Bremerhaven Caisson for intertidal field studies

Abstract

The Bremerhaven Caisson has been designed for field experiments in ecosystem research and pollution studies in intertidal areas. The container-like construction (5.6 m long, 2.35 m wide, 2 m high) has an open top and bottom and encloses an area of 13 m2. The interior of the caisson is connected with the surrounding tidal area by a tube system which functions as water inletloutlet resulting in total water exchange twice a day according to the tidal rhythm. In long-term pollution studies the pollutant must be added every flood tide. Automatic dosing is used for pollutant addition. Due to periodic water exchange, only short-term investigations are possible on seston and plankton, while long-term studies may be carried out on the enclosed sediment and benthic organisms. The caisson is mobile and can be towed by detachable floats to or from the experimental site. As no severe disturbances of the site occur upon refloating, postexperimental effects can be studied under natural environmental conditions. Microor mesocoms have increasingly received attention during the last decade. In the definitions of Banse (1982) and Grice and Reeve (1982), mesocosms must be larger than benchtop container systems but smaller than large empondments, bays, or small lakes. Mesocosms used in plankton research, for example, are the Kiel Plankton Tower, the Hamburg Enclosure, and the CEPEX Enclosure (see Grice and Reeve, 1982). Only a few mesocosms like the land-based MERLtanks incorporate planktonic and benthic systems and allow the study of exchange processes between both (Banse, 1982). With a volume of ca. 13 m3 water at high tide and an enclosed area of 13 m2 the Bremerhaven Caisson (Fig. 1) is a mesocosm. The system is designed for field experiments on ecosystem or pollution research in intertidal areas, but use in shallow parts of lakes or rivers is also possible. The caisson allows short-term plankton and seston investigations, long-term benthos research, and studies on the interrelations of planktonic and benthic systems. The unit is mobile, and experiments can be conducted from a few days up to 1 or 2 mo. Related systems are the 'Large indoor tidal mud-flat ecosystem' at the Netherlands Institute of Sea Research, Texel (de Wilde and Kuipers, 1977), and an immobile system of wooden basins enclosing an intertidal mussel bed, which was used in the early seventies for heavy metal studies (de Wolf et al., 1972). Fig. 1. Bremerhaven Caissons in intertidal area, German Bight . S m O Inter-Research/Printed in F. R. Germany Mar. Ecol. Prog. Ser. 16: 193-197, 1984 Technical description and operation. The Bremerhaand held in place by supports. Once in position the ven Caisson is a rectangular, container-like construecaisson strands with the receding tide and the floats tion 5.6 m long, 2.35 m wide, and 2 m high with open will be detached. At the end of the experimental top and bottom. It encompasses an area of ca. 13 m2 period the caisson is refloated. The area previously (Fig. 2). The main parts of the caisson, which can be occupied by the caisson can be marked and further Ca~sson s ~ d e view