A mass balance assessment of the contribution of floating-leaved macrophytes in nutrient stocks in an eutrophic macrophyte-dominated lake

In this study we examine the contribution of three floating-leaved macrophyte species, Nymphaea alba, Trapa natans and Nymphoides peltata (covering about 700–750 ha), in the N and P stocks in a shallow eutrophic lake (4000–6300 ha) during a 3-year period (1995–1997). The monthly nutrient standing stocks in the above-ground structures of the plants were derived from biomass, nutrient content and area covered by each plant species during the growth period. These N and P quantities were compared to the net nutrient inputs of the lake waters from the catchment area recorded in the winter (water discharging phase), preceding the vegetation period. N and P contents reached high values for the three species (2.16–3.23% N and 0.29–0.68% P) showing the ability of the plants to accumulate large nutrient contents. Despite significant annual fluctuations in nutrient contents and biomass, not always in the same way for each floating-leaved macrophyte, and changes in the area they covered, the maximum monthly P stocks in the plants were globally uniform between the 3 years (7.10–8.85 tonnes). Maximum values for N ranged from 40.85 to 54.55 tonnes in summer. The vast beds of N. alba (until 75% of the total area in 1997), contributed largely to these annual nutrient stocks (50–72% and 46–70%, respectively, for N and P). Whereas the nutrient stocks estimated in the plants during the biomass peak represented little proportions of the annual net flux of N in the lake waters (3.35–5.00%), they reached high proportions for P sometimes exceeding the annual net loading of P in the lake according to years (16.80–156.25%). These results suggest that the nutrient-enriched sediments of the lake (6–53 and 2–6 times the net annual P and N inputs) probably compensate for annual variations in the external nutrient loading, notably for P, and floating-leaved macrophytes are likely not limited when the inputs from the catchment area are small.