Research - Category II: Aquatic Ecosystems

Effects of Fine Sediment Storage on Food Web Structure and Juvenile Salmonid Rearing in North Coast California Rivers
(Funded 2000-2001)

Principal Investigator: Mary Power
Integrative Biology
UC Berkeley
(510) 643-7776
mepower@socrates.berkeley.edu

Executive Summary:
Problem Statement: Excessive loading of fine sediments into salmon rearing habitat is one of the most serious environmental problems affecting Northern Coastal California rivers. Deleterious sediment loading, due in part to past and ongoing timber harvests and road construction, is exacerbated by the steep dissected terrain and weak parent materials of these drainages. The influence of fine sediment content on salmonid eggs and larvae in redds has been well studied. There are fewer data, however, that relate the sediment composition of river beds to growth and survivorship of juvenile salmonids, or the structure and dynamics of food webs supporting them. Redds and juvenile rearing habitats are often in different positions in drainage networks. Reviews of the assessment of Cumulative Watershed Effects, and recent assessment of a number of northern California rivers by the Environmental Protection Agency point to our need to understand the site specific impacts of land use in a whole watershed context. Field Research is needed to link quantitatively juvenile salmonid rearing performance to sediment composition of river beds, to contribute to efforts to set reasonable targets for watershed management and channel rehabilitation in this region.

Summary of Research Approach: Our proposed study combines a field survey of salmonid habitat use with a study of juvenile salmonid performance under experimentally manipulated bed conditions. In the field experiment, we will rear 2-3 juvenile steelhead in stream channels with mixed cobble pebble beds representative of ambient coarse sediments characteristic of juvenile rearing habitats in pool-riffle transitions in the upper Southern Fork Eel River. Local sand will be added to enclosure beds to produce five levels of embeddedness, ranging from clean (no sand) to enough sand so that larger cobbles at the bed surface are 75% embedded. These conditions will bracket the fine sediment loading in the ambient river bed. Over the summer rearing season, we will document changes in the abundance and composition of algae, detritus, and invertebrates, components of the food webs that potentially support salmonid grown. River bed texture is expected to affect salmonid energetics directly by providing different quantities and qualities of feeding, resting, and hiding habitat. Bed texture will also influence juvenile salmonids through the food web. Access to downstream, and the growth of algae and accumulation of detritus which support invertebrate production, should all vary with bed composition. We will examine these food web effects, as well as the influence of upstream conditions (bed texture, channel productivity, riffle length) on invertebrate drift into our experimental channels. Our field survey (focusing on the upper South Fork Eel River and its tributaries) will relate habitat characteristics to abundances and size structure of local salmonid assemblages, as well as the abundance and composition of other vertebrates and of lower tropic levels in food webs (detritus, algae, invertebrates). The habitat characteristics we quantify would include watershed position (slope, aspect, drainage area), bed composition (pebble counts and small scale maps of sediment facies) and infiltration rates of water measured with a standardized permeability standpipe. WE would also document other habitat features (depth, velocity, canopy over, temperature regimes) affecting fish. We would carry out surveys early (May) and late (August) in the summer, to document large scale movements and changes in size and distributions and densities of fish over the summer rearing period.

Statement of results, benefits, and information expected: We anticipate that performance of juvenile salmonids in our experimental channels should provide useful quantitative information on how their feeding, space use, and growth are affected by bed sediment composition. Surveys would give larger scale information documenting ontogenetic changes in salmonid use of habitats with different sediment composition. This information should be useful to managers in developing targets for sediment loading and bed compositions of salmon rearing habitat in streams throughout coastal Northern California, where timber harvest and road building activities are being increasingly scrutinized for their impacts on fine sediment in salmon-bearing channels (EPA 1996, 1997).