hexachloran levels in the water here are likely due to their high 

 volatility, which appears to be driving most of the HCH present 

 in these hot climates into the atmospheric compartment. 



The rather unusual distribution of HCH within the water 

 column — that is, the marked drop in its level that occurs at 

 considerable depths — is explained by the relatively high 

 solubility of this pollutant in seawater and by the fact that it is 

 present largely in dissolved form (in contrast to other globally- 

 occurring CH's. which are largely sorbed and precipitate 

 together with suspended matter). Hexachlorocyclohexane 

 isomers are practically the only common pollutants whose 

 behaviorin the open ocean can be explained largely on the basis 

 of hydrochemical factors. 



Also worthy of note is the fact that there was almost a 

 twofold lower concentration level of HCH in the waters of the 

 atoll lagoon versus the ocean water surrounding the lagoon. 

 The only obvious explanation was the presence of a temperature 

 gradient, which entailed differing rates of photochemical and 

 microbial transformation of the cyclohexane ring, and the 

 accelerated evaporation that was taking place in the lagoon. 



Another interesting finding was that the lagoon water of 

 Caroline Atoll contained roughly one-half as much PCB as did 

 the surrounding ocean water. Moreover, the composition of 

 the CH's was significantly different: the lagoon water contained 

 virtually no highly chlorinated PCB congeners, which was 

 probably due to the higher rates of photochemical processes in 

 the thoroughly heated shallow water of the lagoon. It should be 

 noted, however, that as a general rule, PCB levels in the 

 equatorial Pacific were only slightly lower than in the northern 

 Pacific, even though sample composition turned out to be 

 considerably different. 



Whereas most of the PCB' s in the Bering Sea consisted of 

 di- and trichlorobiphenyls, the major constituents of PCB's 

 in the equatorial waters were tri- and tetrachlorobiphenyls 

 as well as heptachlorobiphenyls. Analysis of the CH's sorbed 

 by suspended matter revealed a clear dependence of pollutant 

 levels on latitude. For example, the content of HCH isomers in 

 suspensions from the equatorial Pacific was almost 50 times 

 lower than in the circumpolar parts of the ocean (Table 3 ). This 

 may have been due to the significant shift in sorption-process 

 equilibria associated with a 25°C rise in temperature. There 

 was a marked (almost tenfold) change in PCB levels in 

 suspended matter, whereas the levels in the water layer remained 

 virtually constant. It is a curious fact that the equatorial Pacific 

 is a unique region of the World Ocean where the PCB mixture 

 appears to be equally apportioned between the suspended 

 matter and the dissolved phase. 



Of special interest is the distribution of the extensively 

 used pesticide DDT in Pacific Ocean ecosystems. Dichloro- 

 dipheny ltrichloroethane levels in water samples from the Bering 

 and Chukchi Seas have decreased considerably over the past 

 decade due to restrictions on the use of this compound imposed 

 by a number of industrialized countries. In fact, in some 

 instances these levels come close to analytical zero (Chernyak 

 et al.. 1989). In the equatorial Pacific, however, the DDT 



TABLE 3 



Chlorinated hydrocarbon levels (Ug/g dry weight) in suspended 

 matter in the Pacific Ocean. 



hazard remains considerable: its levels average 0. 1 ng/1, which 

 is typical of the areas most severely impacted by human 

 activities, namely the North Atlantic and the Indian Ocean 

 (Tanabe etal.. 1982). 



Results for the microbial and photochemical degradation 

 of PCB's in the equatorial Pacific are presented in Figs. 2-4. 

 The gradual loss of several of the PCB congeners are plotted 

 over time in Fig. 2. On incubation with natural populations of 

 microbes from these central Pacific waters, some congeners 

 were reduced by more than 50% in 10 days (e.g., BZ#s. 

 Ballschmitter and Zell numbering system for PCB congeners: 

 Ballschmitter&Zell, 1980)7, 16. 49, 52, and 42. In Fig. 3, the 

 degree of microbial degradation of the PCB's is organized by 

 the PCB homologue group. From this presentation it is 

 observed that the greater the degree of chlorine substitution of 

 the bipheny 1 ring, the more resistant it is to microbial breakdown. 

 Plotted for comparison are the relative rates of breakdown of 

 the homologous groups for the two regions that were studied, 

 the Bering Sea. and the central Pacific Ocean. Between these 

 two locations, the rates of degradation of especially the mono- 

 and dichlorobiphenyl homologues are faster in the warmer 

 water of the Pacific. Figure 4 presents the times loss of total 

 PCB's for the central Pacific based on a comparison between 

 microbial degradation, photochemical degradation, and 

 photochemical processes under the influence of added PAH's. 

 It is apparent here that microbial degradation accounts for most 

 of the breakdown in this area and that PAH's may have the 

 capacity to inhibit photochemical breakdown of PCB's. 



