GLOETRICHIA-ECHINULATA, A COLONIAL CYANOBACTERIUM WITH A UNIQUE PHOSPHORUS UPTAKE AND LIFE STRATEGY

Epilimnetic colonies of Gloeotrichia echinulata were harvested from 20 0-300 l of water in Lake Erken with filtration through appropriate pla nkton nets (200 or 70 mum). Phosphate uptake characteristics, phosphor us (P) status and photosynthesis of the colonies were determined twice a week during July and August 1991. Phosphate uptake was analysed acc ording to the simple force-flow relationship of Falkner et al. (Arch. Microbiol., 152, 353-361, 1989). The threshold concentration of P upta ke below which uptake ceases for energetic reasons, was constantly muc h higher than the epilimnetic soluble reactive phosphorus (SRP) concen tration, so that the planktonic colonies were unable to acquire any P in the epilimnion. Neither did organic P seem to be a source of P for planktonic colonies. Gloeotrichia echinulata has a unique life strateg y in comparison to other common genera of bloom-forming cynanobacteria . Its P assimilation and growth are completely separated both in time and space; growth is preceded by benthic P assimilation. Epilimnetic g rowth was based solely on internally stored P and growth rates fitted the Droop model well. Depletion of stored P restricts the length of th e planktonic phase to 15-20 days under 'optimal' growth conditions. Wi nd-induced surface drift seemed to be the most important loss factor f rom the epilimnion. Massive recruitment of P-rich benthic colonies acc ounted for two-thirds of total net internal P loading observed between mid-July and mid-August (3.8 mg P m-2 day-1).