkin-Elmer 2400 CHN elemental analyzer. The inorganic carbon was removed through vapour phase acidification (Hedges and Stern, 1984). After sampling, particle concentration and size distribution were immediately determined by means of a Coulter Counter Model TA II in the range from 2 to 40 µm. Phytoplankton abundances and species composition were determined on selected samples utilizing a Zeiss ICM 405 inverted microscope (Utermöhl, 1958).

Current field.

The experimental design of long-term monitoring of the water flow variability across the Strait consisted of six currentmeter moorings, two of them coupled with sediment traps. They were deployed along the section 3, at 39 50' N (Fig. 2, upper), in the southernmost portion of the Strait. The current monitoring was conducted by Osservatorio Geofisico Sperimentale, Trieste (Italy), in collaboration with National Centre for Marine Research, Athens (Greece) and Saclantcen, La Spezia (Italy).

The satisfactory spatial coverage with current time series, essential for the water flux calculations, regards almost a year long interval, from December 1994 to November 1995 (Fig. 2, below). Regarding the transect location and the Strait orientation, the north component is taken as a representative of the flow perpendicular to the section across the Strait, thus suitable for water transport computation.

Tidal and inertial oscillation were filtered out applying a digital filter on the mean hourly currents. The obtained low passed north hourly current field formed the base for the water flux calculations.

Dissolved oxygen, nutrients and particulate matter distributions show generally a four layers subdivision of the vertical section through the strait: the surface layer (from 0 to 50 meters), the gradient zone (from 50 to about 200 meters), the intermediate layer (from 200 to about 600 meters), and the bottom layer.

From the classical hydrological point of view, the strait is characterized by the presence of four main water masses: the ASW (Adriatic Surface Water) outflowing from the Adriatic Sea along the western side of the strait, the ISW (Ionian Surface Water), inflowing into the Adriatic along the eastern side, the LIW (Levantine Intermediate Water), wich flows into the Adriatic at intermediate depths, with the core centered in the eastern half of the section, and the ADW (Adriatic Deep Water) at the bottom, that outflows from the Adriatic, feeding the deep layers of the Eastern Mediterranean Sea.

The surface layer.

The first 50 meters of the water column are characterized by a significative seasonal variability both in dissolved nutrients and particulate matter distributions, due to the seasonal modulation of physical forcing factors and of biological processes. In Winter the surface layer shows a strong West-East gradient for nitrate, oxygen, particles and particulate carbon and nitrogen concentrations, the western side being richer (>260 µM for oxygen, up to 3 µM for nitrate, about 17,000 particle/cm3 , 0.5 mg/dm3 of TSM, 30 % of organic matter, 3.5 µM POC, 0.5 µM PN, C/N molar ratio of 6.9) than the eastern one (5000 particle/cm3, 0.1 mg/dm3 TSM, 60 % of organic matter, 2.4 µM POC, 0.3 µM PN, C/N molar ratio 7.6). The same doesn’t occur for phosphate and silicate: the first is present at very low levels (about 0.05 µM), near the detection limits, in the whole surface layer; the second one shows concentration less than 2 µM without any evident West-East gradient.

In Spring the surface layer is going towards the typical post-bloom nutrients depletion that will be completely established in summer, where the whole layer appears completely nutrients-exhausted and exhibits the oxygen maxima due to the biological activity. The sub-surface oxygen maximum is well correlated with presence of a noncontinuous chlorophyll sub-surface maximum, coincident with phytoplankton biomass maxima (about 4 µg C/dm3) and spots of biogenic Calcium (Price, pers. comm.), possibly related to the development of Coccolithophorids populations. Also from the point of view of particulate matter, the surface layer appears homogeneous in horizontal (values of 4000 particle/cm3, 0.1 mg/dm3, POC 3.4 µM, PN 0.5 µM, C/N molar ratio 7.4), without any evident differentiation between western and eastern sides.

The following Winter (February 1995) presents the same characteristics of the previous one, with the nutricline partially destroyed by the seasonal mixing.

