Volume 13, Issue 1 Jan - June 2014 Chief Editor: Michael H. Schleyer ISSN: 0856–860X Western Indian Ocean Journal of Marine Science CHIEF EDITOR: Prof. Michael H. Schleyer, Oceanographic Research Institute, Durban, South Africa COPY EDITOR: Gordon Arara EDITORIAL ASSISTANT: Kheira Kortenbout EDITORIAL BOARD Dr Jared O. Bosire, Kenya Prof. Kassim Kulindwa, Tanzania Dr Blandina R. Lugendo, Tanzania Dr Francis Marsac, Reunion Dr Nyawira A. Muthiga, Kenya Dr Brent K. Newman, South Africa Prof. José P. Paula, Portugal Prof. Chris C. Reason, South Africa Mr Jan Robinson, Seychelles Dr Jacqueline N. Uku, Kenya PUBLISHED BIANNUALLY Aims and scope: The Western Indian Ocean Journal of Marine Science provides an avenue for the wide dissemination of high quality research generated in the Western Indian Ocean (WIO) region, in particular on the sustainable use of coastal and marine resources. 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No. 1, pp. 1 - 10, 2014 © 2015 WIOMSA Morphology of the Zambezi River Plume on the Sofala Bank, Mozambique Fialho P.J. Nehama1, 2 and Chris J.C. Reason2 1Escola Superior de Ciências Marinhas e Costeiras, Universidade Eduardo Mondlane, Chuabo Dembe, Quelimane, Mozambique; 2Department of Oceanography, University of Cape Town, Rondebosch 7700, South Africa. Keywords: Sofala Bank, Zambezi River, dispersion patterns, plume, density- driven current Abstract — Hydrographic data collected in the vicinity of the Zambezi River plume between 2004 and 2007 is discussed alongside historical data to infer the plume morphology. Our strategy involved the establishment of 73 CTD stations. Satellite-derived wind speed data and river discharge measurements at an upriver gauging station were also analysed. The plume dispersion patterns indicated a tendency in its progressive propagation to move both equatorward and poleward. This tendency was not explored in previous studies and places the Zambezi River plume in a short list of plumes across the globe that propagate in a direction opposite to Kelvin or shelf waves. Visual inspection of the salinity profiles revealed that the Zambezi plume is super-critical, indicative of a faster freshwater inflow compared to the phase speed of long internal waves. The plume’s vertical structure was found to be surface-advected when the freshwater discharge measured at Tete was less than 2000 m3s-1, and bottom-advected under larger discharges. A clear distinction was found between the plumes of the Zambezi and Licungo Rivers, characterised by a seaward bending of the salinity contours as the Zambezi freshwater flows downstream past the mouth of the Licungo River. INTRODUCTION Freshwater discharges from river basins various consequences. The most profound of into the ocean have an important influence on these occur within the region of freshwater the dynamics of many coastal regions. In these influence and vary from the physical regions, the input of freshwater generates a alteration of the coastline, to changes in distinct physical regime, characterised by productivity and the availability of biological a surface layer of less saline water flowing resources. For example, evidence of seaward over the denser ambient seawater. Following extension of the Zambezi Delta in geological plume generation, a vast range of dissolved timescales caused primarily by high sediment and suspended materials reach the ocean with flux is presented in Walford et al. (2005), Corresponding author: FPJN Email: [email protected] 2 Fialho P.J. Nehama and Chris J.C. Reason and the impact of the Zambezi freshwater METHODS on secondary production, in particular the shrimp fisheries in the adjacent coastal sea, Study Site is discussed by Mann and Lazier (2013). The Zambezi River meets the Indian Ocean An accurate description of the dispersion of on the Sofala Bank (Fig. 1). The connection plume waters as well as of the plume structure is made through a relatively large delta, is needed to elucidate these changes. characterised by weak navigability due River plumes are often used as indicators to obstruction by grass and reeds. The of the spread of materials transported by rivers course of the Zambezi River is marked by (e.g. nutrients, pollutants, sediments) to the a number of artificial lakes impounded by ocean. For instance, Brakel (1984) analyzed dams (Kariba and Cahora Bassa), which satellite images and found distinctive patches are mostly used for power generation rather of sediment dispersal in the Malindi Bay in than water retention during the wet season. Kenya, indicating a northward transport of Despite regulation of the river flow at the the largest sediment plumes, promoted by the dams, the historical flood seasonality in the prevailing southern monsoon. A typical pattern lower Zambezi remains undisturbed, with in a positively buoyant plume is one in which annual floods occurring during the rainy a layer of freshwater spreads over the ambient season. However, the current maximum waters. Within an estuary and near a river mouth, extent of the flooded area is reported to be transport in the primary direction is dominated less than half what it was before the dams by advection of the river’s momentum and were constructed (Scodanibbio & Mañez, depends largely on the volume, timing and 2005). According to Beilfuss and dos Santos intensity of the river discharge (Fong & Stacey, (2001), rainfall in the Zambezi river basin 2003; Nezlin et al., 2005). is strongly influenced by the movement of Despite numerous studies conducted along the Intertropical Convergence Zone, and in the Sofala Bank in recent years, as well as the the delta region, this movement translates proposed linkages between Zambezi River into a rainy season 4-6 months long between discharge and local secondary production, October and April. This region is also highly dispersal patterns of freshwater outflow from susceptible to torrential rain from tropical the Zambezi River have not been elucidated. cyclones and depressions that can occur However, a concise review of the coastal between November and April (Mavume features along the coast of Mozambique has et al., 2009). During the dry season, tidal been provided by Lutjeharms (2006), and influence is still noticed 80 km upriver, as includes evidence that the seaward intrusion of noted during field work in September 2009 freshwater from the Zambezi River can reach in this study. up to 50 km offshore, and is confined to a water depth of 15-30 m. The mean outflow of the Data sampling Zambezi is 3000 m3s-1 (Gammelsrød, 1992) and its freshwater discharge is believed to influence The Mozambican National Institute of not only the near-shore hydrodynamics and Fisheries Research (IIP) carried out an ecosystems, but also the offshore mesoscale oceanographic cruise once every year circulation. This is particularly noticeable when between 2004 and 2007 on the Sofala Bank freshwater runoff dominates the water masses using vessels of the semi-industrial shrimp within the continental shelf (Schumann, 1998; fishery fleet. Hydrographic measurements Sætre & da Silva, 1984). were collected at the stations illustrated in The Zambezi River plume is documented Figure 1 by deploying an enhanced Seabird in this paper, based on data published in CTD system with conductivity, temperature, a number of reports as well as in situ data depth, turbidity, dissolved oxygen and collected, to describe its patterns of dispersion. fluorescence sensors. The first measurement Morphology of the Zambezi River Plume on the Sofala Bank, Mozambique 3 Figure 1. Hydrographic transects and stations in the vicinity of the Zambezi River delta. was taken as close to the surface as possible Ancillary data at each station and subsequent readings were Freshwater