A flow through centrifuge is a large volume centrifuge that separates solids from liquids that are pumped into its separation chamber. This can be configured with pumps and hoses to pump water and sediment from rivers and lakes to separate and isolate suspended sediment. As many analytical methods require large amounts of sediment the use of a continuous flow centrifuge aids the environmental evaluation of sediment transported nutrients and contaminants. The purchase of a flow through centrifuge with our FSWP funds allowed us to collect suspended sediment from the outflow plume of the Horsefly River for the analysis of marine derived nutrients (MDN) during 2011.
We are interested in determining the amount of MDN that originates from the spawning and die-off of salmon that is subsequently flushed out of the river. The timing of this movement of nutrients is important to the functioning of the lake as it is a source of nutrients for primary production, the bottom of the food chain which supports the food web in Quesnel Lake. 2011 was our first attempt at this sampling approach and we collaborated with DFO who installed instrumentation in the bay to track the movement of the plume and some aspects of its water quality. Our efforts in this preliminary year were rewarded in that we timing our sampling perfectly to capture the snowmelt freshet plume and had all our instrumentation working for the full open water period.
The most significant finding to date is that the signal of marine-derived nutrients is apparent at all of the times we sampled. While we expected to see the salmon nutrient signal being moved downriver during the fall spawning period, as this is the time that most researchers believe the transfers occur, we noted a significant amount of MDN was associated with the sediment that was flushed from river bed storage in the spring melt. This means the MDN is being stored in the river overwinter and potentially being incorporated into the food chain between the September spawn thorugh to the May springmelt. Another point of interest is that we are identifying MDN in the water column during July sampling which occurred in a low flow period for the Horsefly River. This suggests that the nutrients are either recirculating in the lake water column or dectectable amounts are moving downstream in low flows, likely associated with in-river primary production.