INTRODUCTION AND SETTING

Cruise LTES1-10, coordinated by ISMAR-CNR of Bologna, has been dedicated to the maintanance of the observational sites already included (buoy S1) or next to be included (buoy E1) on the network LTER-Italia. LTER-Italy is coordinated by CNR and Dr. Ravaioli is member of the Coordinating Committe. It participates to the EU project Life + Enveurope, thank to the data provided by the S1 Buoy. The cruise was planned also to further acquire oceanographic data for the biogeochemical modeling designed within the EMMA project (Life 2004-2007) and aiming at the monitoring of extension and timings of hypoxic and anoxic phenomena on the Adriatic Basin, i.e. by repeating classical and well known transects (Venezia, Adige, Po-Rovinij, Casal Borsetti, Rimini, Senigallia, Ancona) and other ones.

This paper reports the shipboard activities during cruise LTES1-10, including description of the ship, equipment and their usage, along with details of the general settings, performances and some scientific and technical results.

GEOLOGICAL AND OCEANOGRAPHICAL SETTING

Geological setting

The Adriatic Sea (Fig.1) is an epicontinental sea showing two margin configurations, north and south of the Gargano Promontory RIDENTE2005 and references therein).

The northern Adriatic (NA) Sea is bounded by the Italian peninsula to the east and by the Balkans to the east (Fig. 1) and is the northernmost part of the Mediterranean Sea. It is characterized by very shallow environment, with an average depth of $\sim$35 m, and regularly and gently slopes toward the south until the 120 m isobath, taken as its southern open boundary, approximately north of 43:20 ARTEGIANI1997-I, RUSSO1996, POULAIN2001-NAS. Other authors consider Ancona or Rimini to be the southern limit of the NA.

The Central area is characterized by the Mid Adriatic Deep (MAD), a remnant basin, 260 m deep, separated in 2 depocenters by the Central Adriatic deformation belt ARGNANI1997, and bordered by the Gallignani and Pelagosa (Palagruza) ridges to the south and by the structural high of the Tremiti Islands. The two depressions of the MAD are likely to be filled by the NadDW.

Figure 1: Adriatic Sea setting.

Oceanographical setting

Being an epicontinental basin, the hydrology and dynamics of the NA are primarily influenced by meteorological forcing, thermal variations and river runoff. Climatological studies (see CUSHMAN-ROISIN2001 and citations therein) indicate that prominent weather situations in the NA include unperturbed weather or airflow from the northwestern, northeastern and south-eastern quadrants (respectively Etesian, Bora and Sirocco winds). Bora and Sirocco are the most frequent winds in the area and often trigger severe wind-storms. Despite its limited volume, the NA receives approximately 20 % of the total Mediterranean river runoff RUSSO1996. This runoff comes mainly from the Po River, whose annual average flow rate is approximately 1500 m$^{3}$/y ARTEGIANI1981, RAICICH1994. This leads to a net gain of fresh water. In autumn, intense cooling and evaporation processes, usually associated with Bora wind events over the NA, create conditions for dense water formation during the winter VIBILIC2005.

Due to runoff and heating in the late spring and summer and to autumn-winter cooling, gradient currents are established leading to a cyclonic circulation system ZORE-ARMANDA1956, BULJAN1976, FRANCO1982, ORLIC1992, ARTEGIANI1997-I, ARTEGIANI1997-II, RUSSO1996, HOPKINS1999, POULAIN2001-CI consisting of an entering NW-ward current (the Eastern Adriatic Current, EAC), on the eastern coast, and an exiting SE-ward current (the Western Adriatic Current, WAC) on the western coast. EAC introduces warmer and saltier water into the sub-basin, while WAC pushes fresher water towards the southern regions. The general circulation pattern in the NA is also largely affected by wind. Bora episodes can generate a transient double gyre circulation consisting of a cyclone north of Po delta and an anticyclone to the south, driving the upwind extension river plume filaments JEFFRIES2007; an anticyclonic circulation also develops along the southern Istrian coast POULAIN2001-BOOK, POULAIN2001-CI, while Bora enforce flow in the WAC BOOK2007, URSELLA2007.

The NA Sea is one of the most biologically productive regions of the whole Mediterranean. The rate of oxygen consumption due to biogeochemical processes is the largest of the entire Adriatic basin, with a maximum occurring around the Po River delta area ARTEGIANI1997-II. This region can thus be regarded as a favourable environment for the development of hypoxic conditions. The formation of a hypoxic bottom layer in wide areas of the basin DEGOBBIS1993, DEGOBBIS2000 can cause major ecological problems such as the mass mortality of marine animals, defaunation of benthic populations and a decline in fisheries production.

Hypoxia is usually defined as occurring in regions where dissolved oxygen concentrations are less than 2 ml l$^{-1}$ (equivalent to 2.8 mg l$^{-1}$). This concentration is the lower tolerance limit for many benthic species SIMUNOVIC1999, RABALAIS2000, WU2002.