Eulerian sampling of zooplankton from Marseilles' Vieux Port over a 24 h period

A 24 h continuous study of hydrological parameters and zooplanktonic populations was carried out in Marseilles' Vieux-Port, a semi-closed area supposed to be a fairly homogeneous body of water. Sampling was performed in spring 1980 with a pump system. Samples were taken every 30 min over a period of 24 h, during which time only slight temperature variations were recorded (min.=14.5°C, max.=15.1°C); salinities were high and fairly stable (min.=37.49, max.=37.63%.). It can thus be assumed that the water was homogeneous. Concentrations of suspended particulate matter were high and fairly stable (x = 11.8 mg 1^-1). In most samples, nitrate levels were fairly low (x = 0.84 g-at 1^-1); conversely, phosphate contents fluctuated greatly (min.=0.24 g-at 1^-1, max.=2.24 g-at 1^-1). As a result of these fluctuating nutrient-salt values, the N:P ratios were often unbalanced (between 0.5 and 2.0). Detergent values were low (often less than 0.2 ppm). Zooplanktonic populations were made up of a few eurybiotic species, the most thriving of which were the cladoceran Podon polyphemoides and the copepod Acartia clausi. These two species perform nychthemeral migrations towards the surface: P. polyphemoides during the day and A. clausi at night. As a result of these opposite patterns of migrations, the planktonic biomass remained fairly stable near the surface. There were also some meroplanktonic species (especially cirripede larvae) and a small number of other copepods present. This study revealed a regular hydrological and faunal cycle with a return to initial conditions at the end of the 24 h period.