Structures hydrologiques à petite échelle dans la zone de divergence de la Mer Ligure (Méditerranée nord-occidentale): Etude par l'analyse en composantes principales de séries spatio-temporelles multiples

Abstract

During the “Pros IV Cruise” of the R. V. “Korotneff” (March, 1979), continuous surface measurements of hydrobiological parameters were made across the divergence zone of the Ligurian Sea (North-Western Mediterranean). Principal component analysis (PCA) applied to data of two transects provided a graphic description with a high resolution of their spatial structure. Data from the first transect (Marth 12) confirmed the well known classification of the area into coastal, intermediate and central zones. The distribution of the data points reveals a simple and direct gradient between the coastal and intermediate zones and a great complexity of hydrological structures and chlorophyll a in the intermediate zone and its transition with the central area. Projection of the variables enabled this distribution to be interpreted as an irregular succession of small areas where vertical mixing is indicated by high salinity and nutrient concentration and patches of high chlorophyll a concentration. The second transect (March 15) included two parts: one parallel to the coastline, the other perpendicular to the coast. The PCA analyses show that similar hydrobiological structures exist along the two parts of the transect, and reveal the existence of either an isotropic patch of water with the characteristics of the intermediate zone or a meander in the front between the coastal and intermediate zones. Moreover, the analyses distinguish small phytoplankton patches (500 to 700 m wide) and group them according to their interrelationships between the variables. The resolution of PCA graphs is equivalent to several sampling steps, enabling similar interpretation of the major features from a smaller set of data and thus the simultaneous analysis of all the cruise data. Throughout the 15 d duration of the cruise the delimitation of the divergence area into 3 zones persisted. The spatial structure of the coastal and central areas was very stable, while the intermediate area displayed an important time-dependent variability in hydrobiological patterns.