ý.,,: V. ý ýý . - Nottingham eTheses - University of Nottingham

et al., 1999). No melt out was observed in Ace Lake and as such no known wind mixing
occurred. However, mixing of the mixolimnion
does occur with ice formation. A
thermohaline convection cell is set up under new ice formation, due to the exclusion of
brine increasing sub-ice salinity and thus density, producing a cell which is heated by
transfer from deeper waters resulting in a convection cell (Gibson, 1999). Such a
situation was seen with the increase in ice cover from July through November (Fig 3.1 a);
a layer of isohaline water forms, which becomes more saline and penetrates deeper as the
winter progresses (Gibson & Burton, 1996), leading to complete mixing of the
mixolimnion.
This explains the isohaline state during September and November (Fig
3.4a).
Due to the lack of atmospheric CO2 input and allochthonous input from the
catchment area, DOC is entirely derived from carbon fixation by photosynthesis
(Laybourn-Parry et al., 2002). Parker et al. (1977), working on Lake Hoare (Dry Valleys)
showed that 75% of the total photosynthetically fixed organic matter appeared as
extracellular products. In the current study, DOC ranged from an annual average of 2mg
L-1 at the surface to 9mg L-1 at 10m (Fig 3.9a), except for a peak in concentration (30.079
mg U) at IOm during January, which was possibly due to accidental inclusion of water
from the monimolimnion,
but high DOC concentrations in January have been reported by
Laybourn-Parry et al. (2002). DOC concentration reaches peaks at l Om, probably due to
an increase in photosynthetic activity, producing DOC as by-products from
photosynthesis. This peak may be brought on by deep light penetration to l Om in January
when the snow coverage of the lake ice was minimal (Laybourn-Parry et al., 2002).
Photosynthetically active radiation (PAR) levels are much higher in summer (11.5m)
compared with winter (2m), due to changes in incident light levels. High PAR during the
summer may inhibit photosynthetic production in the surface waters. Therefore, motile
organisms e. g. flagellates and ciliates (e. g. Mesodinium rubidium), move to a depth
where they are not light inhibited (Wright & Burton, 1981; Bell & Laybourn-Parry.
1999b). The majority of phytoplankton are shade adapted due to the low light levels
experienced for long periods during the Antarctic year. This is substantiated by the fact
that the phytoplankton at 10m can still photosynthesise at