400 



R. J. Barsdate et al. 



500- 



Pond E 



Rhodomonas minuta 



,'^vUroglena sp 



FIGURE 9-9. Biomass (mg wet weight wV of 

 Uroglena sp. and Rhodomonas minuta in Pond E, 

 1970. 



found in Chlamydomonas cultures (Soto et al. 1975) and was caused by 

 volatile fractions of the oil. 



The most dramatic effect of the oil experiment in the subponds 

 (Figure 9-7) was the replacement of the cryptomonad species of algae by 

 chrysophytes (Federle et al. 1979). The same replacement of species 

 occurred in Pond E in 1970 and 1971 (Figure 9-8) and in Pond Omega in 

 1975 (Miller et al. 1978a). In all the unaltered ponds we studied, the usual 

 seasonal progression of forms was an early chrysophyte domination 

 followed by a cryptophyte peak; a typical pattern is given for Pond C, 

 1971, in Figure 9-8. One of the obvious changes is a complete replacement 

 of the small flagellate Rhodomonas minuta by Uroglena (Figure 9-9). This 

 replacement took place in Pond E in 1970 and the Rhodomonas did not 

 return to this pond until 1976, the same year that Daphnia and fairyshrimp 

 returned in any numbers (Federle et al. 1979). The same replacement 

 occurred in Pond Omega in 1975 (Miller et al. 1978b) and in the oiled 

 experimental subponds (Figure 9-7). Similarly, Barsdate et al. (1973) 

 noted decreased densities of cryptophytes in ponds at the natural oil seeps 

 at Cape Simpson. 



It is likely that the Rhodomonas minuta are eliminated because the 

 zooplankton are killed rather than because of special sensitivity to oil. This 

 species, a worldwide planktonic form, has never been cultured so we could 

 not test it in the laboratory. We did run experiments in which the only 

 treatment was removal of the zooplankton and concluded that this 

 removal was enough to eliminate the Rhodomonas (Figure 9-7, treatment 

 2). Other workers have also found that zooplankton control the species 

 composition of the phytoplankton (Porter 1973, Weers and Zaret 1975) 

 but the mechanism of this interaction in the arctic ponds remains 

 unknown. It is possible that the grazing pressure and the zooplankton's 



