Relationship Between Standing Crop and Primary Productivity Along a Hot Spring Thermal Gradient

Quantitative measurements of photosynthesis (by carbon—14) and chlorophyll were made for algal mats which had developed at a series of stations along the thermal gradient formed by the outflow channel of a hot spring at Yellowstone National Park. The temperature range studied was from 30°—70°. Comparisons were made of photosynthetic efficiency, which had been measured and calculated in different ways. There was no simple correlation between standing crop (as measured by chlorophyll, protein or RNA per unit area) and photosynthesis. Animal grazers on the algal mats occur only at temperatures below 50°; the low standing crop at the lower temperatures may be due to the comsumption of algae and bacteria by these animals. At a temperature of 58 o C, very thick mats are found, composed mostly of filamentous bacteria with a surface layer of algae. It is suggested that the organic carbon originating from the algae is assimilated by these bacteria and is not released because of the absence of grazers. At temperatures close to the upper limit for algal growth, standing crop is low although photosynthetic efficiency is high, and the algae are optimally adapted to the temperatures at which they are living. Some physiological factor other than photosynthesis may limit the growth of algae at these highest temperatures. The value of hot springs as models to study aspects of ecosystem trophodynamics is shown by this work.