Impact of Human Activities on Nutrient and Trophic Status of some Selected Lagoons in Ghana

The nutrient and trophic status were used to assess the impact of human activities on lagoon by determining the trophic status of the lagoons and then using nutrient parameters and transparency in a form of model to predict the trophic status index (TSI) of the lagoon waters. The trophic index of Benya, Nakwa, Amisa and Nyanya ranges between 35.0– 39.0 and falls within Carlson’s TSI range scale of 30–40, which implies that their waters are oligotrophic. The Oyibi lagoon is mildly eutrophic (Carlson’s TSI scale of 56) in the riverine portion. However, at the seaward end it has TSI of 42, which is moderately clear. Similarly, Muni lagoon is moderately clear with TSI range of 35.8–43.1. The Sakumo II lagoon is hypereutrophic at the riverine end (Carlson’s TSI of 87.8). The seaward end is oligotrophic. The high phosphorus (231.9 kg/day) and nitrogen (927.9 kg/day) recorded at Nyanya lagoon are as a result of domestic activities mainly from the wastes generated from humans at the lagoon and its surroundings. The use of nutrients and transparency, in predicting the trophic status of lagoons and coastal waters is a new and emerging scientific way of looking at lagoon trophic status. The nutrient inputs into lagoons, as well as transparency, are direct impacts of human activities. The model has proved to be useful in explaining the impact of human activities on the trophic status of lagoons in Ghana. The model was assessed by predicting the observed versus predicted, and this showed a performance index (R) of 95.1%. This shows that the model is able to use various nutrient and transparency values to predict the trophic status of lagoons. The model is able to predict lower or oligotrophic TSI values better than higher or hypereutrophic values. The model has then been used successfully to predict various TSI values from the lagoons. Introduction There are two types of lagoons fringing the West African coast. The coastal lagoons of Ghana have been categorised into ‘open’ and ‘closed’ by Boughey (1957). The open lagoon, often referred to as an estuary, has sufficient volume of water at all seasons to maintain a permanent outflow from its mouth into the sea. In Ghana, such lagoons occur more in the western part of the coastline where rainfall is heavy (mean of about 1250 mm per annum) and the lagoons are fed continuously by rivers. The closed lagoons are fed by seasonal rivers and streams. They usually lie behind a sand barrier, which separates them from the sea and are normally opened for one or two months of the year during the rainy season (May–September). Most of these lagoons are located on the eastern coastal region where the rainfall is low. Associated with all lagoons in tropical areas are coastal wetland mangrove swamps. Lagoons have been found to be extremely vulnerable to build up of contaminants from the terrestrial, freshwater and marine environment. Because lagoons are rich in nutrient salt and an elementary productivity force resource, they have become an area for developing sea transport, reclamation, aquaculture and tourism (Zhuang, 1992). Therefore, there are more human activities in lagoonal coast than in the other kinds of coasts. There have been increases in deposition of sediments in lagoons as a result of land use changes, overgrazing and urbanization. Sewage discharges are also causing eutrophication, increasing the frequency of toxic algal blooms and creating health hazards. Lagoons in Ghana have a great influence on the socio-economic well-being and health of communities that live close to and beyond them. The existing balance between the input of inorganic nutrients from different sources and their use or transport through assimilation, sedimentation or export from the system is important in evaluating coastal trophic state of lagoons. All trophic classifications are based on arbitrary