Determination of the Effect of
Re-Suspended Shoreline Materials on Primary Productivity using Large Enclosures, Dworshak
Reservoir, Idaho
by R.E. Wierenga
M.S. Thesis. Environmental Science and Regional Planning.
Dworshak Reservoir is a deep, cold-water
reservoir on the North Fork of the Clearwater River in northern Idaho. The reservoir
extends 86.8 km into the Clearwater and Bitterroot mountains, draining nearly 6,320 km2 of
a primarily forested watershed. A regionally significant fishery is maintained in the
reservoir, consisting of primarily kokanee, rainbow trout, and smallmouth bass. Dworshak
Reservoirıs limnology is characteristic of a system of low productivity: relatively few
available nutrients; high, stable dissolved oxygen levels; and low algal biomass.
Recent declines in the Northwestıs salmon and steelhead populations have forced changes in the management of the regionıs water resources. A large volume of water was released from Dworshak Reservoir during the summers of 1993-1996 for flow augmentation in the Lower Snake and Columbia Rivers. These late-summer releases have affected the in-reservoir limnology and aquatic organism interactions. Areas of concern include shoreline instability, and organism entrainment in the outflow.
This thesis research focused on the effect of shoreline erosion on in-reservoir primary productivity in the Elk Creek Tributary of the reservoir. Shoreline sediment was added to large enclosures and the response in primary productivity was monitored using a modified 14C light-dark bottle method. The effects of the sediment additions on nutrient levels, light penetration, physical parameters (temperature, dissolved oxygen, etc.), and algal biomass were also assessed.
Regression analysis showed that a low level
sediment treatment (25% reduction in Secchi depth) had no significant effect (p<<0.05) on the algal growth rate in the enclosures. The heavier sediment treatment (50% Secchi depth reduction) significantly reduced primary productivity (p="0.08)" relative to the controls. Correlation between productivity and inorganic nitrogen was significant in the enclosures and in the reservoir, supporting conclusions from earlier studies that the reservoir is mainly nutrient limited. Light penetration may have influenced the primary productivity only when greatly reduce, a result of phytoplanktonıs ability to adapt to short term disturbances.
<<
0.05) on the algal growth rate in the enclosures. The heavier sediment treatment
(50% Secchi depth reduction) significantly reduced primary productivity
(p="0.08") relative to the controls. Correlation between productivity and
inorganic nitrogen was significant in the enclosures and in th ereservoir, supporting
conclusions from earlier studies that the reservoir is mainly nutrient limited.
Light penetration may have influenced the primary productivity only when greatly reduced,
a result of phytoplankton's ability to adapt to short term disturbances.