Photosynthesis by Fontinalis Antipyretica

S ummary Environmental variables in the North Bay of Esthwaite Water, where the aquatic moss Fontinalis antipyretica Hedw. grows, were measured over a year. Steep depth profiles often existed in the water column of about 1 m, with lower pH and higher alkalinity 01m above the sediment where F. antipyretica was found. Consequently, CO 2 concentration was higher at depth than at the surface. The North Bay generally had higher CO 2 concentrations than the centre of the lake. The survival of F. antipyretica in this productive lake probably results partly from these elevated concentrations of CO 2 . Nutrient limitation of production was discounted since tissue levels of phosphorus and nitrogen were about six and four times greater respectively than suggested limiting levels of these nutrients. Rates of photosynthesis, respiration and production were calculated from the field data and the previously‐determined measurements of the effect of photon irradiance, CO 2 concentration and temperature on photosynthesis. In all the four months studied, no single factor appeared to be predominantly limiting. Calculations suggested that in November, photon irradiance and in March, temperature were the major factors limiting production, both as a result of physical conditions. In August, low CO 2 concentration, caused by the activity of dense phytoplankton, was the major factor limiting production.