The gradient zone.

The gradient zone extends from 50 to 200 meters (400 meters for silicate distribution). In this layer the concentrations rise from 0.5 to 5.4 µM for nitrate, from 0.05 to about 0.24 µM for phosphate and from 1.5 to about 8 µM for silicate. Dissolved oxygen levels decrease from their maximum values of about 250 µM to less than 195 µM at the base of the layer. For what concerns the particulate matter, from 100 to 200 m the concentrations remain unchanged, with particle concentration between 2000 and 3000 particle/cm3 and 0.8 µM of POC.

The intermediate layer.

The large body of water extending from 200 to about 600-800 meters is characterized by the presence of the LIW (Levantine Intermediate Water) that occupies the eastern half of the section with a core with salinity higher than 38.75 psu. The dissolved oxygen levels decrease to their minimum values (185-190 µM) at a depth of about 400 meters below that they start to increase due to the influence of the oxygen-rich Adriatic Deep Water. In an opposite way nutrients increase their concentration up to 5.5-6.0 µM for nitrate, 0.24-0.26 µM for phosphate and 8.5-9.0 µM for silicate, whose maxima values lie deeper (about 600 meters) than the other nutrients. The decrease of concentration towards the bottom is not always clear except for silicate.

The LIW is characterized by almost constant concentrations both for particle (less than 2000 particle/cm3) and POC (0.3 - 0.7 µM) and PN (0.03 - 0.1 µM), with C/N molar ratio from 8 to 12 along the year. These values are the absolute minima for the area.

The bottom layer.

The bottom layer is occupied by the Adriatic Deep Water, a water mass characterized by relatively higher concentration of dissolved oxygen (maximum value of 218 µM in Winter) and lower values of nutrients (4.5-5.0, less than 0.22, and less than 8.5 µM for nitrate, phosphate and silicate, respectively). The vertical gradients for oxygen and silicate are particularly evident and are well correlated with the extension of the ADW layer.

Particle concentration, POC and PN show more or less constant properties in time (about 2000 particle/cm3, 0.6 - 1.6 µM for POC and 0.05 - 0.15 µM for PN, with a C/N molar ratio from 8 to 16). The observed gradient for oxygen and silicate is also present in particle concentration, POC and PN, that slightly increase in ADW. This relative enrichment is maintained southward even at the bottom of the Ionian basin (Boldrin et al., 1990), confirming the signal of the southward advection of dense waters of Adriatic origin.

In the bottom layer over the Italian continental shelf and at the beginning of the slope a nepheloid layer presenting relative high particulate matter concentrations near the bottom occurs. This layer is confined toward the shelf and its extension varies during the seasons, being particularly pronounced during summer, with a particle concentration of 8000 particle/cm3 (twice the surface layer), POC 1.3 µM , PN 0.2 µM and a C/N molar ratio of 13.4. This layer is not always evidenced by classical hydrological properties, nevertheless it can be considered relevant for the exchanges in the area, being relatively rich in suspended matter.

The Adriatic Sea and the N:P ratio anomaly of the Eastern Mediterranean.
As previously stated, only the surface layer exhibits a significant seasonal variability connected either with the hydrological regime or the occurrence and development of biological activity.
During Winter the shelf along the Italian coasts is occupied by a relatively fresh and cold water stream of ASW flowing southward. ASW is selectively enriched in NO3, as also showed by the vertical distribution of NO3:PO4 ratio , in which the relative excess of nitrate is clearly recognizable on the western side and in a small lens in the center of the section due to the cyclonic recirculation. Also the area interested by the ADW outflow shows a slight but significant increase of the ratios. This behaviour is confirmed by previous investigations carried out in the Southern Adriatic (Civitarese, pers. comm.) and it is always associated with strong outflows of ASW and ADW. The selective enrichment in NO3 is particularly noteworthy because it suggests a possible role of the Adriatic Sea in supporting the well known N:P ratio anomaly in the Eastern Mediterranean. Recently, some theories have been developped to explain the deviation of the N:P ratio for the Mediterranean in comparison with the oceans: we can mention, among the others, nitrogen fixation (Bethoux and Copin-Montegut, 1986) and phosphate removal from the water column due to adsorption onto Saharian dust particles blown across the Mediterranean (Kromm at al., 1991). Our results suggest that the anomaly in the N:P ratio could originate directly from of the source of ventilated deep waters of the Eastern Mediterranean, i.e. the Adriatic Sea, because of a selective enrichment in nitrate due to freshwater inputs and the active role of Northern Adriatic Shelf in the efficient removal of phosphorus from the water column via burial in the bottom sediments. Due to its size, the presence of eutrophicated regions, and the scale of global physical processes, the Mediterranean, at least its eastern basin, can be viewed as a small ocean whose biogeochemistry is significantly influenced by terrestrial inputs.

