Phytoplankton funcitonal distribution in peri-alpine lakes: a close up on the morphological plasticity of picocyanobacteria.

CELLAMARE, M.; DE TEZANOS PINTO P.; FONTANARROSA M.S.; JACQUET, S.

Kastoria

Workshop; ? 17th Workshop of the International Association of Phytoplankton Taxonomy and Ecology (IAP); 2014

Picocyanobacteria can contribute up to 80% of the primary production in lakes. However, this group has been ignored for many years because of its small size (≤2 µm), and therefore unseen by conventional microscopy. Picocyanobacteria may exhibit two morphology types: single cells (coccoid, rods) and microcolonies. The dominance of one of these morphotypes at specific periods or depths may reflect an adaptive significance. We aimed to characterize the distribution of these two picocyanobacterial forms during the onset of the warm season in peri-alpine lakes, and compare their occurrence to that of large-sized phytoplankton. Samples were collected in three French peri-alpine lakes (Geneva, Bourget and Annecy) characterized by different trophic status, between February and June 2005, at three depths. Large-sized phytoplankton (>2 µm) was analysed by inverted microscope, and small-sized (≤2 µm) by epifluorescence microscope and flow cytometer. We classified the phytoplankton into functional groups and explored their relationship with environmental variables using multivariate analyses. In the most oligotrophic lake, picocyanobacteria both single cells and microcolonies, dominated  in abundance, but its biomass was almost imperceptible compared to the large-sized phytoplankton. In both mesotrophic lakes mainly single cells picocyanobacteria were found, and in low abundances. High biomass of both picocyanobacteria morphologies occurred at the lowest phosphorus and nitrogen concentrations. Based on our results we suggest to place the picocyanobacteria microcolonies (usually placed into the group Lo) in a new functional group (here called LoCPcy), because it displayed a different ecology from the codon Lo. Regardless of size, phytoplankton functional groups responded to the trophic status of the lakes. Functional groups containing large phytoplankton (>2 µm) were associated to the mesotrophic lakes, whereas those containing picocyanobacteria (single cells and microcolonies) characterized the oligotrophic lake.