Bioturbation of intertidal sediments : an experimental approach involving the amphipod Corophium volutator (Pallas)
The effect of dense accumulations of macroalgae on the mudflat invertebrate populations, was investigated in Newburgh South Quay. In order to partition the effects of the physical presence of weed and the consequent anoxic conditions of the sediment, the distribution of macro- and meiofauna was examined within a range of redox potential (Eh) and weed cover. No obvious trend was found for any species within the gradient of Eh. However, the number of individuals of Capitella, Tubificoides and Pygospio were significantly more abundant under weed cover, whereas Corophium disappeared almost completely in areas covered by weed. The relationship between Corophium burrows and depth of the chemocline (defined here as the line of colour change dividing the oxic from the anoxic zones of the sediment), was investigated by means of a photographic survey of sediment profiles. The depth of the chemocline was found to be closely associated with the bioturbational processes of Corophium, considered here only as the number and depth of its burrows. To demonstrate that Corophium is a significant bioturbator, and through bioturbation it might affect the invertebrate mudflat community, field and laboratory experiments were carried out, where the densities of Corophium were manipulated. The field experiment showed an increase in the organic matter of the sediment in the treatment with high density of Corophium; also, a decrease in the number of young Hydrobia was found in the same treatment. This field experiment revealed an adult-recruit interaction in Corophium. In the laboratory experiment, Corophium showed a significant increase in the redox potential, and organic matter of the sediment. There was also, an increased percentage of nematodes in deeper sections of the sediment in the treatment with high density of Corophium. This might be associated with a greater availability of oxygen near the burrows, or to an enhancement of the bacterial population upon which nematodes feed. Following the indications that Corophium by bioturbation can affect the depth of the chemocline, redox potential and distribution of nematodes with depth, its potential in the recovery of organically polluted sediments was investigated. Field and laboratory experiments were carried out in which Corophium was added to almost azoic and grossly enriched mud. In the laboratory experiment the chemocline was located significantly deeper in the treatment with Corophium, and in the field, numbers of nematodes, Manayunkia (and possibly Pygospio) increased in the treatment with Corophium. This field experiment demonstrated again an adult-recruit interaction in Corophium. It was concluded that the increase in areas covered by weed could lead eventually to the disappearance from the estuary of Corophium, which is the major prey item of many of the shorebirds and fish. Corophium is also an important sediment bioturbator and probably plays a significant role in the recovery of organically polluted sediments.