An estimation of the nitrogen and phosphorus loading by wet deposition over Lake Maracaibo, Venezuela

In order to make an estimation of the wet deposition levels of the major ni trogen compounds 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 p Hs averaged about 5 to 6. NH4+ levels were significantly higher (up to 1.44 mg L-1 NH4-N) than NO3- plus NO2- ions which resulted in the highest pHs v alues in the strait Maracaibo. The strait is the nearest zone to Ammonia Pl ant located upwind at the Tablazo Petrochemical Complex. At the strait, the inorganic-nitrogen ((NH4 + NO3 + NO2)-N) concentrations were greater than organic nitrogen levels, but at the other sites were lower. Phosphorus leve ls were low, ranging from undetectable to 1.5 mg L-1; organic-P was estimat ed to account for about 40% of total-P. Total mean nitrogen wet depositions were 6.93 kg ha(-1) yr(-1) (approximate to 59% as NH4-N) at the strait and 9.85-16.84 kg ha(-1) yr(-1) (approximate to 55% as organic-N) around the l akeshore. Inorganic and organic nitrogen account for about equal portions o f the total nitrogen loading. These amounts represents approximate to 24% o f the annual total-N inputs by the tributary rivers to the lake and are sub stantially higher than those reported in the shore of Lake Valencia (north- central part of Venezuela) and other tropical areas. The contribution of pr ecipitation to P in lake is very small; approximate to0.6 kg ha(-1) of tota l-P are added annually. The N and P inputs were greater than the sewage con tributions. The mean molar ratio N/P is approximate to 22 at the strait sit e whilst at lake sites the N/P ratio is consistently > 22 (about 35 similar to1 to 100 similar to1). The results of this preliminary study show that t he atmosphere represents a significant factor for the total nitrogen loadin g to Lake Maracaibo system.