Trophic relationships of benthic fauna at the Oman Margin: the impact of mass jelly falls.

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The Arabian Sea is one of the most biologically productive areas of the world's oceans. It is also characterised by an oxygen minimum zone (OMZ); OMZ's are areas of water that have very low oxygen concentrations. Our study area is the Oman continental margin, here the OMZ impinges on the sediment community. This has implications for the fauna living here. Where oxygen concentrations are low, the faunal community will be characterised by soft bodied animals that are able to withstand these conditions e.g. polychaete worms and not molluscs or crustaceans. The Oman margin is subject to two monsoon seasons; these lead to mixing of the water column which brings nutrients to the surface waters feeding microscopic plants, which when they sink and die form a food source for the sediment faunal community. Additionally under conditions of low oxygen concentrations, bacteria can use sulphur to fix carbon for energy. This is known as chemosynthesis and these bacteria also provide a potential food source for the sediment community. The bacteria can be free living, or can form mats on the sediment surface for the fauna to graze on. The balance between food sources and oxygen concentrations is likely to be critical to the benthos and any perturbation may lead to a shift in community structure. We intend to investigate the food sources available to the sediment community and to understand the trophic linkages that exist within the sediment community. We believe that where oxygen concentrations are low, bacterial food sources may play an important role in trophic dynamics and where oxygen is high the dominant food source will be the microscopic plants. We have a unique opportunity to study the impact of enhanced hypoxia on sedimentary communities, as the result of a huge fall of decaying jellyfish across a large area of the Oman Margin, within and below the OMZ which led to the development of chemosynthesis and a potential food source for the benthic animals. We will compare the benthic community structure with that at the Pakistan Margin, where we have an existing data set for an OMZ where samples were collected at a similar time, but where there was falls of jelly fish were absent.