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Primary production and the settling flux in two fjords of British Columbia, Canada Timothy, David Andrew

Abstract

A time series of primary production and sediment trap flux measurements was carried out in two fjords of British Columbia, Canada between 1983 and 1989. The fjords, periodically anoxic Saanich Inlet and oxygen-replete Jervis Inlet, were chosen in order to compare organic matter formation and particle flux in these environments with largely differing redox conditions. Two sediment-trap moorings were deployed in each fjord, and each mooring had sediment traps at three depths. The moorings were serviced monthly, when primary production was also measured using the ¹⁴C-uptake technique. Hydrographic and nutrient data were collected during portions of the experiment, and ²¹⁰Pb profiling of bottom sediments allowed comparison of water-column fluxes and sedimentary accumulation rates. Saanich Inlet (490 g C m⁻² y⁻¹ ) was 1.7 times more productive than Jervis Inlet (290 g C m⁻² y⁻¹ ) and primary production toward the mouths of both fjords was 1.4 times higher than at the heads of the fjords. The elevated rates of primary production in Saanich Inlet were probably due to exchange with the nutrient-rich surface waters of the passages leading to the Pacific Ocean, and the up-inlet gradients in both fjords reflected the relative nutrient supply. The sediment-trap material was dominated by biogenic silica, especially in the spring and early summer but also in the late summer and fall, while organic carbon fluxes tended to peak in the summer. While winter fluxes were usually dominated by aluminosilicates, at the mouth of Jervis Inlet organic matter often comprised most of the mass flux to the 50 m sediment traps, as wintertime sources of biogenic silica and aluminosilicates were small. At the head of Saanich Inlet, the aluminosilicate flux closely followed the pattern of local rainfall and flow from the Cowichan River, a distinct difference from the other stations where turbulent resuspension from topographic boundaries and particle focusing appear to have dominated the lithogenic flux. δ¹³C of the trapped material was heavier in the summer than in the winter, reflecting a higher ratio of marine to terrestrial organic matter at that time. The relationship between stable carbon isotope ratios and BSi content revealed that 70-80% of the marine OC in these fjords is diatomaceous. This relationship was furthermore used to estimate the

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