Mangrove Zonation as a Tool to Infer the Freshwater Inflow Regime in the Data-Poor Ruvu Estuary, Tanzania

Estuaries provide numerous ecosystem services, including fisheries, coastal community livelihoods, and resistance to saltwater intrusion. Despite this knowledge, estuaries worldwide are threatened by decreasing and/or aseasonal freshwater inflows, which negatively affect ecosystem structure and function. Sound estuarine management requires an understanding of the natural freshwater inflow regime and knowledge of the salinity tolerances of local plant and animal communities—data that are completely lacking in most estuaries. This paper describes a 2-week field survey of mangrove zonation in the Ruvu River estuary carried out during the wet–dry season transition to obtain a multi-decadal proxy for the salinity regime within the estuary. Salinity conditions arising from the mixing of freshwater inflows and sea tides influence the species composition of mangroves. The mouth of the estuary (highest salinity −35 ppt) had monospecific stands of Sonneratia alba—the mangrove with the highest salinity tolerance. Salinity decreased going upriver, from 30 ppt to 5 ppt over 13 km, with 7 other mangrove species progressively appearing in the riverbank forests, ultimately transitioning to palms and other trees intolerant of salinity (<5 ppt). The resulting map relating mangrove zonation with salinity can then be used to calibrate estuary salinity mixing models for calculating minimum freshwater inflows necessary to maintain the estuarine ecosystem. Such periodic surveys and maps can also serve to calibrate/validate remote sensing products for continued coastal vegetation monitoring. The study also reviews available information on climate and land use relating to river flow in the Ruvu basin to summarize the hydrologic vulnerability of the Ruvu estuary to climate change, land use change, and river water demands in the Basin.