Chlorophyll a seasonality in four shallow eutrophic lakes (northern British Columbia, Canada) and the critical roles of internal phosphorus loading and temperature

Chlorophyll a (Chl a) seasonality was investigated in four shallow eutrophic lakes located in north-central British Columbia (western Canada). Chlorophyll a concentration maxima in all four lakes occurred during the late summer/early autumn when near-surface total phosphorus ([Tot-P]) and total dissolved P concentrations, pH, and water temperature were highest. Mass balance and inferential analyses showed that bloom-triggering P loads came mostly from within-lake sources, but that mechanisms controlling internal loading in Charlie and Tabor (lakes having hypolimnetic oxygen deficits during summer) were fundamentally different than those in Nulki and Tachick (isothermal, well oxygenated lakes). Although the timing and intensity of major blooms were associated with late summer/early autumn P loads, average summer [Chl a] were predicted well by previously developed models based solely on spring overturn [Tot-P]. Instantaneous within-lake [Chl a] were best predicted by models incorporating both surface [Tot-P] and temperature (r(2) = 0.57-0.70). Moreover, [Tot-P] and temperature combined accounted for 57% of among-lake variations in instantaneous [Chl a]: log [Chl a] = 0.038 (degrees C) + 0.006 ([Tot-P]) + 0.203 (P < 0.001), where [Chl a] and [Tot-P] are in mu g l(-1). Positive associations between instantaneous [Chl a] and temperature support climate change models that forecast changes in phytoplankton productivity even if nutrient loading rates remain constant.