406 R. J. Barsdateetal. 



The number of bacteria in the plankton was unaffected by the 

 addition of oil but their activity increased during the first year. Two years 

 after the spill in Pond Omega, there were no differences in the types and 

 numbers of bacteria in the oiled and unoiled parts of the pond (plate count 

 technique, nutrient and oil agar). Thus it appears that a single addition of 

 crude oil briefly stimulated microbial activity but the microflora were 

 back to control-pond levels within 2 years. 



The effect of oil on benthic algae was only briefly studied; 1 year after 

 the spill the photosynthesis in Pond E was 50% that of a control pond. In 

 contrast, the phytoplankton algae were intensively studied and we found 

 that the water-soluble fraction of the oil strongly reduced photosynthesis 

 for several days. The amount of algae, however, did not change as a result 

 of the spill and productivity reached normal levels within several months. 



The added oil drastically changed the species composition of the 

 planktonic algae in both the ponds and in experimental chambers. This 

 change, a rapid replacement of the cryptophyte Rhodomonas by the 

 chrysophyte Uroglena, continued for 6 years. It is likely that the 

 Rhodomonas are eliminated because the zooplankton are killed; 

 experimental removal of the zooplankton caused the same elimination. It 

 is not known whether the algae responded to a release of grazing pressure 

 or to a cessation of the zooplankton's recycling of nutrients. 



No damage to the vascular plants was observed in the ponds during 

 the first year, but growth of Carex aquatilis was reduced in later years. 

 Much of this reduction was caused when new leaves encountered a barrier 

 of floating oil. 



In experiments in the laboratory, the fairyshrimps were most sensitive 

 to oil, Daphnia were next, Heterocope next, and the Cyclops were least 

 sensitive. This sequence duplicates the field results; all the Daphnia and 

 fairyshrimps were killed immediately by the whole-pond treatments and 

 did not return for 7 years. The less-sensitive copepods returned within a 

 year. 



In laboratory tests, aquatic insects and other invertebrates were not 

 sensitive to oil. In the field, the spill had no major effect on the numbers 

 and production of chironomids, but there were some minor effects on their 

 emergence. One genus, Tanytarsus, was nearly eliminated from the ponds. 

 Beetles, caddisflies, stoneflies, and snails were also drastically affected; 

 most of these animals live only in the plant beds and may have become 

 trapped in the oil on plant stems and in the floating oil. These insects were 

 still absent in Pond E 6 years after the spill. 



When the oil spills are relatively light, as in these experiments, then 

 the best treatment would be to absorb the floating oil and perhaps to flood 

 the marsh to float oil away from the littoral plants. The biota of ponds will 

 recover within a few years with this simple treatment. More drastic clean- 

 up measures will induce greater changes into the ecology of ponds. 



