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Since it was introduced to Zanzibar (Tanzania), seaweed farming has significantly contributed to local, socio-economic development. However, several investigations have shown impacts on the coastal environment near where the farms are located. As many seaweed farms are located on seagrass beds, there is a risk that seaweed farming could affect seagrass beds, and thereby disturb important ecosystem functions and the flow of ecological goods and services. This study compares characteristics of macrophytes (focusing on seagrasses), benthic macrofauna and sediment in seagrass beds, with and without seaweed farms, and a sand bank without vegetation in Chwaka Bay, Zanzibar. The results showed that seagrass beds underneath seaweed farms generally had less seagrass and macroalgae, finer sediment, lower sediment organic matter content and a reduced abundance and biomass of macrofauna, than seagrass beds without seaweed farms. Further, the macrofaunal community structure in seaweed farms showed more similarities to that on the sand bank than in the unfarmed seagrass beds. Most of the dissimilarity was attributable to Lucinidae (suspension-feeding bivalves), which were almost absent in the seaweed farms, resulting in the large difference in biomass between the seaweed farms and the unfarmed seagrass beds. When interpreted together with information from farmers, the observed pattern is believed to be caused by the seaweed farming activities. This indicates that more research is needed to establish the effects of seaweed farming on seagrass beds, and that more attention should be given to the location of farms and the choice of farming methods.

A three-dimensional coupled hydrodynamic-sediment transport model for the Texas–Louisiana continental shelf was developed using the Regional Ocean Modeling System (ROMS) and used to represent fluvial sediment transport and deposition for the year 1993. The model included water and sediment discharge from the Mississippi River and Atchafalaya Bay, seabed resuspension, and suspended transport by currents. Input wave properties were provided by the Simulating WAves Nearshore (SWAN) model so that ROMS could estimate wave-driven bed stresses, critical to shallow-water sediment suspension. The model used temporally variable but spatially uniform winds, spatially variable seabed grain size distributions, and six sediment tracers from rivers and seabed.

At the end of the year 1993, much of the modeled fluvial sediment accumulation was localized with deposition focused near sediment sources. Mississippi sediment remained within 20–40 km of the Mississippi Delta. Most Atchafalaya sediment remained landward of the 10-m isobath in the inner-most shelf south of Atchafalaya Bay. Atchafalaya sediment displayed an elongated westward dispersal pattern toward the Chenier Plain, reflecting the importance of wave resuspension and perennially westward depth-averaged currents in the shallow waters (o10 m). Due to relatively high settling velocities assumed for sediment from the Mississippi River as well as the shallowness of the shelf south of Atchafalaya Bay, most sediment traveled only a short distance before initial deposition. Little fluvial sediment could be transported into the vicinity of the ‘‘Dead Zone’’ (low-oxygen area) within a seasonal–annual timeframe. Near the Mississippi Delta and Atchafalaya Bay, alongshore sediment-transport fluxes always exceeded cross-shore fluxes. Estimated cumulative sediment fluxes next to Atchafalaya Bay were episodic and ‘‘stepwise-like’’ compared to the relatively gradual transport around the Mississippi Delta. During a large storm in March 1993, strong winds helped vertically mix the water column over the entire shelf (up to 100-m isobath), and wave shear stress dominated total bed stress. During fair-weather conditions in May 1993, however, the freshwater plumes spread onto a stratified water column, and combined wave–current shear stress only exceeded the threshold for suspending sediment in the inner-most part of the shelf.

Resources survey of algae and seagrasses in 63 estuaries and backwaters existing from Madras to Athankarai in Tamil Nadu and Pondichery was made during 1988-89. Among these water bodies, only 44 supported vegetation. Totally 36 species of algae belonging to 23 genera under the groups Chlorophyta, Phaeoph~ta,R. hodophyta and Cyanophyta, and 5 species of seagrasses belonging to 3genera were recorded from these estuaries. The agar yielding seaweeds Gracilaria arcuata and G. verrucosa and carrageenan yielding seaweed Hypnea valentiae occurred in harvestable quantities in some estuaries.

The present paper deals with the distribution of seaweeds and seagrasses during the deep sea survey conducted in the first sector from Kattapadu to Tiruchendur in Tamil Nadu coast between December 1986 and March 1987 covering an area of 650 sq.km. In this survey 58 species of marine algae were recorded of which 7 belong to Chlorophyta 12 to Phaeophyta and 39 to Rhodophyta. Three species of seagrasses viz. Cymodocea serrulata. Halophila ovails and H. ovala were also recorded at the depths ranging from 5.5 to 21.5 III Halim eda macroloba, Dictyota barlayresiana, D. Maxima, Gracliaria corticata var. corticala, G. edulis, Sarcodia indica, Sarconema filiform e, Soliena robusta, flypnea esperi and H. Valentiae were found to be dominant and widely distributed.