Abstract:
Crawford Lake is a meromictic lake, which is 24 m deep and has
an area of 2.5 ha, and has never been reported to have mixed below
16 m. Lady Evelyn Lake, which became a reservoir when a dam was
built in 1916, is dimictic with a maximum depth of about 35 m.
1
My research proved that both native chlorophylls and the ratio of
chlorophyll derivatives to total carotenoids were better preserved in
the shallower lake (Crawford Lake) because it was meromictic. Thus the
anaerobic conditions in Crawford Lake below 16 m (monimolimnion)
provide excellent conditions for pigment preservation. Under such
conditions, the preservation of both chlorophylls and carotenoids,
including oscillaxanthin and myxoxanthophyll, are extremely good
compared with those of Lady Evelyn Reservoir, in which anaerobic
conditions are rarely encountered at the mud-water interface.
During the period from 1500 to 1900 A. D. in Crawford Lake, the
accumulation rates of oscillaxanthin and myxoxanthophyll were
extremely high, but those of chlorophyll derivatives and total
carotenoids were relatively low. This was correlated with the presence
of a dense benthic mat of cyanobacteria near the lake's chemocline.
Competition for light between the deep dwelling cyanobacteria and
overlying phytoplankton in this meromictic lake would have been
intensified as the lake became more and more eutrophic
(1955-1991 A. D.).
During the period from 1955 to 1991 A. D., the accumulation rates
of chlorophyll derivatives and total carotenoids in the sediment core
from Crawford Lake (0-7.5 cm, 1955-present) increased. During this
same period, the accumulation rates of cyanobacterial pigments (Le. oscillaxanthin and myxoxanthophyll) declined as the lake became more
eutrophic.
Because the major cyanobacteria in Crawford Lake are benthic
mat forming Lyngbya and Oscillatoria and not phytoplankton,
eutrophication resulted in a decline of the mat forming algal pigments.
This is important because in previous palaeolimnological studies the
concentrations of oscillaxanthin and myxoxanthophyll have been used
as correlates with lake trophic levels.
The results of organic carbon a13c analysis on the Crawford Lake
sediment core supported the conclusions from the pigment study as
noted above. High values of a13c at the depth of 34-48 cm
(1500-1760 A. D.) were related to a dense population of benthic
Oscillatoria and Lyngbya living on the bottom of the lake during that
period. The Oscillatoria and Lyngbya utilized the bicarbonate, which had
a high a 13C value. Very low values were found at 0-7 cm in the
Crawford sediment core. At this time phytoplankton was the main
primary producer, which enriched 12C by photosynthetic assimilation.
