Land use changes between 1972 and 2008 and current water quality of wetlands in Harare, Zimbabwe.

Abstract

Agriculture and other land use changes such as settlement adversely affect water levels and nutrient status hence influences wetlands ecosystems. To assess the impacts of land use changes in three selected urban wetlands Monavale, Mabvuku and Honeydew in Harare, Zimbabwe, aerial photographs (contact prints) for three time periods (1972, 1984 and 1995) were transferred into a Geographic Information Systems (GIS) environment. For a fourth time period, 2008 Google Earth SPOT images were used. These remote sensed images were used to analyse the temporal changes in the size of the wetland and land uses within the wetlands. Class level landscape metrics were calculated using Arcview patch analyst, in order to characterize wetland fragmentation. Results showed that dry land agriculture and settlement increased whilst wetland area decreased in all wetlands. The landscape became more fragmented with time as indicated by an increase in the patch number (NP) and a decrease in the mean patch size (MPS) of the unused wetland and wetland agriculture classes. Anthropogenic activities especially increase of agricultural land use were the main causes of wetland fragmentation. These human activities have contributed to urban wetland size shrinkage in the study area. Groundwater samples were collected from the wetlands in April and June 2009 to assess quality of the water and relate it to land uses. Water quality results showed that there was significance difference in TDS, conductivity, pH, salinity, and TP between the three urban wetlands in both sampling months April and June 2009. Sewage ponds were associated with high total dissolved solids, conductivity and nitrogen. A microbial analysis was carried out and all samples were positive for Escherichia coli, and faecal coliforms were present in varying amounts. Faecal coliform levels counted using the Most Probable Number (MPN) were between 3 and greater than 1100 and classified between low and very high risk in terms of the WHO drinking water guidelines. Both samples taken in April and June showed that all land uses in the study area had high faecal coliform counts except in a conserved wetland area. Results suggest that water from shallow wells would be unfit for drinking and quality can be improved by removing access by humans. The study provides information on the current status of the wetlands which can be the basis for motivating management and setting up a management plan.