Temporal variation in the biomass and nutrient status of Azolla filiculoides Lam. (Salviniaceae) in a small shallow dystrophic lake Varia o temporal de biomassa e estado nutricional de Azolla filiculoides Lam (Salviniaceae) em um pequeno lago raso distrófico

AIM: This study determined the temporal variation of the biomass and the concentrations of carbon, nitrogen and phosphorus in Azolla filiculoides Lam. in a small (0.5 ha) shallow dystrophic lake located in the city of Rio Grande (Rio Grande do Sul State, Brazil). METHOD: Sampling was conducted monthly between November 2000 and October 2001. The macrophytes were collected randomly in three replicates with a circular collector 0.3 m in diameter and subsequently washed with tap water and oven-dried at 60 oC for determination of the dry weight and the nutrient status (i.e., carbon, nitrogen and phosphorus). Primary productivity was estimated by the variation in biomass among successive samples. RESULTS: A. filiculoides was present in the lake throughout the year and occupied between 50 and 80% of the surface area. The biomass values ranged from 34.2 g DW.m-2, recorded in May (autumn), to 170.9 g DW.m-2 in January (summer). The highest rate of primary productivity was 3.3 g DW.m-2.d-1, observed in June. The concentrations of carbon, nitrogen and phosphorus in the plant ranged between 403 and 551 g.kg-1, 13.4 and 25.7 g.kg-1 and 0.5 and 1.9 g.kg-1, respectively. The water N:P ratio ranged between 19:1 and 368:1. CONCLUSION: The coverage of the surface of the lake by A. filiculoides throughout the study period and the nutritional status of the plant demonstrate the importance of the cycling of nutrients by macrophytes in this aquatic environment. The higher N:P ratio in the water column, compared with other neighboring environments without macrophytes, shows that the enrichment of the lake may result from the biological N-fixation activity produced by A. filiculoides. OBJETIVO: Este estudo determinou a varia o temporal da biomassa e as concentra es de carbono, nitrogênio e fósforo de Azolla filiculoides Lam., em um pequeno lago raso distrófico situado no município do Rio Grande (Rio Grande do Sul, Brasil). MéTODO: As amostragens foram realizadas mensalmente no período entre novembro de 2000 e outubro de 2001. As macrófitas foram coletadas aleatoriamente em três repeti es, utilizando-se coletor circular de 0,3 m de diametro e posteriormente lavadas com água corrente e secas em estufa 60 oC para determina o do peso seco e dos nutrientes: carbono, nitrogênio e fósforo. A produ o primária foi estimada pela varia o de biomassa entre coletas sucessivas. RESULTADOS: A. filiculoides esteve presente no lago durante todo o ano, ocupando sempre entre 50 e 80% da superfície do lago. Os valores de biomassa variaram entre 34,2 g PS.m-2, registrado em maio (outono) e 1