The relationship between autotrophic picoplankton (APP) - the smallest autotrophic component of food web and the trophic status and the depth of lakes

The occurrence and dynamics of autotrophic picoplankton (APP) in relation to the trophic status of a lake is discussed basing on results from 9 lakes situated in northeastern Poland and southern Finland. The lakes varied in the concentration of basic nutrients, chlorophyll a, phytoplankton biomass, pH depth and the composition of APP. A positive relationship between the productivity of the lakes and the APP biomass was confirmed. Shallow lakes were characterised by a significantly higher abundance and biomass of APP than deeper lakes, regardless of their trophic status. The abundance of APP as well as its contribution to the total phytoplankton biomass in the studied shallow lakes was also higher, than it could be expected from the level of basic nutrients in those lakes. Results imply that, when compared to deep lakes of a comparable trophic level, shallow and occasionally turbid water environment may create favourable conditions for APP, enabling it to occur in high numbers and thus influence the carbon cycling causing an increase of the carbon flow through the microbial food webs.