Biomass allocation in Cyperus papyrus in a tropical wetland, Lake Naivasha, Kenya

The nutrient status of Lake Naivasha, a freshwater lake in southeastern Kenya, has been rising since at lease 1982. A potential effect of increases in nutrient supply to the lake's floating papyrus is increasing of the plants' investment in above-water material and reduction of the amount of energy invested in uptake and storage. Biomass and its allocation between culms, panicles, roots, and rhizomes was measured in 17 sites around the 150-km(2) lake. Although above-water biomass was greatest in sites closest to the lake's major nutrient inflow the River Malewa, there was little evidence of corresponding decreases in the biomass of uptake and storage tissues. In August 1995, the average +/- SD biomass of papyrus in the lake was 11,540 +/- 3020 g/m(2), with the papyrus containing about 4500 +/- 1900 g total carbon/m(2) and about 100 +/- 70 g coral nitrogen/m(2). Plant nitrogen contents did not vary with distance from the main external nutrient supply. Together with low nitrogen concentrations in the plants (0.60 +/- 0.26 in culms and 0.99 +/- 0.50% in rhizomes), very high carbon to nitrogen ratios (49 +/- 20:1) and nitrogen fixation in the rhizosphere explaining only about half of the plants' nitrogen, papyrus is a likely net sink for nitrogen supplied from the lake's increasingly cultivated watershed. Despite this role, clearance of papyrus in favor of agriculture partly explains the reductions in the area of papyrus within the lake basin from 48 km(2) in the late 1960s to 14 km(2) in 1995.