Charophytes as a nutrient and energy reservoir in a tropical coastal lagoon impacted by humans (RJ, Brazil)

Abstract

In the Imboassica Lagoon (22o 24’S and 42o 42’W) (Macaé, RJ, Brazil), two species of macroalgae of the genus Chara (C. angolensis and C. fibrosa) registered in large areas were identified. The lagoon is subject to several relevant anthropic impacts, of which untreated sewage is one of the most important. Results of the biomass distribution as analyzed in different areas of the lagoon showed values ranging from 172 to 510 gDW.m–2 . As for the nutrient concentration, the results presented large fluctuations, with carbon values ranging from 325 to 392 mg.g–1 DW, those of nitrogen, from 14 to 27 mg.g–1 DW, and of phosphorus from 0.45 to 0.85 mg.g–1 DW. This led to large fluctuations of the C:N:P ratio (from 387:24:1 to 872:47:1). The minimum and maximum energetic values of the biomass were 9.54 e 12.34 kJ.g–1 DW. Smaller C:N:P ratios tended to occur in more eutrophic areas. The association between biomass and nu- trient concentration showed that the highest quantities in the beds are found in oligotrophic areas. The Spearman correlation between nutrient concentration in the water column and biomass was r s = 0.45 (p < 0.05) for nitrogen and r s = 0.50 (p < 0.01) for phosphorus (n = 30). The large total biomass asso- ciated with nutrient concentration and energy content indicates that Charophytes are an important bio- logical compartment in the structure and metabolism of the lagoon.