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

The seasonal distributions of phytoplankton biovolume and chlorophyll a content were monitored for 14 months in a deep oligotrophic, high mountain lake (Redo, Pyrenees). An allometric relationship of chlorophyll with biovolume was found throughout the period studied, with a correlation coefficient of 0.66. However, the relationship changed with season and the taxonomic composition of the phytoplankton. Both parameters showed a similar seasonal pattern, but differences in space and time were observed. The chlorophyll maximum was recorded deeper and later than that of phytoplankton biovolume. While the biovolume maximum was related to an improvement in conditions for growth (nutrient input during column mixing periods), and reflected an increase in biomass, the chlorophyll maximum was related to changes in cell pigment content, and to spatial or successional trends in species dominance. Flagellated chrysophytes predominated at the chlorophyll maxima. Chlorophyll content per unit of phytoplankton biovolume fluctuated greatly throughout the year, depending on light intensity, temperature and phytoplankton composition. Of the main groups of phytoplankton in the lake, the dinoflagellates, which dominated the summer epilimnion phytoplankton community, recorded the lowest pigment content per biovolume (which is consistent with their size). Higher chlorophyll contents per biovolume were found in the deep hypolimnion and during the winter cover period associated with small cells such as some species of chlorococcales chlorophytes. When flagellated chrysophytes were predominant, a broad range of chlorophyll values per biovolume was found and there was no significant correlation between the two biomass indices. These findings reaffirm the need to treat phytoplankton biomass estimates with caution, in particular when conducting primary production studies. While our results show that changes in chlorophyll content per cell occur as a photoacclimation response along a vertical profile, they also point out a component of the successional trends which appear in a phytoplankton growth phase in a lake.