As part of a diagnostic restoration/feasibility study of La Plata Lake in Puerto Rico, we have examined seasonal, vertical, and (some) horizontal patterns of nutrient distribution. The eutrophic state of this reservoir is evidenced in a series of characteristics including epilimnetic (0–4 m) chlorophyll concentrations averaging 15 mg m−3, virtually total absence of dissolved oxygen below 4m, and water hyacinth areal coverage estimated at 40 percent. Annual loadings of nitrogen and phosphorus have been calculated in 107 and 30 gm−2yr−1respectively, giving N:P loading ratios of 8:1. The ratio of N to P in the entire water column crop averaged 10:1 during the study period. The epilimnetic (0–4m) ratio averaged only 6:1. During a stratified dry period N:P ratios as low as 0.9:1 were observed whereas during periods following heavy runoff and lake mixing, the ratios approached values of 16:1. This pattern of inorganic nitrogen distribution suggests surface depletion from assimilation by hyacinths and algae. The suggestions currently being examined are that nitrogen is limiting to primary productivity in the La Plata system and that the principal controller of lake function is the pattern of hydrological events. Any plan to manage this and similar reservoir systems in Puerto Rico must take these factors into account.