AN ESTIMATION OF THE NITROGEN AND PHOSPHORUS LOADING BY WET DEPOSITION OVER LAKE MARACAIBO, VENEZUELA J. A. MORALES 1∗ , A. ALBORNOZ 2 , E. SOCORRO 2 and A. MORILLO 2 1 Lab. Química Ambiental, Facultad Experimental de Ciencias, Universidad del Zulia, Maracaibo, Venezuela; 2 Instituto para el Control y la Conservación de la Cuenca del Lago de Maracaibo (ICLAM), Gerencia de Investigación Ambiental, Maracaibo, Venezuela ( ∗ author for correspondence, e-mail: jmora@mail.luz.ve) (Received 9 June 1999; accepted 13 December 1999) Abstract. In order to make an estimation of the wet deposition levels of the major nitrogen com- pounds in Lake Maracaibo system, the precipitation was collected by events at five sites located on shore around the Lake from 1991 to 1994. Also the phosphorus concentrations were determined in rain samples. Rain pHs averaged about 5 to 6. NH + 4 levels were significantly higher (up to 1.44 mg L −1 NH 4 -N) than NO − 3 plus NO − 2 ions which resulted in the highest pHs values in the strait Mara- caibo. The strait is the nearest zone to Ammonia Plant located upwind at the Tablazo Petrochemical Complex. At the strait, the inorganic-nitrogen ((NH 4 + NO 3 + NO 2 )-N) concentrations were greater than organic nitrogen levels, but at the other sites were lower. Phosphorus levels were low, ranging from undetectable to 1.5 mg L −1 ; organic-P was estimated to account for about 40% of total-P. Total mean nitrogen wet depositions were 6.93 kg ha −1 yr −1 (≈59% as NH 4 -N) at the strait and 9.85– 16.84 kg ha −1 yr −1 (≈55% as organic-N) around the lakeshore. Inorganic and organic nitrogen account for about equal portions of the total nitrogen loading. These amounts represents ≈24% of the annual total-N inputs by the tributary rivers to the lake and are substantially higher than those reported in the shore of Lake Valencia (north-central part of Venezuela) and other tropical areas. The contribution of precipitation to P in lake is very small; ≈0.6 kg ha −1 of total-P are added annually. The N and P inputs were greater than the sewage contributions. The mean molar ratio N/P is ≈22 at the strait site whilst at lake sites the N/P ratio is consistently >22 (about 35∼1 to 100∼1). The results of this preliminary study show that the atmosphere represents a significant factor for the total nitrogen loading to Lake Maracaibo system. Keywords: Lake Maracaibo, nitrogen, phosphorus, Venezuela, wet deposition 1. Introduction Rain and snow have long been known as nature’s way of cleaning the atmosphere and much research has resulted from the realization that knowledge of atmospheric deposition is fundamental to understanding the biogeochemical cycling of many elements. For example, atmospheric input may account for a significant proportion of the nutrient flux to lake (Schindler et al., 1976; Manny and Owens, 1983) and terrestrial ecosystems (Lindberg et al., 1986). Almost a third of the rain which fall on the Earth’s continents is deposited within the narrow equatorial belt between 10 ◦ N and 10 ◦ S latitude. However, in spite of the Water, Air, and Soil Pollution 128: 207–221, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands.