Anthropogenic Enrichment and Nutrients in Some Tropical Lagoons of Ghana, West Africa
As part of a larger study of demographic change in coastal Ghana, we measured the concentrations of major plant nutrients and phytoplankton chlorophyll in eight coastal lagoons with different land use and human population density. The purpose of our study was to relate human activities to water quality in coastal receiving waters. We also carried out preliminary measurements of stable nitrogen isotopes to quantify the contribution of sewage and fertilizer to fish production in the lagoons. Annual mean concentrations of ammonia, nitrite plus nitrate, dissolved inorganic phosphate (DIP), and water column chlorophyll a varied by factors of over 1400, 15, 315, and 125, respectively, among the eight small coastal lagoons in the Central and Greater Accra Regions of Ghana. Concentrations of ammonia (mean of – 1 mmol/L) and phosphate (mean of -60 umol/L) in Korle lagoon, near the capital city of Accra, may be the highest yet reported for an estuarine system. In most of the lagoons, nutrient concentrations throughout the study period were much lower than previously reported, perhaps due to analytical problems in earlier measurements. Dissolved inorganic nutrients varied markedly over the year but showed no evidence of a regular seasonal cycle such as commonly observed in temperate coastal systems. Nutrient concentrations did increase sharply and briefly in many lagoons during the wet season. Water column chlorophyll increased greatly during August and September. Since water column inorganic nutrients were very low during July, it is possible that the apparent bloom was stimulated by nutrients brought into the lagoons during coastal upwelling or that the elevated chlorophyll was itself brought into the lagoons from offshore. Human population densities in the watersheds of the lagoons were high and varied widely, from -150 individuals/km2 to almost 3300 individuals/km2. Ratios of watershed area to open lagoon area ranged from -30 to almost 2500, much higher than is common in most well-studied estuaries and lagoons in the United States. We found a strong correlation between human population density and mean annual dissolved inorganic nitrogen (DIN) concentrations among the wide range of systems studied. Mean annual DIN concentrations in the Ghana lagoons were higher than found in a number of temperate lagoons with similar population density. The relationship between population density and DIP and chlorophyll was weak. The latter may be due to toxic effects in Korle lagoon and abundant filter feeders in another lagoon. Stable nitrogen isotope ratios in two species of fish, a shrimp, and a crab varied consistently among the lagoons, with higher 515N values indicative of human sewage in lagoons downstream of more densely populated watersheds.