AMONG-LAKE AND WITHIN-LAKE VARIATIONS OF AUTOTROPHIC PICO-PLANKTON AND NANOPLANKTON BIOMASS IN 6 QUEBEC LAKES

The biomass of autotrophic picoplankton (APP) and nanoplankton (ANP) w as measured in six lakes from the Laurentian and Eastern Townships reg ions of Quebec, which vary in trophic status, watershed geology, and w ater chemistry. We examined the variations in the biomass of both phyt oplanktonic components, in relation to changes in limnological variabl es. Each lake was sampled at three depths, corresponding to the epi-, meta- and hypolimnion strata, and at five times over the summer season . The APP biomass and its contribution to phototrophic biomass of alga e <20 mu m decreased with lake trophy when expressed as total phosphor us (TP), while ANP dominated the <20 mu m algae components in more eut rophic lakes. The critical TP level at which ANP biomass exceeded APP biomass (ANP/APP biomass ratio >1) was around 20-30 mu g . L(-1). The mean values found for APP (75-365 mu g . L(-1) or 16-80 mu g C . L(-1) ) and ANP (46-249 mu g . L(-1) or 7-37 mu g C . L(-1)) biomass in Queb ec lakes over our range of trophy (TP: 7-73 mu g . L(-1)) were of the same order as those reported in other freshwater environments. Most of the variation in APP and ANP biomass was related to among-lake differ ences in nutrients (NO3, TP, TDP) and conductivity or pH.