Bernard Coste

Biological and geochemical signatures associated with the water circulation through the Strait of Gibraltar and in the western Alboran Sea

Previous hydrological investigations of the Gibraltar Strait have left some questions unanswered as regards the composition of the Mediterranean outflow into the Atlantic Ocean. In the present study, nutrient, oxygen, and chlorophyll distributions are used to characterize the main water masses flowing through the Strait of Gibraltar and the Alboran Sea, with particular reference to Levantine Intermediate Water (LIW) and Western Mediterranean Deep Water (WMDW). High chlorophyll a concentrations, reaching 3 μg L−1 at the subsurface, are observed in the northern part of the anticyclonic Alboran Sea gyre. The salinity subsurface minimum (≃36), located temporarily in the strait, corresponds to high nutrient concentrations (7–8 μg at. L−1). Diagram analysis shows that in the upwelling area off the Spanish coast, nutrients are largely transported into the photic zone, and that photosynthetic nitrate consumption actually takes place. Consequently, the formation of the Alboran Sea O2 extraminimum can be related to the enhanced new production. Most of the lowest O2 values (<4.00 mL L−1) also correspond to a slight nutrient maximum (PO4 and NO3). Comparative diagram analysis of biologically affected parameters (oxygen and nutrients) versus salinity as a conservative property confirms and expands results from the classical θ-S analysis. Oxygen and silicate exhibit the best tracer qualities for the circulation pattern, demonstrating in particular that during the period of observations, water rich in LIW feeds the Mediterranean outflow.

Re-evaluation of the nutrient exchanges in the strait of gibraltar

Abstract New data of inorganic, particulate and dissolved organic forms of nitrogen, phosphorus and silicon from the Mediterranean Sea near the Strait of Gibraltar allow an estimate of the exchanges of these elements between the Mediterranean Sea and the Atlantic Ocean. Input by Atlantic waters is lower than output by the Mediterranean waters. The difference accounts for about 10% of the total nitrogen and phosphorus outflow and for about 50% of the total silicon outflow. The deficit appears to be balanced by the nutrient supply from land drainage. The dissolved organic nitrogen and phosphorus are about 50% of the total nitrogen and phosphorus inflow and less than 30% of the outflow. The Mediterranean Sea appears to be an active basin of mineralization.

The input of nutrients by the Rhône river into the Mediterranean Sea: Recent observations and comparison with earlier data

The nutrient input by the Rhone river into the Mediterranean Sea was measured from June 1994 to May 1995. The various fractions of N and P (dissolved and particulate, organic and inorganic) as well as chlorophyll and dissolved silicate concentrations were measured twice a month at Arles, 40 km upstream of the river mouth, in this period. In addition, some samples were taken when the river was in high flood.The study shows that nitrogen is mainly present as nitrate (76%). Dissolved and particulate organic nitrogen represent 8 and 9%, ammonium 5% and nitrite 1% of total nitrogen respectively. Almost half of the phosphate is particulate phosphate, the largest part of which is calcium-bound phosphate. Orthophosphate represents 31% of total phosphate.We estimated the total input of nitrogen, phosphate and silicate as 115–127 kt y-1 (N), 6.5–12.2 kt y-1 (P) and 135–139 kt y-1 (Si) by different methods. For nitrogen an annual input of 92.3 to 96.1 kt of nitrate is found, 1.3 to 1.5 kt of nitrite, 6.3 to 6.7 kt of ammonium, 9.7 to 9.8 kt of dissolved organic nitrogen and 5.3 to 12.7 kt of particulate nitrogen; for phosphate the annual imput was 2.7 to 3.0 kt of orthophosphate, 1.3 to 1.7 kt of dissolved organic phosphate & polyphosphate and 2.5 to 7.5 kt of particulate phosphate. While the N-input was mainly in the form of nitrate, a large part of the phosphate input was particulate-P. Comparisons to previous estimations show that the mean annual nitrate concentration in the Rhone has increased by about 50% during the last two decades.

Origin of high phytoplankton concentration in deep chlorophyll maximum (DCM) in a frontal region of the Southwestern Mediterranean Sea (algerian current)

Abstract In June 1990, we investigated the Atlantic flow along the Algerian coast in the Western Mediterranean. Intensive and fine resolution sampling along 13 longitudinal transects, consisting of 4–12 stations (3–5 nautical miles apart), was carried out with physical, chemical and biological measurements. This sampling enabled us to describe quasi-synoptically the spatial distribution over a large region between 1° and 4°E. Two oligotrophic systems, where the integrated chlorophyll was lower than 30 mg m −2 , were separated by an almost continuous rich band characterized by integrated chlorophyll higher than 60 mg m −2 . This chlorophyll-rich band, with concentrations greater than 2 mg m −3 (up to 15.8 mg m −3 ), was found along the boundary of the Atlantic flow, indicating that the phytoplanktonic biomass distribution is determined by hydrodynamics. The patchiness of phytoplankton, both in chlorophyll concentration and in species abundance, along a density surface resulted from interactions between Atlantic and Mediterranean waters. Also, a first assessment of the biological response to the mesoscale hydrodynamical features can be deduced from the comparative analysis of the physical, chemical and biological parameters. Large anomalies in nutrient and chlorophyll distributions indicated strong vertical advections along the front, either following density surface (isopycnal mixing) or crossing it (diapycnal mixing). Important isopycnal mixing of nutrients were evidenced on the inshore side of the current, while diapycnal mixing was observed only on the seaward side. These two processes seem to bring nutrients into the photic zone, leading to an enhanced in situ growth. A cross-front circulation is proposed to explain (i) the vertical distribution of chemical parameters near the front and (ii) the associated phytoplankton patchiness.

Seasonal water mass distribution in the Indonesian throughflow entering the Indian Ocean

A multiparametric approach is used to analyze the seasonal properties of water masses in the eastern Indian Ocean. The data were measured during two cruises of the Java Australia Dynamic Experiment (JADE) program carried out during two opposite seasons: August 1989 (SE monsoon) and February-March 1992 (NW monsoon). These cruises took place at the end of a La Nifia event and during an E1 Nifio episode, respectively. Seven sources have been identified in the studied region for the 200-800 m layer: the Subtropical Indian Water, the Indian Central Water, the modified Antarctic Intermediate Water, the Indonesian Subsurface Water, the Indonesian Intermediate Water, the Arabian Sea-Persian Gulf Water (AS-PGW), and the Arabian Sea-Red Sea Water (AS-RSW). The selected tracers are potential temperature, salinity and oxygen with mass conservation and positive mixing coefficients as constraints. The analysis indicates the proportion of each water source along the Australia-Bali section and into the Indonesian channels. Although no large changes are observed for Indonesian waters, significant seasonal variations are found for the southern and northern Indian Ocean water. During the NW monsoon, the contribution of the AS-RSW increases at the entrance of the Indonesian archipelago whereas the contribution of the south Indian waters decreases in the northwest Australia basin. In a complementary study, nutrients are introduced into the multiparametric analysis in order to more clearly separate the signature of the north Indian waters (AS-PGW, AS-RSW) and to provide supplementary information on the biological history of the water masses, which is compared to large-scale primary production estimates.