Freshwater picocyanobacteria along a trophic gradient and light quality range

Abstract

Picocyanobacterial abundance and their contribution to the total phytoplankton biomass, estimated as chlorophyll a, was investigated in 32 deep and shallow lakes. The lake series covered a wide range of natural and artificial lakes, from high altitude clear lakes and deep, large subalpine lakes through large shallow lakes, small reservoirs and fish ponds. The chlorophyll a concentrations ranged between 0.2 and 390 µg l^-1. No simple relation existed between trophic state and the abundance of picocyanobacterial, mainly represented by Synechococcus spp. Below 10 µg Chl a l^-1 the percentage contribution of picocyanobacteria biomass to total phytoplankton biomass exceeded 70% in some cases, whereas above 100 µg Chl a l^-1, the highest contribution was only 10%. At low chlorophyll a concentrations phycoerythrin-rich picocyanobacteria (PE) dominated almost exclusively but their contribution never exceeded 10% of the total picocyanobacterial abundance when chlorophyll a concentration was higher than 50 µg l^-1. Above this value there was high light attenuation and a shift of the maximum light penetration from the blue-green towards the red portion of the spectrum. In this underwater light climate phycocyanin-rich picocyanobacteria (PC) dominated. In the lakes chosen for their representation of ranges in trophy and light quality, PC cells prevailed over the PE when vertical attenuation coefficient of PAR was higher than 2.25 m^-1 and red light penetrated farthest. PE cells were 100% when the vertical attenuation coefficient of PAR was lower than 0.55 m^-1 and the green and blue were the most penetrating lights.