Dynamics and Succession of the Phytoplankton in a Tropical Lake: Lake Lanao, Philippines

Abstract

SUMMARY (1) Phytoplankton dynamics in Lake Lanao, Philippines, were studied over a 16-month period on the basis of weekly abundance measurements for seventy species and a number of physical and chemical variables. (2) An analysis of growth pulses and growth correlations in the major classes of phytoplankton provides evidence that the niche space is divided temporally on the basis of nutrient and light availability. Growth maxima of diatoms and cryptomonads occur during periods of relatively low light availability and high nutrient availability; pulses of green algae, blue-green algae and finally dinoflagellates occur successively towards the high light and low nutrient end of the spectrum. (3) Variations in total autotroph biomass over the 16-month period were more closely related to factors affecting growth than to factors affecting loss of biomass. Loss control factors nevertheless influenced succession by their differential effects on individual species. (4) Major factors controlling phytoplankton growth include light availability (computed from measurements of incident light, mixing depth and transparency) and nutrient availability (computed from measurements of nutrient concentrations, turbulence and change in depth of mixing). Individual species were arranged by statistical methods according to their growth patterns along gradients of these two factors. There is an identifiable taxonomic trend in the separation similar to that observed in the class-level analysis of succession, but a number of species showed divergent behaviour from others of their taxonomic group. (5) Measurable factors governing loss of autotroph biomass include grazing rate (analysed