LAUSR

Phytoplankton size has an important functional role in ecosystem processes, such as nutrient production and cycling. It has been suggested that warmer temperatures may favor the smaller organisms in biological communities. We evaluate the mean size of the phytoplankton organisms and apply a morphology-based functional groups (MBFG) approach. We use time series of two floodplain lakes that present mean annual temperature difference of 10°C and similar hydrodynamic, area, and mean depth. We expected that a smaller mean size of MBFGs would be associated with higher temperatures. The Akaike Information Criterion was used to investigate environmental factors predicting the mean size of MBFGs within each lake. The mean size was most associated to nutrients and dissolved oxygen in the subtropical lake, and to temperature in the temperate lake. Large filaments with aerotopes and the small flagellated with siliceous exoskeletal structures showed high mean size at higher temperatures, thus contradicting temperature–size rules. Probably the high variability of mean size was provided by the high variability observed around environmental factors. Our findings reveal that other functional traits may be associated with the phytoplankters mean size, which result in adaptation to high variability of various environmental factors.