TEMPORAL AND SPATIAL VARIATIONS IN PHYTOPLANKTON CARBON ISOTOPES IN APOLYMICTIC SUBTROPICAL LAKE

Stable carbon isotopes (delta(13)C) were determined for phytoplankton and dissolved inorganic carbon (DIC) from Lake Apopka, a shallow, poly mictic and hypereutrophic lake in Florida, USA. Bulk plankton dominate d by pico- and nanocyanobacteria were enriched in C-13 (-13.1 +/- 1.1 parts per thousand) as a result of assimilation of extremely C-13 rich DIC (delta(13)C = 9.6 +/- 3.0 parts per thousand). Diatoms (Aulacosei ra spp.) had a delta(13)C of -14.3 +/- 0.6 parts per thousand that was slightly more negative than that of small cyanobacteria. Meroplankton ic diatoms had a delta(13)C (-13.6 +/- 1.8 parts per thousand), simila r to their planktonic counterparts. The delta(13)C of a colonial cyano bacterium (Microcystis incerta) was exceptionally heavy (-3.0 +/- 1.0 parts per thousand) and attributed to localized carbon limitation. Sea sonal variation in delta(13)C of bulk plankton was small (<3 parts per thousand ) relative to reports for other lacustrine systems. No diffe rence in the delta(13)C of bulk plankton from surface water between st ratified and non-stratified periods was found. No measurable changes i n delta(13)C of bulk plankton were indicated in light and dark incubat ion experiments. Frequent wind mixing of the water column, high DIC co ncentration, and consistently high lake productivity were used to expl ain the temporal and spatial isotope consistency of phytoplankton in t his lake.