NITROGEN FIXATION IN CLEAR LAKE, CALIFORNIA. I. SEASONAL VARIATION AND THE ROLE OF HETEROCYSTS1

Abstract

The annual contribution of N 2 fixation to Clear Lake in 1970 was about 550 tonnes, 500 Mg (megagrams) or 18 kg ha −1 , 43% of the lake’s yearly nitrogen inflow. Biological N 2 fixation can provide the nitrogen for almost half the blue‐green algal standing crop during blooms and is implicated as the main cause of large algal nuisance blooms on this lake. The large quantity of nitrogen fixed may be typical for large, shallow, cyanophyceandominated lakes. A sustained spring peak of fixation was associated with a simultaneous bloom of Aphanizomenon flos‐aquae in all three basins, an autumn peak with an ephemeral bloom of Anabaena circinalis occurring at a different time in each basin. A stepwise multiple regression analysis showed that fluctuations in N 2 fixation were best described by variations in heterocysts, quantities of blue‐green algae, PO 4 ‐P, NO 3 ‐N, and temperature. Annual rates of N 2 fixation were correlated with the proportion of heteroeysts to vegetative cells in Aphanizomenon and with total number of heterocysts in Anabaena. Previous nitrogen budgets for Clear Lake have shown a large excess of nitrogen in outflow over inflow, which is accounted for by the levels of N 2 fixation measured.