ABSTRACT

In marine sediments macrofaunal organisms often produce deep reaching tubes or burrows that greatly influence the biogeochermstry of the inhabited sediment (Hylleberg & Hennksen 1980, Aller 1982, Huettel 1990) Among the burrowing organisms thalassinidean shnmps are a group of decapod crustaceans that are often abundant in coastal sediments (Suchanek 1985, Gnffis & Suchanek 1992) and build complex burrow systems reaching up to 2.5 m or more into the sediment (Pemberton 1976) Recent studies focused on the species Callranassa truncata that occurs at high densities (120 rnd m−1) in shallow-water sediments off the coast of the Italian island Giglio in the Mediterranean Sea and constructs elaborate burrows to a sediment depth of 80–100 cm (Ziebis et al 1996a). Like many burrowing organisms it produces a ventilation current through its burrow system Many attempts to study this ventilation n in laboratory systems were difficult due to the constraints of aquariums and the difficulty of measuring inside burrows without destroying the sediment structure (Witbaard & Duineveld 1989, Forster & Graf 1992, 1995) Our interest was to gain information on the in-situ burrowing behaviour and to find out how deep oxygen-rich water is actually pumped into the sediment by bio irrigation and how this is affecting the sediment chemistry

We report here the construction of a diver observatory and its deployment in the field for in-situ investigations of deep-burrowing organisms and their effects on the sedimentary environment The large, hexagonal container (1 2 m high, 2 m in diameter) was built of 6 transparent acrylic walls that were held by a stainless steel frame and was covered by a lid made of PVC to avoid light penetration It was buried in the sediment so that the lid was level with the sediment surface The sediment from inside was removed to allow divers to enter through a door in the lid in order to perform observations and measurements from inside the observatory into the surrounding sediment We demonstrate the unique possibilities of observing the behaviour of burrowing animals in their natural habitat down to a sediment depth of 1 m and show the opportunities of direct sampling of pore- and burrow-water in intact systems as well as detailed in-situ measurements through silicone-filled ports in the walls of the observatory

INTRODUCTION

It has always been an a m of benthic ecologists to perform investigations and measurements reflecting as close as possible the real conditions in the investigated environment Therefore many studies have been carried out in aquaria, mesocosms and flow tanks (Kristensen 1985, van Duyl et al 1992, Huettel & Gust 1992) simulating in-situsituations in order to gain information on the interaction of biological or physical factors with the geochemistry of ocean floors.

In interdisciplinary projects, where parallel field and laboratory studies have been carried out to gain information on a defined marine system, it was realised that there often exists a gap between laboratory and in-situmeasurements (Rumohr et al 1987) It was difficult to relate laboratory findings to the environment and a problem to measure directly in the sediment without disturbing the structure

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