242 A
. de Zwaan et al. J. Exp. Mar. Biol. Ecol. 256 2001 241 –251
1. Introduction
Benthic macrofaunal species show great variability in anoxic tolerance, even within closely related taxonomic groups. A comparison of studies is often complicated by
differences in experimental set-up and conditions used to estimate this tolerance Groenendaal, 1980. In a study of four bivalve species from the Mediterranean zone we
showed that the experimental set-up had a great impact on anoxic survival time De Zwaan and Eertman, 1996. In closed systems, a decrease in pH of the incubation
medium and accumulation of ammonium and sulphide occurred. Both a high dose of cadmium and the broad-spectrum antibiotic chloramphenicol increased survival time.
Also a postponed shell blackening and biotic sulphide accumulation was observed.
In the present survey we extend our study to anoxic survival of Macoma balthica, a bivalve from temperate Baltic and Atlantic coastal area. Past reports of mortality rate of
this clam show wide variability in anoxic survival. Brafield 1963 observed that this clam only tolerated anoxia for 2 or 3 days temperature not given. Dries and Theede
1974 reported LT values of specimens collected in the Western Baltic Sea of 22 and
50
20 days at 10 and 158C, respectively. More recent, Jahn and Theede 1997 reported median mortality values of 8–12 days at 108C for clams from the same location. The
large variation in data for M . balthica raises the question whether the results obtained in
closed systems indeed reflect artefacts or tolerance to anoxia. Understanding of the actual mechanisms causing death may help explain the inconsistency in literature data.
Clams Macoma balthica were collected from a mudflat of the Dutch coastal waters. The role of biotic sulphide and the influence of exogenous sulphide were studied in
semi-static systems. We also compared the effect of chloramphenicol and a number of other antibacterial agents applied in aquaculture suppressing bacterial activity. The
activity of bacteria was followed indirectly by estimating the release of sulphide and ammonium and the recording of pH, as well as by counting bacterial numbers in the
systems.
2. Material and methods
