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Production and consumption of organic carbon and oxygen in Sechelt Inlet, British Columbia Timothy, David Andrew


In an effort to better understand the relationships of hydrodynamic, biological and chem ical processes in British Columbia fjords, multi-disciplinary oceanographic data were col lected from Sechelt Inlet between the end of January and the end of June, 1991. The purpose of this thesis is to provide depth dependent rates of oxygen production and consumption. Oxygen dynamics are inferred from changes in organic carbon content. Autotrophic ‘4C uptake is normalized to estimates of the daily average irradiance to which phytoplankton had been exposed before being collected for incubation. The normalized 1C4 assimilation values are used to estimate daily rates of oxygen production in the euphotic zone and they are correlated with chlorophyll a concentration. The relationship between ‘4C uptake and chla is not directly used for estimates of oxygen production during the five month study period, but may be applied where chla but not photosynthetic carbon assimilation data exist. Oxygen consumption in the water column below the euphotic zone and in the sedi ments is estimated from analyses of sediment trap data collected monthly at three sta tions. Water column oxygen consumption is determined using an algorithm which esti mates the rate of decay with depth of the organic carbon flux, despite measured increases in flux with depth. The algorithm also provides estimates of the composition of the ma terial caught in lower but not upper sediment traps. Following a discussion of possible causes of measured flux increases with depth, it is concluded that changes in trapping efficiency were largely responsible for the pattern of flux observed in Sechelt Inlet during the experiment. Benthic oxygen demand is estimated by degrading a constant fraction of the trap-measured flux of organic carbon to the sediments. The estimates of sediment oxygen demand are found to agree with direct measurements of benthic oxygen demand in other temperate fjords. This work is concluded by comparing predicted and trap measured fluxes of organic carbon, where predicted fluxes are estimated from primary production and an estimate of the f-ratio (= new production/total production) during the experiment. A method is described in which fNQ- (= nitrate uptake/total nitrogen uptake) can be used to pre dict the export flux of organic carbon during ecologically and hydrographically dynamic periods. A large discrepancy is found between predicted and measured fluxes of organic carbon during the study period in Sechelt Inlet. It is suggested that this discrepancy was caused by dissolved and/or fine organic matter that was not caught by the sediment traps and was eventually consumed in the water column, probably by free-living, motile bacteria.

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