A Compositional, Phytogeographical and Elementary Structural Analysis of the Phytoplankton in a Tropical Lake: Lake Lanao, Philippines

(1) A 15-month study of the phytoplankton of Lake Lanao, Philippines, shows that the community is composed of seventy euplanktonic autotroph species, including Cyanophyta (12), Euglenophyta (4), Chlorophyta (44), Chrysophyceae (1), Bacillariophyceae (4), Dinophyceae (3), and Cryptophyceae (2). (2) A comparison of the year-round composition of Lake Lanao phytoplankton with survey data taken in other studies indicates that in South-east Asia a lacustrine plankton community will most often contain 50-100 euplanktonic autotroph species that reach detectable abundances (> 1 per ml) at least once per year. (3) Species-richness appears to be considerably lower in tropical than in temperate phytoplankton communities. In lakes of South-east Asia, including Lanao, about half of the species are chlorophytes, and over half of all species are found in a single integrated aliquot of I ml taken at any time of year. (4) South-east Asian phytoplankton communities have a higher fl-diversity than communities of east alpine lakes, but there is nevertheless a generic overlap of at least 80% between tropical Asian lakes chosen at random. The generic overlap between tropical and temperate lakes is much lower (45%). (5) The taxonomic overlap between tropical phytoplankton communities does not differ detectably between continents, and this is given as evidence for existence of a pantropical phytoplankton assemblage, of which that found in Lake Lanao is representative. (6) A survey of the literature shows that phytoplankton biomass is considerably higher in Lake Lanao and other tropical lakes on average than in temperate lakes, either on an annual basis or during the ice-free season. (7) In Lake Lanao, thirteen species account for 90% of annual biomass, and ten species account for 90% of the annual number of individuals. A statistical test shows no evidence that dominants are more likely to come from one taxonomic division than another.