A partial budget of primary organic carbon flows in the littoral zone of a hardwater lake

Primary flows of particulate organic carbon were analysed within a stand of Phragmites australis (Cav.) Trin. ex Steud, in the littoral zone of a temperate, medium-sized, nutrient-rich hardwater lake. Primary carbon flows refer to those fluxes of carbon which directly relate to the supply and disappearance of organic matter (essentially primary production, allochthonous input, decomposition and export), but without considering the channelling of carbon through the food web. Epiphytic and planktonic production was estimated with the C-14 technique. Net above-ground production of Phragmites was inferred from measurements of above-ground biomass, and allochthonous particulate inputs were estimated from the amounts of litter collected in traps at the lake margin. Decomposition was studied with the litter-bag method, whereas the export of coarse particulate organic matter to and its import from the pelagic zone were assumed to be insignificant. Annual above-ground reed production was 689 g C m(-2) of reed bed and accounted for 88% of the carbon becoming available within the littoral zone. Almost two-thirds of the material consisted of stems and leaf sheaths, and although half of that fraction remained after 1 year, Phragmites also contributed substantially to heterotrophic lake metabolism (83% of the total), Extrapolation of littoral and pelagic primary production indicated that 22% of total production in the lake was achieved in the littoral zone, which covered only 4.5% of the lake area. Taken together, these findings illustrate the great potential of littoral zones to influence overall. lake metabolism.