NITROGEN FIXATION IN CLEAR LAKE, CALIFORNIA. II. SYNOPTIC STUDIES ON THE AUTUMN <i>ANABAENA</i> BLOOM1

Abstract

Nitrogen fixation at three stages of an autumnal bloom of Anabaena circinalis was measured after almost simultaneous collection at up to 32 stations in Clear Lake and algal heterocysts, phytoplankton cell numbers, NO 3 ‐N, NH 4 ‐N, dissolved organic‐N, PO 4 ‐P, Fe, primary production, particulate carbon, and chlorophyll a were also measured. Nitrogen fixation was significantly and positively correlated to Anabaena heterocyst numbers ( P &gt; 0.001), negatively correlated to NO 3 ‐N ( P &gt; 0.01), and positively correlated to dissolved organic‐N ( P &gt; 0.01) and PO 4 ‐P ( P &gt; 0.05). A negative correlation with NH 4 is probable; no significant relationship was found with the other variables measured. An explanation of the apparent restriction of substantial cyanophycean N 2 fixation to nonoligotrophic waters is proposed. The results are consistent with the theory that heterocysts are responsible for N 2 fixation in situ under oxic conditions.