The temporal variability of hydrological variables in the Guadalquivir estuary was examined during three years through a real-time remote monitoring network (RTRM). The network was developed with the aim of studying the influence of hydrodynamical and hydrological features within the estuary on the functioning of the pelagic ecosystem. Completing this data-gathering network, monthly cruises were performed in order to measure biogeochemical variables that are indicative of the trophic status of the aquatic environment. The results showed that several sources of physical forcing, such as wind, tide-associated currents and river discharge were responsible for the spatio-temporal patterns of dissolved oxygen, salinity and turbidity in the estuary. The analysis was conducted under tidal and flood regime, which allowed us to identify river discharge as the main forcing agent of the hydrology inside the estuary. In particular, episodes of elevated turbidity detected by the network, together with episodes of low salinity and dissolved oxygen were closely related to the increase in water supply from a dam located upstream. The network installed provided accurate data that can be rapidly used for research or educational applications and by policy-makers or agencies in charge of the management of the coastal area.

A significant fraction of anthropogenic carbon dioxide (CO2) released to the atmosphere is absorbed by the oceans, leading to a range of chemical changes and causing ocean acidification (OA). Assessing the impact of OA on marine ecosystems requires the accurate detection of the rate of seawater pH change. This work reports the results of nearly 3 years of continuous pH measurements in the Mediterranean Sea at the Strait of Gibraltar GIFT time series station. We document a remarkable decreasing annual trend of −0.0044 ± 0.00006 in the Mediterranean pH, which can be interpreted as an indicator of acidification in the basin based on high frequency records. Modeling pH data of the Mediterranean outflow allowed to discriminate between the pH values of its two main constituent water masses, the Levantine Intermediate Water (LIW) and the Western Mediterranean Deep Water (WMDW). Both water masses also exhibited a decline in pH with time, particularly the WMDW, which can be related to their different biogeochemical nature and processes occurring during transit time from formation sites to the Strait of Gibraltar.

16 17The North Atlantic is the most important sink for atmospheric CO 2 although there still 18 remain uncertainties about the total amount stored by this region and the contribution of 19 the anthropogenic CO 2 (C ANT ) that is exchanged between the Mediterranean Sea and the 20 Atlantic Ocean. During the P 3 A 2 cruise performed in October 2008 throughout the 21 oceanic area covered by the Gulf of Cádiz and the Strait of Gibraltar, which channelizes 22 the water exchange between the Atlantic and the Mediterranean, extensive 23 measurements of the carbon system parameters (pH, total alkalinity and total inorganic 24 carbon) and others related (dissolved oxygen and nutrients) were carried out to analyse 25 their distribution in the area. In order to study the C ANT spatial variability, three 26 observational methods for C ANT concentration assessment (φC T º, ∆C* and TrOCA) 27 were applied. The three water masses identified in the area, North Atlantic Central 28Water (NACW), North Atlantic Deep Water (NADW) and Mediterranean Outflow 29Water (MOW), were shown to contain different C ANT concentration. NADW exhibited 30 the lowest C ANT levels whereas NACW was the most C ANT enriched. Data also indicate 31 a net import of C ANT from the Atlantic towards the Mediterranean through Gibraltar. 32

The Mediterranean Outflow Water (MOW) spills from the Mediterranean Sea (east North Atlantic basin) west off the Strait of Gibraltar. As MOW outflows, it entrains eastern North Atlantic Central Waters (ENACW) and Intermediate Waters to form the neutrally buoyant Mediterranean Water (MW) that can be traced over the entire North Atlantic basin. Its high salinity content influences the thermohaline properties of the intermediatedeep water column in the North Atlantic and its dynamics. Here, the composition of MW in its source region (the Gulf of Cádiz, west off Strait of Gibraltar) is investigated on the basis of an optimum multiparameter analysis. The results obtained indicate that mixing of MOW (34.1% 6 0.3%) occurs mainly with overlying ENACW (57.1% 6 0.8%) in a process broadly known as central water entrainment. A diluted form (80% of dilution) of the Antarctic Intermediate Water (AAIW) reaches the region and also takes part in MW formation (8.3% 6 0.5%). Finally, the underlying Labrador Sea Water (LSW) also contributes (0.4% 6 0.1%) to the characteristics of MW. From these results and considering 0.74 Sverdrups (Sv; 1 Sv [ 10 6 m 3 s 21) as the mean outflow of MOW, the MW exportation rate was inferred (2.2 Sv), which, decomposing MW, means that the MOW outflow is accompanied by 1.24 Sv of entrained ENACW, 0.18 Sv of AAIW, and ,0.01 Sv of LSW.

A selection of ASAR images have been analyzed, together with instantaneous images of surface chlorophyll recorded with MERIS and MODIS, in order to study the relationship between the physical and biological processes associated with internal waves in the Strait of Gibraltar and Alborán Sea. The images show peak levels of chlorophyll at the coastal edges to the north and south of the Camarinal Sill (CS) during the generation of internal waves, and peak levels of chlorophyll associated with the wave fronts as they travel into the Alborán Sea. The images have been compared with in‐situ data. The results seem to indicate that, during the generation of the internal waves, a suction process takes place by which coastal waters rich in chlorophyll are drawn towards the center of the channel and then accompany the internal waves as they move towards the Alborán Sea.