Original paper

Population dynamics of Botryococcus braunii Kutzing 1849 in a shallow tropical eutrophic lake

Ramírez R., John J.; Corbacho, Mónica O.

Abstract

A total of 51 samples, one per week, were taken from March 1999 to February 2000 at a sampling station in Parque Norte lake. Values for physical, chemical, biological and weather variables related to the population dynamics of Botryococcus braunii were estimated. Significant density changes among samples were determined by Anova; temporal arrangement by Dispersion Index (DI = 28.2 %; contagious) and Coefficient of Variation (CV = 35.8 %; contagious), and niche width by Shannon and Weaver Standard Index (A = 97.0 %; high). Additionally, the species growth rate (0.12 ind per day), generation time (5.9 d) and its change rate (generally low) were estimated. Variable correlation was analyzed by Principal Component Analysis (component I = 62.1 %, component II = 16.7 %, from total variance). B. braunii was the dominant species (mean density = 108 000 ind per l) and was found together with two other species from the same genus Botryococcus sp. and B. terribilis. The latter increased its density at the times B. braunii's decreased. The number of colonies of B. braunii outnumbered the other colonies. The most frequent number of globules per colony of B. braunii was from 1 to 3; a greater number of globules would imply greater weight and greater energy expenditure to remain in suspension. Its success in this body of water is due to the low rates of hydraulic washout (residence time high), settling (the species abounds with oils to float), and grazing (e.g. Arctodiaptomus dorsalis, Brachionus plicatilis and Moina macrophtalma; all filtering zooplankton found in Parque Norte lake). Physiological death was the main cause of biomass reduction. B. braunii was positively correlated to NO3- (r = 0.76), free CO2 (r = 0.76) and total alkalinity (r = 0.53) and was negatively correlated to light attenuation, pH (both with r = -0.76), pigment ratio; chlorophyll a (r = -0.45), and light radiation (r = -0.32). B. braunii is an S strategic species, it blooms twice a year; its density is high and it has a wide niche range. Its niche overlapped those of Botryococcus sp. and B. terribilis. B. braunii was found to have a contagious temporal disposition with individuals randomly distributed within the patches. Its generation time was high and its turn over rate was low too, as it is characteristic of S strategic species.

Keywords

shallow lakeeutrophic lakepopulation dynamicschlorococcalesbotryococcusb. braunii