Algal nitrogen fixation in Californian streams: seasonal cycles

Using the acetylene reduction technique, nitrogen fixation was measured in Rocky Creek, a small seasonally dry Californian stream. In the 3 years since 1970 nitrogen fixation varied seasonally and spatially, being highest in the early stages of colony growth in shallow, clear regions where there was little shade. The annual rate of N₂-fixation was similar to that found recently for Arctic tundra, but was greater than rates for Antarctic rivulets and less than rates for temperate rocky shores. A hetero-cystous Nostoc appeared to be the sole organism responsible for this fixation. Nostoc occurred in variously sized gelatinous clumps on the stable boulders on which the co-dominant alga, Ulothrix zonata, also grew. Maximum nitrogenase activity yielded 70 nmoles C₂H₄ mg^-1 drywt day^-1 and 123 nmoles C₂H₄cm^-2 day^-1. Estimated annual amounts of nitrogen fixed in 1971 ranged from 42 in shade to 360 mg N fixed m^-2y^-1 in the most favourable areas. Nostoc biomass reached 33 g dry wt m^-2 and peeled off each year in June-July. Because Nostoc was confined to stable rocks the biomass per unit area of the stream as a whole was much less, ranging from 0.054 to 1.26 g dry wt m^-2 in the most favourable site. Nostoc was common throughout the length of Rocky Creek and also plentiful in eight out of ten adjacent streams but not in the main river (Eel River South Fork), probably due to high turbulence and turbidity. N₂-fixation in these streams makes a significant, but probably small contribution to the nitrogen income of the nitrogen-deficient Eel River system. Nostoc colony establishment appears to be controlled by a combination of reduced turbulence and firmer attachment of the colonies to the substrate. Nostoc colony disappearance in June is probably due to nutrient depletion.