General patterns in the seasonal developlment of chlorophyll a for temperate lakes

The seasonal cycle of phytoplankton biomass in temperate lakes is often represented as spring and fall blooms with intervening biomass minima, but the generality of this pattern has never been critically evaluated. Chlorophyll a time series from 56 north-temperate lakes and reservoirs were collected and general patterns in the seasonal distribution of Chl a determined for oligotrophic and eutrophic lakes separately. The magnitude of temporal variation is greater in eutrophic lakes (mean [Chl a]> 12 mg m-j) and is partitioned into discrete spring and fall blooms. Chlorophyll a concentrations in oligotrophic lakes (mean [Chl a] < 7 mg m-3) are low for the first 4 months of the year, then increase through April and May. Synchronous summer depressions and fall blooms are not evident for oligotrophic lakes overall. Mean [Chl a] and mean annual air temperature were negatively correlated with the date of spring bloom. The date of spring bloom was not correlated with mean depth. The seasonal pattern of phytoplankton biomass in temperate lakes is frequently de-scribed as a pronounced spring bloom fol-lowed by a summer depression, a subse-quent fall bloom, and low levels through winter (Riley 1947; Hutchinson 1967; Goldman and Hornc 1983; Taub 1984). Al-though a few studies have attempted to es-tablish the presence or absence of scason-ality in a group of lakes (Walker 1980