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increase in ammonium is probably due to the deamination of proteins which accumulate
due to a slow organic matter mineralisation rate (Rankin et al.. 1999). Figure 3.8a shows
levels of ammonium to be close to the level of detection, except in July and at l Om in
September, when massive increases in ammonium concentration are seen (Fig 3.7a).
Increases in bacterial abundance from March to July could have been influenced by-
increasing ammonium concentrations (Fig 3.17a); however, the peak in bacterial
abundance during September seems to coincide with a reduction in ammonium and other
nitrogen compounds to levels below the limit of detection, which might indicate that
bacteria were using their preferred nitrogen source, free amino acids.
Inorganic phosphate (SRP, Fig. 3.8a) is relatively high in the upper mixolimnion
and peaks at 1 Om. SRP has been shown to not limit biological activity in the upper
mixolimnion and to increase in concentration after 7m (Hand & Burton, 1981; Bell &
Laybourn-Parry, 1999b; Rankin et al., 1999).
DOC generally shows an increase with depth, but remains relatively stable
throughout the year (Fig 3.9a). Hand and Burton (1981) also show an increase in TOC
(total organic carbon) with depth. In February, the DOC was comprised of only 28% of
dissolved free amino acids (DFAA) and dissolved free monosaccharides (DFCHO), much
lower than Pendant Lake (Laybourn-Parry et al., 2002), which may explain why bacterial
abundance was considerably lower in Ace Lake compared to Pendant Lake despite the
high concentrations of available DOC. This probably relates to the state of meromixis in
the lake, which imposes severe nutrient restrictions on the euphotic zone (Laybourn-Parry
et al., 2002). High bacterial abundance appeared to coincide with high DOC
concentration (Fig. 3.17b), possibly because bacterial production relies on concentration
and composition of the DOC pool as well as nitrogen and phosphorus (Rankin et al.,
1999). Also, high bacterial abundance at l Om (Fig 3.11 a) coincides with
high DOC,
which is released from the metabolism of H2S by the anaerobic sulphate reducers (e. g.
Desulfovibrio). This DOC pool produced by the sulphate reducers near l Om is also fed on
by other heterotrophs. It is also exploited by heterotrophic bacteria as a direct or indirect
food source and fixed by phototrophic bacteria (Hand & Burton, 1981).
Bacterial abundance was low in the upper mixolimnion (>6m) (Fig. 3.11 a)
compared with abundance in the lower mixolimnion (6-lOm), rarely peaking above 5.5
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