Temporal variability of phytoplankton in tropical lakes.

Temporal variability of ecological systems continues to receive theoretical and empirical attention but remains inadequately documented at low latitudes. Results of my comparative investigation of photosynthetic rates of phytoplankton in 6 equatiorial African lakes and similar information from 20 South American, Asian and African lakes studied by others provide the data for an assessment of the range of seasonal variability (expressed as coefficients of variation, CV) among tropical lakes. Sampling intervals varied from 1 week to 3 months and usually spanned at least one year. Within Africa the coefficient of variation ranged from 15% to 61%, and among all the lakes the coefficient of variation ranged from 15% to 86%. The Spearman rank correlation coefficient of the CV's of photosynthesis versus latitude is 0.24 and is not significant at the 0.05 level.Coefficients of variation of photosynthetic rates of phytoplankton in a diverse set of 45 temperate and arctic lakes ranged from 29% to 155% and were significantly different from the set of 26 tropical lakes by the Mann-Whitney U test. When all 71 lakes are compared, the Spearman rank correlation coefficient of CV's of photosynthesis versus latitude is 0.71 and is significant at the 0.0005 level.Three temporal patterns were recognized among tropical lakes. Most tropical lakes exhibit pronounced seasonal fluctuations that usually correspond with variations in rainfall, river discharges or vertical mixing. A second pattern occurs in lakes with muted fluctuations (coefficient of variation less than 20%) in which diel changes often exceed month to month changes. A third pattern is distinguished by an abrupt change from one persistent algal assemblage (i.e., extant for at least 10 generations) and level of photosynthetic activity to another persistent condition.