There are also no degradation rates estimated for butyltins in the surface 

 microlayer. Each of these habitats has its own microbial (communities with 

 probably differing abilities to degrade tributyltin. Studies of the 

 surface microlayer may have to wait for the development of better or at 

 least standardized sampling procedures. There have been no studies 

 investigating degradation rates within bottom sediments. 



The marine diatom Skeletonema costatum has been identified as a primary 

 agent for the debutylation of TBT at sublethal concentrations. Beaumont 

 et al. (1987) give a 72 hour EC 50 of 0.33 ug/1 of TBT. They further 

 state that Skeletonema virtually ceases to grow when subjected to 0.1 ug/1 

 TBT and several other species of algae have similar significant 

 depressions in their rate of growth at similar concentrations. The most 

 effective debutylation of TBT has been found to occur at algal cell 

 densities higher than those normally found in estuarine waters (Lee, 

 1986) . It may well be that bloom conditions coupled with the cellular 

 uptake of TBT will seguester enough TBT so that exposure concentration 

 from the water stays below lethal levels. 



Lee et al. (1987) suggest that hydroxybutyldibutyltin is formed by algae 

 in the presence of light but not by bacteria under dark conditions. These 

 results suggest a different metabolic pathway for the debutylation in 

 algae and in bacteria. It may be possible to optimize the rate at which 

 tributyltin is debutylated and thus reduced to a lower toxicity by 

 carefully maintained populations of degrading organisms. 



The metabolic pathways by which the debutylation occurs is poorly 

 understood as are the limits to the process. It is possible that the 

 presence of TBT in the water column may provide a selective advantage to 

 bloom conditions, by being more toxic to specific species. 



4.4.3 Adsorption to Sediments 



Diverse results have been reported in studies to determine the persistence 

 of tributyltin in bottom sediments. Table 4.16 summarizes the sorption 

 and desorption coefficients for various sediment investigations conducted 

 to date. Johnson et al. (1987) found that the concentration of TBT in 

 sediment pore water, either decreased slightly or remained unchanged as 

 the depth of sampling in the sediment increased from 10 cm to 20 cm. They 

 reported a concentration of TBT in undisturbed sediment pore water of 

 40 ng/1. Bottom sediments which had been recently dredged were found to 

 contain 10 ng/1 of TBT in pore waters. This study was conducted in Back 

 Creek in Annapolis, Maryland which has marinas lining the perimeter of the 

 creek. 



The ability of particulate and suspended matter to remove TBT from the 

 water column varies with the distribution of turbidity within the 

 estuary. In partially mixed estuaries, there can be well defined 

 turbidity maximum where charge interactions with the saltwater interface 



IV-35 



