RELATIONSHIP BETWEEN N-2-FIXATION AND HETEROCYST ABUNDANCE AND ITS RELEVANCE TO THE NITROGEN BUDGET OF LAKE-227

A significant relationship between seasonal N-2-fixation rate and hete rocyst abundance has been found in Lake 227 and can be modelled to est imate areal rates oi N-2-fixation based on the known historical phytop lankton records. Experimental imposition of low nitrogen (N): phosphor us (P) loading ratios stimulated N-2-fixation. The N:P molar ratio of the internal pool of nutrients oscillates around a mean of 30:1, despi te experimental loading ratios that vary from 13:1 to 0. Maintenance o f this internal nutrient mass ratio requires preferential regeneration of N through time at all loading rates. Heterocystous, cyanobacteria blooms are most predictable during imposition of a very low external l oading N:P ratio and high P loading relative to natural sources. Knowl edge of the internal loading N:P ratio and relative contribution of in ternal and external loadings are required for predicting blooms at int ermediate external N:P loading rates. The internal N:P ratio has fluct uated within narrow limits over 25 yr of nearly constant P loading bec ause of adjustments in N-2-fixation, N sedimentation, and denitrificat ion, which have counterbalanced changing experimental N loading regime s.