Fine-scale distribution of carbohydrates on intertidal sediments in relation to diatom biomass and sediment properties
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Extracellular polymeric substances (EPS) are produced by epipelic diatoms as a function of their locomotive mechanism and also protect cells from desiccation and heavy metal toxicity. EPS are carbohydrate-rich and form an important carbon source for heterotrophic microorganisms. In addition, the polymeric structure of EPS results in sediment inter-grain binding, thereby increasing the resistance of sediments to erosion forces. Despite the importance of generic carbohydrates (measured as an index of EPS), there is little information on their spatial distribution or factors influencing their abundance. In this study, a fine-scale sectioning technique was developed and provided the first depth profiles of sediment carbohydrates at a scale relevant to microphytobenthos. The operational separation of sediment carbohydrates into bulk and colloidal fractions was examined and fractions were shown to vary in spatial and temporal characteristics. Colloidal carbohydrates were concentrated in the surface 200 mum layer of intertidal mud flats and therefore influenced sediment interface processes such as diffusion and sediment erosion. Colloidal carbohydrates were positively correlated to sediment chlorophyll a concentrations and also varied with tidal height and with sediment topography. During the emersion period, colloidal carbohydrate concentrations increase significantly, however, bulk carbohydrate concentrations remain unchanged. Biochemical analysis of the carbohydrate fractions showed marked, but not statistically proven, differences in the proportion of monosaccharides present and suggested these operational fractions may arise from different sources and are subject to varying turnover rates. Bulk sediment carbohydrates increase in concentration with sediment depth in the upper millimetres of intertidal sediments and were positively correlated to increases in sediment density. The increase in density with depth reflects post- depositional compaction and has important implications for models which predict sediment erosion based on sediment density. These findings are discussed in relation to contemporary thinking on sediment processes.
Thesis, PhD Doctor of Philosophy
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