Factors affecting the distribution and abundance of an unpigmented heterotrophic alga Prototheca richardsi

SUMMARY 1. Numbers of the contramensal alga Prototheca richardsi were high in spring in two ponds used for breeding by anuran amphibians, but lower at other times of year and undetectable in two ponds not used by anurans. 2. Prototheca richardsi became abundant in the silt of eight experimental ponds which contained tadpoles, but remained undetectable in four otherwise identical ponds lacking tadpoles. 3. Prototheca richardsi numbers in laboratory microcosms remained stable for many days in sterile tap water, but declined with a half‐life of about 6 days in pond water at 20°C. 4. Further studies with microcosms using antibiotics and electron microscopy indicated that mortality of P. richardsi was caused primarily by pathogenic bacteria.