Exploring urbanization and critical habitat loss through land cover change around the Bons Sinais Estuary, Mozambique

Estuaries supply direct and indirect multi-sectoral opportunities including for transport, natural resource use and climate protection. These provisions support livelihoods and contribute to social and economic development. The Bons Sinais Estuary in Zambézia Province, central Mozambique, is adjacent to the provincial administrative capital Quelimane, some 25 km from the coast. The rapid growth of Quelimane has increased the demand for natural resources from the estuary, including space, food, fuelwood, transport and raw materials for construction and economic activities. Expansion of the built environment has extended into low-lying lands, mostly within the critical estuarine functional zone with inevitable consequences, such as damage to natural habitats and flooding of occupied areas during rainy seasons. The aim of this study was to analyse three decades of change (1991 – 2018) in land use and land cover (LU/LC) in the Bons Sinais Estuary, focussing on the growth of Quelimane city and the transformation of estuarine and surrounding habitats. The method relied on open-access satellite images and a LU/LC change analysis to quantify the spatio-temporal changes brought about by economic development and related human activities. A combination of low-intensity fieldwork and satellite-derived data (Landsat-5, sensor: Thematic Mapper and Landsat-8; sensors: Operational Land Imager, Thermal Infra-Red Scanner) was used to generate LU/LC information classified according to the features: mangrove trees; wetlands; estuary intertidal areas; built-up area; cultivated trees; and cultivated land. From 1991 onwards, there was an overall increase in cultivated crops (66 %), development (79 %) (including rural human settlements) and intertidal mudflats (12 %) with a concomitant decline in critical wetlands (16 %) and mangroves (12 %). The study predicts a worsening of the impacts on the estuarine ecosystem with further growth of Quelimane city. To reverse the negative trend on estuary health, the recommendation is for management interventions that promote sustainable LU, and urban development plans that consider ecosystem conservation and active restoration

The residual circulation profile of the Bons Sinais Estuary in central Mozambique - potential implications for larval dispersal and fisheries

The residual circulation in estuaries determines the net exchange of water, heat, salt, fine sediments and drifting biological organisms between estuarine and nearshore marine waters. The Bons Sinais Estuary in central Mozambique is ~ 30 km long with the city of Quelimane and an industrial port on the northern bank of its upper reaches. To investigate residual circulation in the estuary, seasonal (wet, dry and transition season) CTD profiling data were collected at 11 fixed stations between the upper estuary and mouth, and vertical current profiles were measured over a full tidal cycle at a fixed mid-estuary station. Strong longitudinal gradients in salinity and density indicated that the estuary was river-dominated during the wet season and tide-dominated during the dry season, but the water column remained partially mixed. Tidally averaged vertical profiles from the mid-estuary station revealed: uniform vertical temperatures, warmest during the wet season; depth-stratified salinity during the wet season, but uniform profiles during the dry and transition seasons with highest salinity during the dry season when the density was also highest. The density was uniform and somewhat lower in the transition season, and in the wet season the density was even lower, but stratified. The vertical velocity profile showed a classical two-layer circulation model, with downstream flow intensifying at the surface, and upstream flow at the bottom, during the wet and transitory seasons, when freshwater discharges into the estuary. The flow velocities obtained from a calibrated Hansen and Rattray model fitted the observed data well, confirming that a simplified modelling approach is adequate to describe the residual flow of the Bons Sinais Estuary. The residual circulation model provides insights useful for fisheries research and predicting the spread of water-borne pollutants in the estuary