The West-East gradient of particulate matter.

The particulate matter observed along the western side of the strait is originated by the complex of processes taking place along the Adriatic basin (especially in the coastal belt) that combines lateral advection and transport of material of riverine origin connected to the coastal currents with the seasonally modulated biological activity.

The different mechanisms producing the particulate matter are well evidenced in the time-series of samples collected by sediment traps, in the Italian shelf area and along the continental slope. In fact, in the western station (301), directly interested by the ASW southward flowing, the inorganic fraction prevails, whereas in the eastern one (st. 304) the relative weight of organic part increases from 10 to 13% (Boldrin et al., 1996).

In the particulate phase we observed a significant change in the C/N molar ratio, that in the ADW reaches values up to 16, meanwhile in the Adriatic Surface Waters is less than 7. This fact can be explained as a signal of a relatively older “age” of bottom waters, that loose selectively nitrogen compounds, more easily utilizable.

Phytoplankton abundance and biomass estimates clearly show that this area is characterized by oligotrophic conditions. Compared with the eastern side, the western half of the Strait is characterized by a maximum of phytoplankton biomass in ASW (8 µgC/dm3), in February, with a decreasing eastward gradient (3 µgC/dm3), to be related to the corresponding nutrient availability. The taxonomic group composition reflects in the western part the Adriatic origin, diatoms being more represented (>15%), if compared with the summer composition (<4%) when the flow is reduced and the phytoplankton population shows characteristics typical of the northern Ionian basin (Rabitti et al., 1994).

The comparison between phytoplankton concentrations in the water column and cell countings on sedimen-trap samples suggests the possible existence of a seasonal phytoplankton cycling, with early spring maxima and late summer minima.

The ADW pathway.

The OTRANTO Project allows us for the first time to follow with great details the ADW path just south of the sill of the strait. Two interesting features can be observed from the oxygen distributions at the bottom: the first one is that the presence of two or more maxima of oxygen concentrations located longitudinally demonstrates the pulsating characteristic of the deep outflow processes, as a consequence of the production mechanisms of the new dense waters and the overflowing dynamics.

The second observation is that the oxygen contouring lines reveal a possible bifurcation of the ADW path in two directions: one southwestward, adjacent to the Italian shelf break, the second southeastward, along the Greek side, following the fall-line of the Otranto Valley.
Other investigations in the Northern Ionian Sea carried out in the frame of the POEM Program (Manca, pers. comm.) confirm this bifurcation. Therefore, the deep ventilation of the Eastern Mediterranean seems to follow a more complex pathway than previously thought (Pollak, 1951; Schlitzer et al., 1991).

The fluxes through the strait.

One of the main objectives of the OTRANTO Project was to estimate the water, dissolved and particulate matter fluxes and their seasonal and possibly interannual variability through the Strait. To achieve this, the first step was to measure and calculate the current velocity field in a section of the strait, at least throughout one year. The current field exhibited high variability on the synoptic time scales, and was heavily influenced by transient mesoscale features (Gacic et al., 1996). The spatial resolution of the experiment design was unfortunately not fine enough to resolve these mesoscale eddies, passing through the area of the shear zone between inflow and outflow. As the biogeochemical data are discrete in time, sampled in different seasons, they need to be associated with a seasonally representative flow pattern, with balanced input (northward) and output (southward) fluxes. Therefore, to smooth the current structure and to filter out the influence of mesoscale eddies, and to make it representative for the time in which the biogeochemical sampling was realized, the currents at each measuring site were averaged over a determined interval of time (approximately ten days), in the period corresponding to the sample collection.

