The Distribution of Amphipods in the Severn Estuary and Bristol Channel

Abstract

Gammarus pulex is a species which is intolerant of dissolved salt or of a pronounced tidal rhythm when in competition with other species, and it does not penetrate far into fresh-water at the head of the estuary. G. duebeni lives in water of low salinity but tolerates salinities up to 20 or 25 gm. per 1. It occurs from the zone of tidal fresh water, through the transition zone, into water which is nearly always brackish. Here its distribution overlaps the next species. G. zaddachi lives in salinities below 28 gm./l. and so extends from the zone which is not permanently fresh, i.e., from the zone of maximum penetration of salt water, to a point near the mouth where it is replaced by the next species. G. locusta lives in waters of salinity greater than 28 gm./l. and so occurs near the mouth only. Marinogammarus marinus occurs at mean tide level and lives in zones where the salinity does not fall below 10 gm./l. It extends from the mouth to a point somewhat below the seaward end of the G. duebeni zone. Near the mouth it may be replaced by M. obtusatus, which is believed to be a markedly steno-haline species.