The influence of light and nutrients on buoyancy, filament aggregation and flotation of Anabaena circinalis

At Chaffey Dam, New South Wales, Australia, Anabaena circinalis filaments accumulated at the surface as diurnal surface layer thermal stratification developed. Previously buoyant, homogeneously 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 (I-o). 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. circinalis deployed in bottles at three depths in the reservoir. GALD increased in samples exposed to I and 30% I-o by the following day. The rank order of GALD from smallest to largest grouped samples exposed to 70, 30 and 1% I-o, 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 enlargement of biomass units increased the mean floating velocity, supporting the theory that filament aggregation 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.