Nitrogen fixation in Clear Lake, California. 3. Repetitive synoptic sampling of the spring Aphanizomenon blooms [Algae]

Nitrogen fixation (as acetylene reduction) and factors most likely to influence it were estimated simultaneously for 31 sites at eight stages of the 1971 and 1972 spring blooms of Aphanizomenon in naturally eutrophic Clear Lake. The major factor controlling rates of N2fixation was the number of Aphanizomenon heterocysts (P < 0.001). Environmental influences on fixation were largely mediated through heterocyst induction or repression. Regression analysis showed heterocysts highly negatively related to NO3(P < 0.05). at high (linear regressions) but not at low (log-transformed regressions) numbers, heterocysts were positively correlated with phosphate (P < 0.05). At low rates of N2fixation, heterocysts were also positively related to water clarity, chlorophyll (both P < 0.05), and temperature (P < 0.01). The role of very low levels of nitrate (2-22 μ g· liter-1 NO3-N) in (apparently) indirectly suppressing heterocyst induction was unexpected. Only at high rates as N2fixation correlated with phosphate. Presumably at low rates sufficient phosphorus is available in this P-rich lake (14-43 μ g· liter-1 PO4-P) to permit repression of heterocyst formation by low NO3levels. Ammonium suppressed N2fixation and heterocysts formation only where it was present in relatively large quantities (20-170 μ g· liter-1 NH4-N). Early in the blooms, low rates of vegetative (i.e. nonheterocyst) N2fixation were indicated in the "flake" colonies of Aphanizomenon. These may have an anoxic center like that of Trichodesmium.