Biomass and productivity of the main autotrophic communities, and their contribution to total lake primary production, were studied in two shallow, polymictic lakes of the Karelian Isthmus (north-west Russia). Both lakes are highly productive but with contrasting contributions from macrophytes and algae. In Lake Rakovoe, with very large catchment and high external P loading, macrophytes are estimated to occupy 60% of its area and produce 106 g C m^-2 or 63% of the lake's annual primary production. In Lake Vishnevskoe with high TP content in lake water, determined by internal loading, phytoplankton is the main producer of organic matter; it produces 178 g C m^-2 or 85% of total annual production. The Conditions which determine different types of primary production composition are discussed.
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