Abstract
Stable carbon isotopes (513C) were determined for phytoplankton and dissolved inorganic carbon (DIC) from Lake Apopka, a shallow, polymictic and hypereutrophic lake in Florida, USA. Bulk plankton dominated by pico- and nanocyanobacteria were enriched in 13C (-13.1 ± l.l%o) as a result of assimilation of extremely l3C-rich DIC (513C = 9.6 ± 3.0%o). Diatoms {Aulacoseira spp.) had a 613C of -14.3 ± 0.6%o that was slightly more negative than that of small cyanobacteria. Meroplank- tonic diatoms had a 813C (-13.6 ± 1.8%o), similar to their planktonic counterparts. The 613C of a col- onial cyanobacterium (Microcystis incerta) was exceptionally heavy (-3.0 ± 1.0%o) and attributed to localized carbon limitation. Seasonal variation in 813C of bulk plankton was small (<3%o) relative to reports for other lacustrine systems. No difference in the 8I3C of bulk plankton from surface water between stratified and non-stratified periods was found. No measurable changes in 513C of bulk plank- ton were indicated in light and dark incubation experiments. Frequent wind mixing of the water column, high DIC concentration, and consistently high lake productivity were used to explain the tem- poral and spatial isotope consistency of phytoplankton in this lake.
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