From the experimental results we can derive that the seasonal variability of the transports is mostly due to the variability of the current regime and only partly to the variability of the biogeochemicals distribution in the surface layer. It is clear that the main source of uncertainty in the transports estimate lies in the seasonal representativeness of the used patterns.

The averaged current field was then interpolated on a regular grid and the fluxes of water across the Strait were calculated.
The same kind of interpolating procedure was applied to biogeochemical concentrations, and the resulting values were then multiplied by water fluxes to give the fluxes of matter.

Fluxes computation shows (Tab.1) a net outflow through the strait for nitrate (46,000 x 10-6 moles y-1), phosphate (1,800 x 10-6 moles y-1) and silicate (87,000 x 10-6 moles y-1), while an approximated balance is achieved for POC and PN. The outflowing fluxes are larger than those previously estimated by Zore-Armanda and Pucher-Petkovic (1976) and, for nitrogen, by Civitarese et al. (1996). They are not balanced by the major freshwater inputs in the Northern Adriatic. We think that any budget calculation for the Adriatic sea must take into account the biogeochemicals that actually are available to the advective transport. Since the Northern Adriatic is the region receiving the highest charge from rivers but also the widest shallow area in the Eastern Mediterranean with a real turn for biogeochemicals removal, only a part of the terrestrial inputs, both natural and anthropogenic, is lost by water mass transport southward, the remaining being buried into sediment. As estimated by Degobbis and Gilmartin (1990), only 54% and 59% of the total nitrogen and total phosphorus respectively is lost from the Northern Adriatic. This increases the discrepancy between the export from the North and the negative balance in the Strait. The contribution of the remaining Adriatic rivers (from Croatia, Albania and Italy) is very difficult to assess because of the scarcity of the available data. Although the total nutrient input from these regions is presumably lower if compared to the input from the Northern Adriatic, the lack of a wide shallow shelf area could make horizontal advection more important than burial in sediment. Also the nitrogen input coming from the rainfall, that in the Northern Adriatic was estimated to be 14% of the total nitrogen input (Degobbis and Gilmartin, 1990), could significantly contribute to balance the budget. Further, we must take ito account that the error in water transport estimates due to mesoscale eddies and sub-basin scale recirculation, that don’t actually contribute to the water exchange rate, probably introduces errors in the fluxes calculations.

For what concerns the particulate matter transport, it must be pointed out that the most relevant water masses are the surface waters (both Adriatic and Ionian), and the Adriatic Deep Waters, with a less relevant importance of the waters flowing along the italian shelf.

The Levantine Intermediate Water, even if larger in volume, doesn’t contribute significantly to the total particle budget, because of its very low particle content, all over the year.

The particulate carbon and nitrogen flux computations result in a balanced equilibrium, while there is a net export of suspended particles from Adriatic toward the Ionian. During spring there is a maximum export from Adriatic to Ionian of POC and PN (about 1,600 x 10-6 moles of carbon and 250 x 10-6 moles of nitrogen), mainly associated with ASW; in summer and autumn this export is compensated, producing a balanced flux on a yearly scale. This balance could be explained in terms of efficiency in carbon utilization by the “Adriatic system”. This hypothesis is in fact consistent with some preliminary estimates of vertical carbon flux, that indicate that only 10% of carbon present or produced at surface reaches the layers below 200 m, indicating an almost complete re-utilization of carbon compounds in the upper layers, and a very limited export toward the bottom. However, also these results have to be revisited on the grounds of a more reliable water trasport estimates.

AKNOWLEDGMENTS

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