THE PAUCITY OF BLUE-GREEN ALGAE IN MEROMICTIC BROWNIE LAKE: IRON LIMITATION OR HEAVY-METAL TOXICITY? (EUTROPHIC, PALEOLIMNOLOGY, PIGMENTS)

Meromictic Brownie Lake contains a low proportion of blue-green algae despite eutrophy and evidence for nitrogen-deficiency. Because nitrogen-fixing blue-green algae are known to have a high iron requirement, I hypothesized that low iron availability--resulting from a reduced internal cycling--is responsible for the paucity of blue-green algae.; Bioassays demonstrate iron stimulation of phytoplankton growth in Brownie Lake. Such stimulation could also be caused by adsorption of toxic metals by the added iron, although controls reveal no evidence for the detoxification of heavy metals in Brownie Lake. Available iron in Brownie Lake is therefore in short supply and is likely responsible for the paucity of blue-green algae.; The phytoplankton of diverse aquatic systems have been found to respond positively to iron additions. However, future studies must control for detoxification effects to determine whether the mode of iron stimulation is through nutrition of detoxification.; The strong meromictic stability of the lake is probably responsible for the low availability of iron, which is reduced either by (i) the small proportion of the sediment-surface exposed to the epilimnion, and/or (ii) the absence of seasonal circulation of iron-rich anoxic bottom waters. Because iron is introduced to surface waters by circulation of iron dissolved in anoxic hypolimnia, only those meromictic lakes without seasonal anoxia above the chemocline would be expected to exhibit iron deficiency.; A paleolimnological study was performed to evaluate the hypothesis that meromixis is responsible for the paucity of blue-green algae in the lake. Measurement of two pigments unique to blue-green algae--oscillaxanthin and myxoxanthophyll--was accomplished with a rapid trichromatic technique. The sedimentary record is consistent with the meromixis hypothesis, but does not provide a critical test because the lake became meromictic before it became eutrophic. I had assumed that heavy road-salt use during the 1960's was responsible for meromixis, but paleolimnological and historical evidence indicate that meromixis resulted from a reduction in lake surface area in 1917 A.D.