The relationship between phytoplankton biovolume and chlorophyll in a deepoligotrophic lake: decoupling in their spatial and temporal maxima

The seasonal distributions of phytoplankton biovolume and chlorophyll a con tent were monitored for 14 months in a deep oligotrophic, high mountain lak e (Redo, Pyrenees). An allometric relationship of chlorophyll with biovolum e was found throughout the period studied, with a correlation coefficient o f 0.66. However, the relationship changed with season and the taxonomic com position of the phytoplankton. Both parameters showed a similar seasonal pa ttern, but differences in space and time were observed. The chlorophyll max imum was recorded deeper and later than that of phytoplankton biovolume. Wh ile the biovolume maximum was related to an improvement in conditions for g rowth (nutrient input during column mixing periods), and reflected an incre ase in biomass, the chlorophyll maximum was related to changes in cell pigm ent content, and to spatial or successional trends in species dominance. Fl agellated chrysophytes predominated at the chlorophyll maxima. Chlorophyll content per unit of phytoplankton biovolume fluctuated greatly throughout t he year, depending on light intensity, temperature and phytoplankton compos ition. Of the main groups of phytoplankton in the lake, the dinoflagellates , which dominated the summer epilimnion phytoplankton community, recorded t he lowest pigment content per biovolume (which is consistent with their siz e). Higher chlorophyll contents per biovolume were found in the deep hypoli mnion and during the winter cover period associated with small cells such a s some species of chlorococcales chlorophytes. When flagellated chrysophyte s were predominant, a broad range of chlorophyll values per biovolume was f ound and there was no significant correlation between the two biomass indic es. These findings reaffirm the need to treat phytoplankton biomass estimat es with caution, in particular when conducting primary production studies. While our results show that changes in chlorophyll content per cell occur a s a photoacclimation response along a vertical profile, they also point out a component of the successional trends which appear in a phytoplankton gro wth phase in a lake.