Abstract
At Chaffey Dam, New South Wales, Australia, Anabaena circinalis filaments accumulated at the surface as diurnal surface layer thermal stratification developed. Previously buoyant, homo- geneously distributed colonies accumulated in the top 2 m, but a proportion lost buoyancy. Similarly, a percentage of A.circinalis suspended in bottles lost buoyancy at depths experiencing >30% surface irradiance (Io). Nutrient addition reduced the proportion of filaments that lost buoyancy following a full day of high irradiance. The greatest axial linear dimension (GALD) was measured for A.circi- nalis deployed in bottles at three depths in the reservoir. GALD increased in samples exposed to 1 and 30% Io by the following day. The rank order of GALD from smallest to largest grouped samples exposed to 70, 30 and 1% Io, suggesting that increasing GALD is a function of irradiance. The increased GALD of biomass units was attributed to aggregation of filaments in low light. The enlarge- ment of biomass units increased the mean floating velocity, supporting the theory that filament aggre- gation may be a strategy, utilized by light-limited filaments, to increase light exposure. High irradiance increased the carbohydrate content of cells and decreased the floating velocity of filaments.
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