Nutrient loading by precipitation in the Maracaibo Lake basin, Venezuela

Nutrient content and pH of rain samples collected at nine sites during 1988 -1996 were studied to determine the amounts of N, P and S added by precipit ation over Maracaibo Lake basin. The basin is a wide depression occupied by Lake Maracaibo and extensive alluvial plains and delimited by a mountainou s zone of variable physiographic. characteristics with different cattle ran ching and industrial activities. Rain pHs averaged approximate to 4 to 6, i ndividual samples occasionally reached as low as pH 3 in the south basin. N H4+ levels were significantly higher (up to 2.6 mg/l NH4-N) than NO3- plus NO2-. Phosphor-us levels were low, ranging from undetectable to 1.5 mg/l. T he amounts of inorganic N/ha/yr added ranged from approximate to 5.4 Kg in the north to approximate to 8 Kg in the south basin; around the lakeshore, it was estimated that approximate to 9.8-16.8 Kg TN/ha (approximate to 55 % as organic-N) are annually deposited. The annual amounts of SO4-S/ha added range from approximate to 10 to 13 Kg. The contribution of precipitation t o P in the ecosystem is very small; approximate to 0.02 - 0.8 Kg of PO4-P/h a/yr was added in basin. The results of this study show that precipitation represents a significant factor for the total nitrogen and sulfur loading t o Maracaibo Lake Basin.