| Summary: | The productivity and standing crop of epiphytic algae in a marsh pond and some of the factors controlling the seasonal algal growth were investigated. The use of artificial substrates for the estimation of epiphytic algal production was evaluated. Cellulose-acetate was found to provide a convenient, reliable substrate for epiphytic algal attachment which allowed simple routine application of the C14 technique for productivity estimation. The annual overall primary productivity of epiphytic algae in the marsh pond (Crescent Pond, Delta Marsh, Manitoba) was 1881 kg C yr-1 or 26.8 g C m-2 pond surface area. Productivity per unit colonizable surface area was greatest on Scirpus acutus Muhl. (32.3 g C m-2 yr-1) while Potamogeton pectinatus L. was the most significant substrate in overall pond productivity (1700 kg C yr-1). Analysis of photosynthetic C uptake revealed that the photosynthesis to in situ light intensity relationship of the epiphytic algae was relatively constant throughout the day. A net loss during the night of 30% of C14 assimilated in the previous light period was detected. Apparent uptake of C14 by the epiphytic community was cumulative in the light for periods up to 24 hr... The seasonal growth at the two sites was related to nutrient levels (N, P and Si) and to various physiological indicators of nutrient availability, including chlorophyll content, protein to carbohydrate and lipid ratio, Si debt, storage phosphate levels, alkaline phosphatase activity and nitrogenase activity. Low standing crops in June and early July coincided with indications of deficiency in terms of chlorophyll a content and protein to carbohydrate and lipid ratios at both the Scirpus and Potamogeton sites. Growth of heterocystous, N2-fixing blue-green algae and high alkaline phosphatase activity occurred in July at the Potamogeton site. Increased standing crop of epiphytic algae in September and October at the Scirpus site coincided with higher nutrient levels and absence of deficiency symptoms. A delay in the standing crop increase at the Potamogeton site correlated with low nutrient levels and various physiological deficiency symptoms.
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