SEASONAL NUTRIENT DYNAMICS IN AN AMAZONIAN FLOODPLAIN LAKE

Monthly changes in the content of dissolved nutrients (NO2, NO3, NH4, PO4, Si), SO4 and Of Specific conductance in the Amazon floodplain lake, Lago Camaleao, were measured during a two years period. Comparative studies were done in the Amazon river water seasonally flooding the lake. Seasonal changes in specific conductance and the content Of Si, SO4, and NH4 in the lake were large. During the low water period (October to December) when the lake had receded to its minimum depth, specific conductance increased up to 14 times, Si up to 19 times, SO4 up to 23 times, and NH4 up to 29 times of the respective concentrations in the river water. During rising and high water level (January to September) specific conductance and concentrations Of Si, SO4, and NH4 in lake surface water were closely related to those of the river water during the same period. On the average, specific conductance was 56 %, concentration of Si 55 %, Of SO4 2 %, and Of NH4 0 % higher than in river water. PO4 concentrations in the lake during rising and high water level were about 17 % higher than those in the river water but did not show a marked rise at low water. Short term chemical stratification at high water was only pronounced for NH4 and PO4. The average concentrations in water near the bottom were 91 % and 112 %, respectively, higher than those of surface water. NO3 content in the lake was lower than in the river during the whole two year study period (24.2 +/- 35.8 mug N l-1 versus 101.5 +/- 43.9 mug N l-1); the lake acts as a sink for dissolved nitrate.