Stave River Water Use Plan - BC Hydro

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Stave River Water Use Plan Monitoring Terms of Reference June 13, 2005 1. Pelagic Monitor (Nutrient Load/Total Carbon Levels) 1.0 Program Rationale 1.1 Background During the Water Use Planning (WUP) process, several difficulties were encountered when trying to assess the impacts of facility operations on the overall productivity of Stave and Hayward reservoirs. These difficulties stemmed from the paucity of productivity related information specific to these reservoirs and the lack of resources needed to fill these data gaps. Rather than abandon this component of the WUP trade off process, an evaluation procedure was developed based on surrogate performance measures was developed using general models of lake ecosystem function, general knowledge of ecosystem impacts arising from impoundment practices, published data from other reservoirs through out North America, and the little reservoirspecific data that was available. The result was an impact assessment model that divided the productivity of Stave and Hayward reservoirs into pelagic (open water) and littoral (near shore) components and reported overall productivity in each reservoir in terms of the rate of total annual carbon assimilation (Failing 1999). Though the use of this paradigm allowed the WUP to proceed to a successful conclusion, it was generally acknowledged among CC members that it was rather a simplistic view of reservoir ecology and hence, fraught with uncertainty. Four key elements of uncertainty were identified, two of where are the subject of the present monitor. The first uncertainty is in the assumption that pelagic productivity would remain unaffected by changes in reservoir operations, at least within the range of operations being investigated. This assumption arose because of insufficient information to indicate otherwise and was deemed to be consistent with what is generally known about pelagic ecosystems. Accepting this assumption simplified the interpretation of the carbon assimilation estimates in that noted changes could be directly attributed to changes in littoral productivity. This however, may not be the case if the assumption of ‘pelagic immunity’ is found to be invalid. The other key uncertainty is the method by which total carbon assimilation was calculated and the underlying assumption that it would serve as a reasonable indicator of fish production potential. Annual carbon assimilation rate was calculated from a simple, linear regression equation developed from lake data collected throughout BC (J Stockner Pers comm.). The data set did not include storage reservoirs and was primarily directed towards measures of pelagic productivity. Therefore, its application to a reservoir setting was considered to be suspect, including its use as an overall indicator of reservoir productivity. Also contributing to the uncertainty is the large error associated with the predictions made with this equation (Failing 1999). Finally, it was generally acknowledged that the assumed link between carbon production and fish production potential was a rather tenuous one and fraught with uncertainty. Its adoption into the BC Hydro Page 6
Stave River Water Use Plan Monitoring Terms of Reference June 13, 2005 decision making process was driven primarily by the absence of any other kind of production-based information. In recognition of these uncertainties, the CC recommended that they be addressed by a comprehensive monitoring program design to improve the decision making process in future WUPs. Consultative committee acceptance of Combo 6 as the preferred operating alternative for the Stave Lake generation facility was conditional on the design and implementation of such a monitoring program (Failing 1999). The monitor presented here is focused on the pelagic environment. The other two uncertainties alluded to above, are the subject of a separate monitor geared towards studies in the littoral zone. It address uncertainties associated with the use of the Effective Littoral Zone performance measure to capture changes in littoral productivity and the examines the importance of littoral productivity relative to that of pelagic waters. Because the direct measurement of total annual production (i.e., direct measure of flora and fauna growth) is beyond the budgetary scope of this monitoring program, the CC accepted the use of primary productivity (the annual production of phytoplankton) as an alternative index measure. As a result, the monitor described here is focused primarily on this trophic level of production. 1.2 Management Questions The consultative committee identified four key management questions pertaining to the pelagic productivity of Stave and Hayward reservoirs: a) What is the current level of pelagic productivity in each reservoir, and how does it vary seasonally and annually as a result of climatic, physical and biological processes, including the effect of reservoir fluctuation? This information is required to identify the key determinants that currently govern/constrain the level of productivity in each reservoir. Once these environmental factors have been identified, an assessment can be carried out to determine whether they are susceptible to change given alternative reservoir management strategies. Environmental factors that are susceptible to change are then monitored through time in conjunction with the productivity indicator variable (in this case primary productivity). This information sets up the foundation for the next management question. b) If changes in pelagic productivity are detected through time, can they be attributed to changes in reservoir operations as stipulated in the WUP, or are they the result of change to some other environmental factor? This information allows one to clearly determine whether a causal link between reservoir operations and reservoir pelagic productivity exists, and if so, to describe its nature for use in future WUP processes. BC Hydro Page 7
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Stave River Water Use Plan - BC Hydro

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