Spatial distribution and biological rhythms of suprabenthic mysids from the English Channel

One hundred and ninety-eight suprabenthic hauls from the English Channel and the Seine Estuary were taken with a modified Macer-GIROQ sledge. Numerically, mysids were the dominant group amongst the peracarids collected with the sledge and 28 species were recorded. Mysid densities were higher in the oligohaline zone (>200,000 ind 100 m(-3)) of the Seine Estuary than at other stations in the English Channel (where the density was the highest on medium sand stations, similar to 5000 ind 100 m(-3)). Two main gradients of mysid distribution were identified: (1) a higher species richness in the western part of the Channel compared with the eastern part of the Channel; and (2) an increasing density gradient from west to east during spring and a decreasing gradient from west to east during the autumn (eastern high abundance of Haplostylus spp. during spring, and western high abundance of Schistomysis ornata and Erythrops elegans during autumn). Mysids showed important diel rhythms with maximum abundance at sunrise and sunset, and low density at night. According to their swimming activities, suprabenthic mysids were classified into three groups: upper organisms with a very strong activity, species with a strong activity and occupying the whole water column near the sea-floor, and lower species with limited swimming activity. Nevertheless, the swimming activity could be modified according to the hydrodynamics of each site. A seasonal cycle of abundance (summer recruitment for dominant species Anchialina agilis, Haplostylus lobatus, Haplostylus normani) was observed on two stations where temporal samples along the year were available.