SUMMARY
The purchase of a continuous flow-through centrifuge will allow us to sample the outflow plume of the Horsefly River to collect bulk samples of suspended sediment for the analysis of marine derived nutrients (MDN). We are interested in determining the amount of MDN flushed out of the river in spring and fall 2011 and to track the fate of these nutrients delivered to the nursery lake (Quesnel Lake) which supports the sockeye fry.
DESCRIPTION
Background and Project Rationale: The return of salmon spawners from the ocean to interior lakes and streams forms an important part of the nutrient cycle within interior watersheds. In particular, Fraser River Sockeye salmon (Oncorhynchus nerka) return massive amounts of nutrients far from their oceanic source to contribute substantially to inland ecosystems. Fraser River Sockeye follow four year cyclical patterns of abundance with one high return year followed by three years of lower returns, the reasons for which remain unclear . In the low return years, the number of returning adult salmon confers a direct size and survival benefit to the next generation of juvenile salmon. In a high return year, however, when there are a large number of spawners, the relationship between the number of returning adults and the size and survival of the next generation of juvenile salmon is less direct . The cyclical pattern of salmon abundance is not in phase across lakes throughout the Fraser River basin, suggesting this limitation is driven by rearing lake productivity rather than any ocean limitation.
In interior systems rearing lake productivity may be partially driven by the flux of decayed salmon products from the spawning river to the lake and subsequent lake mixing and uptake processes. For example, in Quesnel Lake the characteristics of the inflowing tributary rivers tend to determine the spatial patterns of lake mixing. The Horsefly River plume water tends to spread down the lake as surface flow while the Mitchell River discharge, derived mostly from high elevation snowmelt, will plunge down in the lake water column to deeper waters . In 2010 sockeye salmon returns to the Fraser River were extreme, reaching 100-year record high values. The 2010 returns to the Horsefly River (120,000 spawners) exceeded the expected high return year of 2009 and therefore provide an excellent and time-sensitive opportunity to evaluate the in-channel, overwinter storage of salmon decay products and their delivery and mixing in Quesnel Lake. Recent work in salmon spawning streams has identified the link between salmon spawning activity and gravelbed nutrient storage . At higher densities of spawners like in 2010, abundant salmon decay products and re-suspended fine sediment increase the potential for the river to act as a gravelbed nutrient sink; fewer nutrients, therefore, are transported downstream to the nursery lake, limiting productivity and potentially restricting size and survival benefits to juvenile salmon.
OBJECTIVES
To purchase the flow through centrifuge and prepare it for use from a boat
Sediment samples to be sent to analytical labs for isotope analysis
DFO to install moorings in Horsefly Bay for 5 months
METHODS
Methodological Approach: To examine the behaviour of the Horsefly River plume two moorings will be installed, one at the mouth of the Horsefly River and one deeper into Quesnel Lake (Figure 1; bay/river). Each mooring will contain a series self-recording thermistors to profile the temperature characteristics of the plume as well as a fluorometer chain to characterize dissolved organic matter (DOM). Moorings will continuously record this information from April until September. On the dates of deployment and retrieval a series of transects will be performed to differentiate the water column into lake and plume water (using temperature, conductivity over depth (CTD) and distance). The flow characteristics of the plume will be characterized by a downward looking 1.2 MHz acoustic doppler current profiler affixed to a buoyant drogue during the spring and fall periods. CTD casts will also be done to profile and sample the Horsefly Plume. All of this equipment will be provided, installed and retrieved by DFO collaborators. Data analyses will occur in the fall of 2011.
To examine the sediment load of the Horsefly River plume, a continuous flow centrifuge will be used to concentrate suspended sediment from the water column. This will occur at a single station on four dates in May 2011 in order to capture the period of the rising limb of the spring-melt hydrograph (historical hydrological data indicates this occurs in May). This period is important as the high water flows will flush the river channels of materials stored from the fall which over-winters. A suspended sediment transect at five stations along the river plume will occur samples near the peak of the Horsefly hydrograph will be used to determine spatial differences. At the same time as continuous flow centrifuging is occurring the size structure of the suspended sediment will be determined using a LISST particle size analyser4. The collected sediment will subsequently be analyzed for a marine nitrogen isotope (15N) to examine the magnitude of salmon inputs4 to the lake and for 13C to determine the proportions of other sources of organic matter delivered to the lake in the spring melt. Concentrations of total carbon, nitrogen and phosphorus associated with the sediment will also be analysed in a laboratory. This second component of the project will be conducted by UNBC researchers working out of the Quesnel River Research Centre (QRRC).
The continuous flow centrifuge is a critical aspect of this project in that it is a field based instrument (in this case on a boat) which draws water from the plume into a large centrifuge bowl that spins to separate the solids from the liquids. The outflow water is drained back into the lake. The system is run for several hours (dependant on the sediment concentrations in the water column) to collect a bulk sample of sediment. This bulk sample can then be used for a range of analytical tests. The ability to obtain large samples for isotope analysis is the important aspect for this project as we want to determine what the strength of the signal is and the variability over the period of high river flow events. This instrument can be used riverside (mounted on a trailer or truck bed) to collect sediment as well as from a boat, but for the 2011 initial year we will work in the bay from a 22’ boat. Please see attached quote from US Centrifuge.
Chemical Analysis of Sediment: Salmon accumulate 95% of their biomass at sea and thus when they enter freshwater systems, are the only significant source of heavy marine isotopes. In particular isotopic, forms of nitrogen and carbon (δ15N & δ13C) can be used to trace marine-derived nutrients bound to sediment. Further analysis of total concentrations of C Nand P, will characterize the nutrient condition of the sediment. The analysis of 55 samples taken from the five centrifuging trips will link the experiment to salmon influence and are thus crucial for this study to provide information on rearing lake productivity. Analysis of isotope and total nutrients will be done at the U of Ottawa, C.C. Hatch Isotope Lab for a cost of $35/sample (UNBC does not have this analytical capability as yet).
BENEFITS
Relevance: Fraser River Sockeye salmon are experiencing an unprecedented level of volatility. Very low numbers in 2009 were followed by extremely high numbers in 2010. This wildly fluctuating pattern is indicative of instability during some portion of the salmon life cycle. It is important to determine what structures the ability of juvenile salmon to take advantage of previous salmon abundance and nutrient delivery during the freshwater phase of their life cycle. Nutrient uptake in lakes is determined by complex and often non-linear interactions between rivers and thermodynamic fluxes interacting over many spatial and temporal scales. Profiling and sampling the Horsefly River plume over these temporal and spatial scales should provide preliminary information on nutrient delivery and distribution in a year with expected high values. The opportunity to do this in the 2011 spring-melt following a large salmon return year will provide us with valuable information to plan future sampling strategies and grant proposals.
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