Biovolume variability in natural populations of picoplankton and nanoplankton from the Canary Current region

Abstract

Picoplankton (0,2-2 µm) and nanoplankton (2-20 µm) populations contribute greatly to total living biomass of planktonic food webs in the oceans. However, quantifying the biomass size-distribution variability across the different autotrophic and heterotrophic groups is a tedious, time-consuming effort that barely is done in oceanographic studies. For this reason, most authors estimate carbon biomass of natural populations from measured cells counts, using fixed biovolumes obtained from the literature. These are mostly derived from laboratory cultures or field studies from a particular site and time of the year, leading to significant inaccuracies. Here, we present data from two field studies where the variability in biovolume of different picoplanktonic (Proclorococcus, Synechococcus, picoeukaryotes, high NA and low NA bacteria) and nanoplanktonic (autotrophic and heterotrophic flagellates) groups were estimated. Water samples for picoplankton biovolumes were collected from 1 coastal and 3 oceanic stations around the Island of Gran Canaria (Canary Islands) at different depth layers (surface, mixed layer, and depth of chlorophyll maximum), during 2011. More than 40 sequential filtration experiments were carried out, filtering through 7 polycarbonate filters (ranging from 0.2 to 3 µm), to obtain the average cell diameter. Cell counts were measured by flow cytometry, using a FACScalibur system (Beckton & Dickinson). Samples for nanoplankton biovolumes were collected during four seasonal oceanographic cruises at seven stations and five depths near Cape Guir (NW Africa), during 2009. Cells were counted and measured by epifluorescence microscopy, before being clustered into 3 size classes (2-6, 6-11 and 11-20 µm) and 2 different morphologies (oval and pear-shaped). Biovolumes were calculated from mathematical equations according to the shape of the cell, and biomasses were derived from carbon conversion factors. Our results show that picoplankton biovolumes are very constant in time and depth, and similar to published studies in the NE Atlantic; except for picoeukarytes, which may range up to 6 fold in the average biovolume (from 0.3 to 1.9 µm3). Average nanoplankton range in biovolume from 14 to 697 µm3 across the whole community, and up to 2-fold across a same size cluster. Our study is the first one that estimates the biovolume variability in the smaller but most abundant size classes of planktonic communities in the Canary Current. This work will allow better estimations of planktonic biomass to address future studies of food webs dynamics in this eastern boundary region.