Institute of Marine Sciences

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    Rates and Causes of Coastal Erosion on the Zanzibar Island.
    (1991) Nyandwi, Ntahondi; Muzuka, Alfred N.
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    Land Reclamation as a Cause of Fine-Grained Sediment Depletion in Backbarrier Tidal Flats
    (1994) Flemming, Burghard W.; Nyandwi, Ntahondi
    Grain size distribution patterns along the East Frisian coast (German Wadden Sea) were evafuated with the aim of identifying potential effects of man-made structures (e.g. dikes) on the textural composition and sediment budget of the area. It was found that the distribution of various grain size fractions and settling velocities revealed a well-defined cross-shore energy gradient. Adjacent to the mainland dike the sediment is dominated by the 3.0–3.5 phi sand fraction (>70% by weight). In comparison to the sedimentary sequences normally observed in unobstructed backbarrier depositional environments, the sediments along the land-ward margin are in this case conspicuously depleted in grain sizes <3.5 phi, the corresponding size fractions contributing <20% by weight. This suggests that the dike interrupts the normal energy gradient, thereby inhibiting the deposition of sediments with settling velocities <0.5 cm s−1. This interpretation is confirmed when comparing individual cross-shore transects. The shorter the transect between the barrier island shore and the dike, the greater the fine sediment depletion. Since many faunal assemblages are grain-size specific, it is further suggested that the backbarrier ecosystem must have undergone significant modifications as a result of the substantial reduction in mud flats since the onset of dike construction some 1000 years BP. It is predicted that in the wake of the persisting and possibly accelerating sealevel rise this process of fine sediment depletion will continue and also affect progressively coarser sediments.
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    A Hydraulic Model for the Shore-Normal Energy Gradient in the East Frisian Wadden Sea
    (1995) Nyandwi, Ntahondi; Flemming, Burghard W.
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    The evolving western indian ocean coral reef programme
    (Int coral reef sym, 1997) salm, R; Mthiga, N; Muhando, Christopher A.
    The region has all reef types from atolls to fringing reefs with many endemic species shared within the Western Indian Ocean (WIO), which suggests that the reefs are linked by currents to make this a discrete biogeographic region. This also means there is a need for regional collaboration among the ten WIO states to manage these reefs. Reef management is not well developed in the WIO, and is focused at the site rather than at national or regional levels. Poorly regulated fisheries and coastal development, together with increasing populations and tourism are major contributors to reef destruction. This is ironic, as both fisheries and coastal tourism are heavily dependent on healthy coral reefs, and make major contributions to the economies of most countries. Many of the reefs in the region are showing distinct signs of damage from human activities, and bleaching in 1998 has been particularly severe in the Seychelles and Kenya. The principal reef management activity is the establishment.....
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    Hydrodynamics of Chwaka Bay, a Shallow Mangrove-Fringed Tropical Embayment, Tanzania
    (Elsevier, 2000) Nyandwi, Ntahondi; Mwaipopo, O. U.
    Time-series data of currents, sea levels and temperatures from Chwaka Bay,Zanzibar were analysed with the view of understanding the water circulation of the bay. The analyses show that there is a tidal asymmetry in the bay, with peak ebb tidal currents in the deep channels (45 cm s−1) being stronger than flood tidal currents (35 cm s−1), and ebb periods (7 hours) being longer than flood periods (5 hours). The velocity and time asymmetry as well as the asymmetry in the current direction are controlled by the morphological variations of the tidal basin. It was found that, as the water flows from the inner bay during the ebbing tide, it first drains towards the main tidal creek which leads to concentrated but delayed flows. The temperature variations in the inner part of the bay are predominantly diurnal, whereas at the mouth of the bay they are semi-diurnal. There is a general temperature gradient between the inner bay and the mouth, the highest temperatures being recorded in the inner bay (30.14°C). This indicates high residence times of the bay waters, presumably resulting from entrapment.
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    Mariculture
    (southwest Madagascar, 2000) Mmochi, Aviti
    Aquaculture is the cultivation of aquatic animals and plants in natural or controlled marine or freshwater environments, whereas mariculture is restricted to marine and estuarine (brackish) waters. On a worldwide scale, aquaculture is a fast-growing industry, showing a 12-fold increase between 1980 and 2010, or average annual growth of 8.8 per cent (FAO 2012). Recent statistics show that growth has slowed, with food fish annual production rates decreasing from 9.5 per cent (1990-2000) to 6.2 per cent (2000-2012) (FAO 2014). World production through aquaculture increased to 90.4 million tonnes in 2012 (66.6 million tonnes food fish and 23.8 million tonnes aquatic plants; FAO 2014) with about 50 per cent of production originating from mariculture (Figure 22.1). This is in stark contrast with the global levelling off in harvests from capture fisheries, at about 80 million tonnes per year since the mid 1990’s (FAO 2014).
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    Mineralogical and Biogenic Composition of the Zanzibar Channel Sediments, Tanzania
    (Estuarine, Coastal and Shelf Science, 2000) Shaghude, Yohanna W.; Wannas, K. O.
    Sediments from the Zanzibar Channel have been analysed for mineralogical and biogenic composition. The main objective of the study was to describe the depositional environment of the sediments based on these parameters. The study was also conducted to give an insight into sediment provenance of the sediments. Quartz, feldspar (both microcline and plagioclase) and hornblende are the main mineralogical components, while zircon, muscovite, biotite and augite occur as accessory minerals. The quartz grains are generally angular to sub-angular and highly strained, indicative of immature sediments, probably derived from highly metamorphosed rocks. The presence of hornblende also supports the conclusion that the sediments are immature. A possible source is the metamorphic rocks of the Mozambique belt, located about 25–50 km from the coastline. Benthic foraminifera are the dominant biogenic constituents. Other common biogenic constituents are molluscs (pelecypods and gastropods) and corals. Bryozoans and ostracods are relatively less common, while sea urchins are accessory biogenic components. The composition and abundance of these biogenic components and morphological, sedimentological and mineralogical data permit construction of a simple model which distinguishes between three bio-physiographic settings in the Zanzibar Channel using depth contours: (1) the coastal zone (0–10 m depth); (2) the reef platform/patch reefs zone (10–20 m depth) and (3) the central channel zone (>20 cm depth). In the coastal zone, the sediments are dominated by fine sand, sized with a very prominent fine sand mode. The proportion of benthic foraminifera is lower than in the other two zones (<45%) as is the biogenic:quartz ratio (<2), but the lithogenic concentration is highest (>25%). Two lithogenic mineral components (quartz and feldspar) occur in significant proportions. In the reef platform/patch reefs zone, the sediments are coarse to medium sand sized with no distinct mode. the proportion of benthic foraminifera is highest (80–85%) there, and so is the biogenic:quartz ratio (>10) but that of lithogenic components is lowest (<5%). In the central channel zone, the sediments are medium to fine sand sized and often bimodal, with a prominent fine mode and a less prominent coarse mode. The proportion of benthic foraminifera (70–75%) is also higher than in the coastal zone but slightly lower than in the reef zone. The proportion of lithogenic components (5–20%) is higher than in the reef zone but lower than in the coastal zone and the biogenic:quartz ratios is intermediate.