Contract Description:

This contracting period represents the fifth year of a second nutrient restoration pilot study on Dworshak Reservoir.  This project is a joint venture between the U.S. Army Corps of Engineers (USACE) and the Idaho Department of Fish and Game (IDFG).  The USACE will apply ammonium nitrate to the reservoir to promote an improved nitrogen to phosphorus ratio and increased food web efficiency.  The IDFG will be responsible for collecting limnological samples, which will be used for in-season management of nutrient supplementation and for evaluating project effectiveness.  The USACE has contracted Advanced Eco-Solutions to provide the nutrient prescription, interpret the results of limnological sampling, and recommend in-season adjustments to the amount of fertilizer to be applied.  Fish monitoring will also be conducted by the IDFG and, combined with limnological data, will allow for an overall assessment of project effectiveness.

This project was initially designed to be a five year pilot study to determine if nutrient restoration would work as a long-term management strategy for Dworshak Reservoir.  However, the original pilot study was interrupted by a legal challenge less than four years into the project.  Many of the benefits of nutrient restoration are believed to be cumulative, working up successive trophic levels each year.  Therefore, it was determined that an additional pilot study of five consecutive years should be conducted in order to properly assess the effects of nutrient restoration.  Furthermore, this scenario has unintentionally resulted in a unique opportunity to evaluate a nutrient restoration project using an interrupted time series study design.  From this, we can track changes to the limnological conditions and productivity of the reservoir in response to adding nutrients for a period of time, discontinuing the additions, and then continuing them again.

The first four years of nutrient restoration produced promising results.  Nitrogen additions resulted in immediate increases of picoplankton densities. By the second year, we observed increases in the proportion of edible plankton concurrent with decreases in the proportion of inedible phytoplankton.  Furthermore, the percentage of the phytoplankton community that was composed of toxigenic cyanobacteria (blue-green algae) was significantly less than non-restoration years by the second year of the project.  Zooplankton densities were also higher by the fourth year of the project than they had been in recent years.  Daphnia biomass and densities were higher during the restoration period than the non-restoration period.  Daphnia were also larger in three out of four restoration years than in pre-restoration years.  Kokanee were larger by the fourth year of the project than in a pre-restoration year of similar density and biomass was at least double that of recent years.  These results were achieved without violating water quality standards set forth in the Consent Order.

Both limnological and fish sampling during this contracting period will be similar to the last contracting period.  We will continue to collect creel data, but at a reduced level due to the available funding.  While we are beginning to get a good understanding of how each trophic level is responding, we need more data to determine how the fishery has benefited.