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14C-labeled tributyltin-chloride (TBT-Cl) was delivered to the water column of seagrass microcosms held in the laboratory under flow-through conditions. Benthic macroinvertebrate abundances across a three treatment, logarithmic dose gradient were compared to untreated control microcosms. Within 3 to 6 weeks, statistically significant mortality appeared in the high treatment. Sensitive species included surface deposit feeders of several phyla, as well as a suspension feeding mollusc. Results suggest that effects can arise because TBT is rapidly accumulated in surface sediments, as well as in Thalassia tissues. Concentration of tracer in plant tissues, animals, and sediments suggests that measurement of TBT (and total butyltin) in these components of seagrass beds would provide a better indicator of exposure regimes than occasional measurements in the water. A propensity for accumulation, along with a biological vulnerability, suggests a sentinel role for seagrass ecosystems in some shallow coastal areas. Experimental findings demonstrate concern for some key invertebrates within beds proximal to TBT sources, and ecological risks could radiate through coastal food webs dependent on these productive vegetated shallows.

Seagrass beds are highly productive ecosystems whose leaves and sediments provide considerable surface area for interactions with seawater; thus, they may be foci for the sorption, accumulation and degradation of pollutants. The fate of the potent biocide tributyltin (TBT) in water that passes through seagrasses and over sediments was studied in marine microcosms containing sediment cores from a subtropical seagrass bed (including Thalassia testudinum and associated fauna) and seawater. Over 3 or 6 weeks, 48 of these microcosms were dosed weekly for 24 h with 14 C-labelled TBT at three different doses (initial concentrations of 0·2, 2 and 20 μg TBT + liter −1 ) and flushed with flowing seawater between dose periods. The TBT was rapidly removed from the water column (half times of 10–20 h), primarily through adsorption onto sediments and seagrass leaves. By contrast, 12 microcosms that received similar TBT doses but that contained only seawater had TBT removal half times of 2–7 days. Accumulation of TBT in sediments and grasses was temporary, however; at harvest, the seagrass microcosms contained just 20–30% of the 14 C that had been adsorbed or assimilated during dose periods, and half of this label was in degradation products. The principal mechanism of TBT loss from solids was degradation followed by desorption of degradation products (largely monobutyltin and CO 2 , which are more soluble than TBT). Despite relatively rapid TBT degradation, TBT accumulated in fauna; at harvest, 2–6% of the 14 C in microcosms was in invertebrates. Thus seagrass beds can be viewed as foci for the concentration of TBT, as processors of TBT to less toxic degradation products, and as vectors for distribution of TBT through coastal food chains.