Influence of environmental conditions on biogeochemical processes in the Arabian Sea : oxygen and benthos

This thesis is concerned primarily with the relationship between oxygen concentrations and benthic fauna on organic matter (OM) cycling at the Pakistan Margin (PM) in the Arabian Sea. The approach involved the analysis of lipids and stable carbon and nitrogen isotopes insediments and benthic fauna. Five sites (140, 300, 940, 1200 and 1850 m) were chosen; above, in the core and below the oxygen minimum zone (OMZ), which was defined by oxygen concentrations being < 0.5 mL Lot at the PM. Samples were collected on two pairs of cruises in order to assess temporal variability in OM and the response of the benthic fauna to the biogenic flux during the presumed low flux the inter-.monsoon period (IMP: MarchMay 2003) and the high flux late-monsoon period (LMP: August-October 2003). Lipid distributions of sediments were highly complex and predominantly marine in origin. Little temporal or spatial variability was noted, suggesting that oxygen is not the • primary controlling factor in lipid biogeochemistry and therefore in surficial sediment OM quality. In order to assess the nutritional quality of the sedimentary OM at the PM a food quality index (FQI) was derived using the lipid data set. The FQI of surficial sediments at the PM was compared to other previously published data sets for particulate OM (PaM). This led to the conclusion that PM surficial sediments had a relatively high nutritional value for the benthic fauna. There was little difference in the FQI of sinking paM collected from the Oman Margin at 500 and 950 m, suggesting that this reflected reworking ofOM in the surface waters of the Arabian Sea. Stable isotope data of surficial sediments supported lipid data, displaying little temporal or spatial variability. Stable isotopic compositions of benthic macrofauna displayed no temporal variability except in the case of one polychaete, Linopherus sp. Temporal variability was noted in 5 species of benthic megafauna suggesting changes in feeding mode and possibly selective ingestion/assimilation of certain components of OM. Benthic fauna generally became enriched in t3C and t5N downslope; application of the trophic level model demonstrated that feeding guilds changed downslope in response to oxygen concentrations, with more surface deposit feeding taxa in the OMZ and more predatory/scavenging taxa below the OMZ. Distributions of lipids in benthic fauna were complex, but provided insight into food sources for the benthic fauna. Strong temporal variability in lipid distributions of certain species e.g. Astropecten sp., Amphiura sp. and a natant decapod suggested that these fauna spawned in concurrence with the biogenic flux. Lipid distributions of the polychaete Linop/zerus sp. displayed spatial variability across the lower OMZ boundary (800 to 940 m). This may be a result of spawning, or a response to oxygen concentrations, as juveniles appeared to inhabit the more hypoxic regions in order to build up lipid reserves.EThOS - Electronic Theses Online ServiceGBUnited Kingdo