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In stratified lakes, dominance of the phytoplankton by cyanobacteria is largely the result of their buoyancy and depth regulation. Bloom‐forming cyanobacteria regulate the gas vesicle and storage polymer contents of their cells in response to interactive environmental factors, especially light and nutrients. While research on the roles of nitrogen and phosphorus in cyanobacterial buoyancy regulation has reached a consensus, evaluations of the roles of carbon have remained open to dispute. We investigated the various effects of changes in carbon availability on cyanobacterial buoyancy with continuous cultures of Microcystis aeruginosa Kuetz. emend. Elenkin (1924), a notorious bloom‐former. Although CO2 limitation of photosynthesis can promote buoyancy in the short term by preventing the collapse of turgor‐sensitive gas vesicles and/or by limiting polysaccharide accumulation, we found that sustained carbon limitation restricts buoyancy regulation by limiting gas vesicle as well as polysaccharide synthesis. These results provide an explanation for the positive effects of bicarbonate enrichment on cyanobacterial nitrogen uptake and bloom formation in lake experiments and may help to explain the pattern of cyanobacterial dominance in phosphorus‐enriched, low‐carbon lakes.

We hypothesize that the pattern of cyanobacterial dominance in experimentally enriched, low‐carbon lakes is related not only to the resultant N:P ratio but also to the availability of carbon for gas‐vesicle synthesis. We tested this hypothesis by determining the buoyancy responses of a highly gas‐vacuolate, N2‐fixing cyanobacterium to P enrichment with and without induced C limitation. Enrichment of samples of Aphanizomenon schindleri (Kling et al. 1994) from blooms in Lake 227 with combinations of C, N, and P produced rapid buoyancy reductions in P treatments, reductions that were reversed within a generation time in treatments that included C or C and N as well as P. These responses are the first of their kind to be observed in experiments with lake populations of cyano‐bacteria. The rapid buoyancy reductions were associated with polyphosphate accumulations in P‐treated A. schindleri. Differences in buoyancy status after one generation time were linked to differences in relative gas vacuolation between samples treated with P only and samples treated with C and N as well as P. These results may explain the relative success of different types of cyanobacteria in newly enriched, low‐carbon lakes. The availability of C for gasvesicle synthesis may determine whether a low N:P ratio induces N2 fixation by benthic or by planktonic cyanobacteria and whether a high NP ratio leads to dominance by non‐gas‐vacuolate or by highly gas‐vacuolate, non‐N2‐fixers.

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