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RESEARCH PRODUCT

Population dynamics of Cyclotella ocellata (Bacillariophyceae): endogenous and exogenous factors

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Population dynamics of Cyclotella ocellata PANTOCSEK were examined from 1991 to 1993 in an oligo-mesotrophic, gravel-pit lake near Madrid (Spain). Monthly and daily sampling was carried out to track seasonal- and auxosporulation dynamics, respectively. This species was the most abundant planktonic diatom in the lake. The population increased steadily in spring until the seasonal peak (3.8.10 6 ind/ L in April) and then abundance declined slowly through late winter. Time series techniques showed that the seasonal dynamics were driven by exogenous factors: there was a relationship between Cyclotella abundance and the chemical environment, both TP (synchronic) and SRSi (with delay). Auxosporulation (studied on a daily scale) occurred in late summer, when 90 % of cells had a diameter of less than 10 μm. The smallest vegetative cells of C. ocellata (preauxospores) presented an endogenous dynamic: their dynamics had no relationship with environmental changes and the cell size dynamic seems to be the most important factor in inducing auxosporulation. The regenerated population (auxospores and initial cells) constituted the largest component of diatom biomass during thermal overturn, and followed the increase in nutrient concentration. The dynamic was also controlled by nutrients in the lake as influenced by mixing, hence it was driven by exogenous factors. The pattern of life history strategies followed by C. ocellata was that of synchronous sexuality under favourable conditions: vegetative growth throughout the year, a reduction in size when nutrients were decreasing until a critical size and a critical percentage of minor cells was reached and then, when more nitrogen became again available, the formation of auxospores for one month.