o Which is more important in the degradation of TBT in the surface 

 microlayer, UV light or phytoplankton? 



o What are the most important photosynthetic organisms for 

 degrading TBT in the water column and under what environmental 

 circumstances does degradation occur? 



o What environmental conditions bring about maximum degradation 

 rates? 



5.2.1.6 Exposure Pathways 



It is very probable that exposure pathways are one of the critical 

 functions in relating the high toxicity of extremely low <»ncentrations of 

 TBT to certain organisms. Whereas the water column concentration may only 

 be a few ng/1, the surface microlayer or suspended sediment levels can be 

 2 or 3 orders of magnitude higher. Therefore the actual dose level to the 

 organism is not directly related to the water column concentration but to 

 concentration mechanisms in specific critical pathways. 



Studies are needed to provide more information on: 



o What are the critical exposure pathways via the surface 

 microlayer, suspended particulate material, sediments, P°re 

 water, water column, and/or food webs which affect specific 

 organisms of concern? 



o What are the effects of exposure to methylated butyltin vs. 

 butyltin in the water and the sediment? This is important 

 because bacterial products of metabolic debutylation of TBT 

 include methyl-dibutyltin and methyl-monobutyltin, and algal 

 metabolic debutylation products include dibutyl and monobutyltin. 



5.2.2 Effects 



Ihe correlation of laboratory toxicity studies to effects found in the 

 field will allow for field dose response estimates and cause and effect 

 relationships. TBT and related organotins are toxic to certain organisms 

 in trace quantities because of the characteristics of the physiology of 

 these organisms and critical pathways as previously discussed. For 

 example, the relative lack of mixed function oxidase s ystem s in the 

 bivalve molluscs does not permit them to purge themselves of TBT loadings 

 as readily as other organisms. Ihe mode of action of TBT is strongly 

 membrane-oriented. To determine nonlethal but detrimental effects, 

 toxicity testing in the field and laboratory will have to become more 

 sophisticated and focused en physiological processes, in particular the 

 reproductive processes. The case in point is the gastropod molluscs such 

 as the dogwhelk, Nucella lapillus. Dogwhelx populations are in jeopardy 



V-6 



