Effects of grazing and light on the growth ofNostoc pruniforme(Cyanobacteria)

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Abstract
Nostoc pruniforme colonies grow unusually large (up to 22 cm diameter) in Mare's Egg Spring, Klamath County, Oregon. Colonies were held in bags with varied mesh size and transmittance for periods of several months in 15 separate experiments done throughout the year. Light was related to growth rate as shown by regression analysis. Mesh size, and length of experiment were not significant (P > 0·05). Other factors such as temperature (4·5°C), pH (7·6), alkalinity (36 mg 1-1 CaCO3) and conductivity (72 μS cm-1) were constant seasonally, indicating light was the most important abiotic factor determining growth in this system. Growth calculations indicated that a colony of average size would be between 3·4 and 5·1 years old. The snail, Vorticifex effusa, appeared to graze epiphytes from the surface of the N. pruniforme colonies. The use of mesh bags to include and exclude snails demonstrated that grazing had a positive effect on the growth of N. pruniforme. Auto-epifluorescent microscopy used in conjunction with computer-assisted image analysis showed that the area covered by epiphytic algae also dropped when normal densities of snail were included. Epiphytes may lower the growth rate of N. pruniforme, although the mechanism is unknown.