Systematic variation in the logarithmic ranges in abundance of freshwater phytoplankton populations

Summary The logarithmic scaling of species abundance in phytoplankton populations enables the relative ranges of species between periodic (e.g. annual) upper and lower limits to be defined and compared. Examples are discussed mainly from published information on four lakes, two tropical (lakes G eorge and V ictoria) and two high‐latitude temperate ( W indermere and E sthwaite W ater), differing in seasonality and predominant environmental factors. Although wide logarithmic ranges (>3 orders, log 10 units) do exist in the tropical lakes, there are many examples of narrow ranges (<1.5 orders). In L ake V ictoria, the latter are characteristic of species with low maximum abundance measured as biomass that do not appreciably deplete the medium (e.g. of nutrients, light) in a density‐dependent manner, but are indirectly regulated by resource depletion caused by more abundant species and by physical mixing, the predominant environmental seasonality. In the seasonally equable tropical L ake G eorge, narrow ranges in numerical abundance are predominant. Some are favoured by recruitment from a considerable past accumulation of viable biomass on the sediment. Annual ranges of under one order are rare in W indermere and E sthwaite W ater, and probably in most other temperate lakes, although this value is approached by a few ‘opportunistic’ species (e.g. some cryptomonads) of wide environmental tolerance and some species (e.g. P lanktothrix agardhii ) of enriched shallow lakes. There is little information for species of very low abundance. Examples are given of logarithmic plots of a measure of total phytoplankton biomass with time. Annual ranges are markedly less than those of the major individual component species, indicating species replacement.