Picoplankton in Lake Maggiore, Italy

In 1992 we examined the morphological characteristics and space‐time distribution of picoplankton cells in Lake Maggiore, a subalpine lake in which oligotrophication is in progress. We measured by image analyser the biovolume of autotrophic (APP), eukaryotic and prokaryotic. and heterotrophic (HPP) picoplankton. Among the APP < 2μm the yellow fluoresceing are the dominating cells in the euphoric zone. The red cells, mainly eukaryotic, on average are only 11% of the total abundance of cells < 5μm. The APP cell numbers range from 9.5 × 10 3 cells ml −1 to 1.3 × 10 5 cells ml −1 (average: 5 × 10 4 cells ml −1 ). Their mean biovolume shows a minimum value of 7.8mm 3 m −3 in March and a maximum of 186.3mm 3 m‐ 3 in September. The contribution of biovolume of yellow cells to total phytoplankton biovolume varies between 0.3% and 27%. suggesting that picocyanobacteria, at this stage of lake recovery, are not yet a dominant component. The HPP cell density is two order of magnitude higher than the APP with a mean value of 2.6 × 10 6 cells ml −1 . APP mean cell size fluctuates from a minimum of 0.5 μm to a maximum of 1.4urn (0.26–1.69μm 3 volume), while HPP range from 0.4 to 0.7 um (0.07–0.57 μm 3 volume), making it easier to distinguish them on a dimensional basis for most of the year. During the period of thermal stratification, a peak in abundance was noted in the central part of the metalimnion at depths receiving less than 10% of surface irradiance. The total picoplanktonic carbon fraction (APP and HPP) varied from 38 to 384 μgC 1 −1 with a mean value of 133μgC 1 −1 which represents 42% of POC collected on GF/C filters. Most of the picoplankton carbon is made up of HPP cells (34% of the total POC).