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The bivalves Thyasira sarsi and T. equalis, with symbiotic sulphur-oxidising bacteria in their gill tissue, were collected from various sites in N. Europe, including the 115 m deep basin in Gullmar fjord, Sweden, between 1986 and 1990. The tissues of most specimens were more depleted in I3C than bivalve species without bacteria, indicating the importance of carbon fixed by the autotrophic endosymbionts to the nutrition of these bivalves. In contrast, thyasirids collected in 1988 in the Gullmar fjord showed a 613C composition indistinguishable from those of heterotrophic bivalves, suggesting that in that year there was little input of carbon from the bacteria. Shell carbonate 6I3C and 6"0 determinations on thyasirids from different sites suggest that the observed isotopic disequilibrium was proportional to growth rate, as for carbonates deposited by corals, foraminifera and algae. The carbon isotope ratios indicated that the thyasirids from the Gullmar fjord collected in 1988 had a slower growth rate than those collected in 1986. Between 1986 and 1988. in the upper 15 cm of sediment in the Gullmar fjord, the elemental sulphur concentration decreased from 0.5-8.0 to only 0.003-0.04 mg-at. S dm-= and total sulphide decreased from 1.0-18.0 to 0.1-3.6 mg-at. S dm-3. Below 20 cm depth, the limit to which T sarsi forms tunnels in the sediment, the reduced sulphur concentrations were still high in 1988, > l 2 mg-at. dm-3 It is proposed that the utilisation and oxidation of reduced sulphur in the sediment, following the population explosion of 7. sarsi post-1980, was greater than the sulphate reduction rate, with the result that the symbiotic bacteria were eventually starved of substrate, and that most of the bivalves died. High sediment sulphide levels had been restored by 1990, following the death of most of the infauna after de-oxygenation of the deep-water.

marine ecology. progress series

Varying nutritional dependence of the thyasirid oivalves Thyasira sarsi and T. equalis on chemoautotrophic symbiotic bacteria, demonstrated by isotope ratios of tissue carbon and shell carbonate