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chemocline (Rankin et al., 1999). Meromixis has been observed in the following sampled
Lakes: Organic, Williams, Abraxas, Ekho, Shield, Oval, Scale, Anderson, Oblong. Laternula
and Angel `2' (Gibson, 1999; Labrenz & Hirsch, 2001). Most of the lakes do demonstrate
some form of thermal stratification during the year (Table 2.1), generally monomixis. For
example the majority of the hypersaline lakes are monomictic (Burton & Barker, 1979;
Hand, 1980; Hand & Burton, 1981; Burton, 1981), being stratified during the summer, due
to salinity and temperature gradients and then mixing in the winter when the upper warm
water is cooled to the temperature of the lower waters. The freshwater lake, Crooked Lake.
becomes inversely stratified when ice covered and only mixes in summer if the ice breaks
out (Laybourn-Parry et al., 1992); it is, therefore, variably monomictic or amictic.
Inorganic nutrients showed variability between the lakes (Tables 3.1 a, b and c).
Dissolved organic carbon showed a correlation coefficient of r=0.895 with salinity (Fig
3.15a) indicating lakes with high salinities generally had a higher DOC concentration (Fig
3.15b). However, the correlation between salinity and DOC concentration was stronger at
lower salinities. The correlation coefficient (r value) was only 0.70 for the hypersaline lakes.
DOC is exuded by the phytoplankton during photosynthesis (Chrost & Faust, 1983),
therefore high PNAN populations may cause DOC levels to increase (PNAN abundance was
not recorded for the summer lakes). Autotrophic bacterioplankton may also exude DOC and
contribute to the DOC pool. The heterotrophic bacterioplankton are the main consumers of
DOC, but HNAN have also been known to exploit it (Laybourn-Parry, 1997), although the
majority of HNAN are consumers of bacterioplankton. The oligotrophic nature of these
lakes coupled with low temperatures results in low microbial biomass and low turnover of
the DOC pool. The higher DOC concentrations recorded in the saline and
hypersaline lakes
could be due to accumulation of photosynthate in the absence of significant microbial
decomposition (Hand, 1980; Barker, 1981; Burton, 1981). Viral lysis of bacteria and
phytoplankton short circuits the carbon cycle in aquatic systems, preventing transfer of
carbon to higher trophic levels (Fuhrman, 1999). Viruses have been identified in the lakes of
the Vestfold Hills (Laybourn-Parry et al., 2001 a) but their role in DOC dynamics has yet to
be determined. However, in microbially dominated communities like those of Antarctic
lakes, viruses are likely to have a significant impact. This same trend can be seen for the
major inorganic nutrients (Tables 3.1a, b and c), nitrate, nitrite, ammonium. and SRP, all of
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