Physical-biological interaction in the Strait of Gibraltarthe Trafalgar-Alborán connection

  1. Iria Sala Martínez
Supervised by:
  1. Fidel Echevarría Navas Director
  2. Carlos M. García Director

Defence university: Universidad de Cádiz

Year of defence: 2021

  1. Emilio Marañón Sainz Chair
  2. Diego Manuel Macías Moy Secretary
  3. Marina Lévy Committee member

Type: Thesis

Teseo: 643242 DIALNET


The Alborán Sea is one of the most productive basins in the Mediterranean Sea. Its productivity is directly related to the water exchange that takes place at the Strait of Gibraltar. The inflowing Atlantic water in the upper layer, also known as the Atlantic Jet (AJ), is the main driver that determines the circulation of this basin, breeding a Coastal Cyclonic Gyre (CsCG) associated with the upwelling of nutrient-rich waters in the north-western sector of the Alborán Sea (Estepona upwelling), and a large oligotrophic gyre in the western sector (WAG, Western Alborán Gyre). On the other hand, the interaction of the tidal current with the sharp topography of the Strait determines an increase of the nutrient concentration in the AJ, which may induce, in turn, an increase up to 40% in the productivity of the Alborán Sea. Most studies carried out in the Strait of Gibraltar have focused on analysing the impact of the tide-topography interaction on Camarinal Sill, located in the main channel of the Strait. Here, the current speed is high and, therefore, the residence time is short. However, along the coastal margins where the current speed is slower (i.e., longer residence time) and hence, it is favoured the accumulation of particles, upwelling processes also occur. This is the case of Cape Trafalgar (NW limit of the Strait), whose singularity lies in the presence of a submarine ridge perpendicular to the coast (Barbate High), which extends up to 2 miles offshore. The interaction of the tide with this ridge has been identified as the main cause of its high productivity. In this Thesis, the main objective was ‘to determine the influence of hydrodynamics on the distribution and dispersion of inorganic and organic fractions, in and from Cape Trafalgar, an area assumed to have a high autotrophic productivity and to act as a biomass source for the region, particularly to Western Alborán Sea’. This general objective was subdivided into 4 specific objectives that have led to four chapters. To better understand the dynamics of the Trafalgar region, firstly, we have analysed 10-years of ocean colour imagery (Chapter 1). Barbate High was characterized by the presence of a quasi-permanent tongue of high chlorophyll concentration. The average concentration of this tongue over the 10-years was 1.44 mg m-3, while in the adjacent platform of the Gulf of Cádiz the chlorophyll concentration was about half (0.79 mg m-3). Furthermore, a strong covariation of chlorophyll concentration with the fortnightly tidal cycle was identified. This fact was confirmed analysing the data outputs of a 3D hydrodynamic numerical model coupled to an ecological model, with a high temporal and spatial resolution (Chapter 2). Analysing these data, it was confirmed that the interaction of the westward tidal current (towards Gulf of Cádiz) with the edge of the submarine ridge results in the pumping of deep, saltier and nutrient-rich Mediterranean waters. However, this tidal-pumping is only effective, reaching the most superficial layers of the water column, when the westward tidal current is ≥ 0.42 m s-1, a condition attained from 2 - 3 days before and after the moment of maximum tidal flow during spring tides. On the other hand, the interaction of the tidal current with the ridge leads to the formation of a cyclonic gyre eastward of Cape Trafalgar, which slows down the water transport between Trafalgar and the Alborán Sea, and increases the residence time of the upwelled waters and the highly-productive tongue over Barbate High. The in situ data collected during the MEGAN campaign, on board the R.V. Sarmiento de Gamboa, also evidenced the difference between the central region of the Strait and the coastal margin, both from a physicochemical point of view and according to the planktonic community composition (Chapters 3 and 4). During this campaign, several characteristic regions or zones influenced by the hydrodynamic connection between the Strait of Gibraltar (and Trafalgar) and the Alborán Sea were analysed: (i) the coastal regions of Cape Trafalgar and Cape Camarinal, in the Strait, highly influenced by the tidal cycle; (ii) the temporal and spatial evolution of the water column following the AJ on its journey to the Alborán Sea, closely related to the processes associated with the frontal dynamics; (iii) the coastal region of Estepona, generally characterized by the presence of an upwelling associated with the cyclonic coastal gyre (CsCG); and (iv) the oligotrophic WAG. In these regions, the spatial and temporal variability of the plankton community composition and the associated metabolic rates were analysed. In all these regions, microplankton was the fraction with the maximum contribution (both in total abundance and biovolume). Centric diatoms were the dominant group (e.g., Skeletonema spp., Chaetoceros spp.), evidencing their adaptability to changing conditions. However, in the coastal regions of Trafalgar and Camarinal, as well as in the CsCG, the contribution of pico- and nanoplankton fraction was relatively higher than in the other oceanic stations. The contribution of mesozooplankton was also slightly higher in the coastal areas of Trafalgar, Camarinal, Estepona and in the oligotrophic WAG. Outside the Strait, during the first ~ 43 hours of the AJ journey, the plankton community showed an increase in abundance and biovolume, followed later by a slow decrease. To study the metabolic rates of the community (Chapter 4), algorithms proposed by the Metabolic Theory of Ecology (MTE) have been used. The current algorithms to estimate the Net Primary Production (NPP) consider the organisms individual size, and the limitation by temperature and light. In order to improve these estimates, we have proposed an adjustment based on nutrient limitation. The values obtained with the original algorithm proved to be much higher than those obtained with the nutrient limitation, although the trends in both cases were similar. The estimation of Gross Primary Production (GPP) in absolute values from the original algorithm and considering the nutrient limitation, evidenced its strong dependence on the community biovolume. However, when the ratio between GPP and community respiration (GPP/CR) is analysed, the difference between estimates with and without nutrient limitation differs both in values and the followed trend. The original GPP/CR ratio showed values > 5 at all stations and depths, pointing out a community with a strong predominance of autotrophy (GPP > CR). Instead, the ratio calculated considering the nutrient limitation showed the maximum values in the Trafalgar region, but then showed a decreasing trend towards the Alborán Sea, following the AJ, reaching values very close to or lower than 1, evidencing a heterotrophic metabolic balance. The results obtained throughout this Thesis help to better understand the close connection between the physical and biological processes that characterize the Strait of Gibraltar and the Alborán Sea. This was possible thanks to a multi-approach, applying different methodologies, including satellite image analysis, in situ data and numerical models, combining the strengths of each one according to the objective to be achieved and obtaining a complementary image of the processes that govern this connection between the coastal areas of the Strait and the Alborán Sea.