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

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 <513C endmember of the marine organic matter and the proportion of terrigenous material to the total organic matter flux. Export ratios of organic carbon were low, likely because of solubilisation within the traps, while export ratios of biogenic silica were high. Sediment-trap fluxes at the mouth of Saanich Inlet were strongly affected by a sediment plume that extended off the nearby sill. However, compared to the other stations, this plume did not result in excess sedimentary accumulation of biogenic silica and organic carbon relative to local primary production. At each station, similar proportions of local primary production (~5%) were buried in the sediments below, suggesting that the bulk of the marine organic matter was not preferentially preserved in the intensely anoxic sediments of Saanich Inlet. The possibility of organic matter solubilisation within the sediment traps, and the excessive water-column fluxes at the mouth of Saanich Inlet, confuse comparison of organic carbon fluxes in Saanich and Jervis Inlets. However, away from the mouth of Saanich Inlet water-column fluxes of biogenic silica were proportional to local primary production. If the biogenic silica carried proportional amounts of organic matter, then the heightened primary production in Saanich Inlet resulted in a large delivery of organic matter to depth. Combined with the high primary production and export flux, low rates of vertical mixing and particle-entrapment within the fjord, factors associated with the weak estuarine circulation and weak winds of Saanich Inlet, may have also stimulated anoxia. Although in Jervis Inlet there is more stagnant water behind the sill and deepwater renewals were less frequent than in Saanich Inlet, the deep sill allows oxidation of a significant fraction of the sinking organic matter before the stagnant waters are reached, reducing the chances of oxygen depletion in the bottom waters. A model that estimates rates of water-column decay from sediment-trap data showing increases in flux with depth was used with the time series from Saanich and Jervis Inlets. Model results from Saanich Inlet were not conclusive, possibly because the depth interval between sediment traps was too small to resolve water-column rates of decay. However, the model fit well to the time series from Jervis Inlet, and rate constants for organic carbon and nitrogen agree well with previous estimates made from oceanic settings. The model has also allowed some of the first estimates of depth-dependent dissolution rates of sinking biogenic silica, and translation to time-dependent dissolution using a nominal sinking rate suggests the diatomaceous opal in Jervis Inlet was dissolving rapidly. Changes with depth of rate constants for organic carbon, nitrogen and biogenic silica are well described by the power function, suggesting that organic matter and biogenic silica are composed of a set of multiple components that decay at varying rates. This model for decay has been explained for organic matter, and for the biogenic silica may be caused by the presence of various diatom species or degrees of frustule fragmentation that result in a number of fractions with different dissolution rates. The model has also allowed a description of the material that causes increases in flux with depth. This sediment was depleted of organic carbon and nitrogen and thus appeared diagenetically altered, and its aluminosilicate and biogenic silica contents were characteristic of hydrodynamically sorted resuspended material. Additional material was delivered to the deepest sediment traps during deepwater renewals, but a continual process such as tidal resuspension, particle focusing, or increases in trapping efficiency with depth resulted in additional fluxes to the mid-depth sediment traps.

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