Distribution and retention of the copepodEurytemora affinis hirundoides in a turbid estuary

The spatial and temporal distribution of the copepodEurytemora affinis hirundoides (Nordquist, 1888) in the Gironde estuary, southwest France, was investigated in 1976, and between 1981 and 1983. The distribution pattern of the population bore a clear relationship to the patterns of water circulation and transport of particulate matter. Laboratory experiments suggested thatE. affinis hirundoides is incapable of autonomous displacement against the direction of general circulation and behaves as passive particles. A major problem faced by species living in estuaries is that of the maintenance of endemic populations. Some authors have suggested that behavioural responses may play an important role in minimising population losses. Our study indicated that hydrodynamic processes are sufficient to explain the distribution and retention ofE. affinis hirundoides, whose populations are retained in the estuary in the same manner as the suspended sediment. Since the outflow of particulate matter from the Gironde estuary to the sea is very sporadic, we hypothesize that most of the production ofE. affinis hirundoides remains within the estuary, and does not fertilize the continental shelf, at least not in the form of particulate organic matter.