Project Title: The Significance of Suspended Organic Sediments to Turbidity and Fish-Feeding Behavior

Investigators: Mary Ann Madej, USGS Western Ecological Research Center Margaret Wilzbach, USGS California Cooperative Fish Research Unit Arcata, CA USA 707-825-5189 FAX: 707-822-8411 mary_ann_madej@usgs.gov

Partners: Redwood National and State Parks Arcata, CA USA 707-464-6101

Many of California’s coastal streams exhibit high turbidity during winter flows. This study investigates the role of suspended sediment particle size and organic content in influencing turbidity and fish-feeding behavior.

PROJECT DESCRIPTION

Salmon need clear water to see their prey, yet suspended organic sediments in the water column provide a food source to these same macroinvertebrates. A common concern in coastal streams is that water clarity is becoming increasingly impaired by land uses such as grazing, mining, timber harvest and road construction. Mary Ann Madej and Margaret Wilzbach of the USGS, with Kenneth Cummins, Samantha Hadden and Colleen Ellis of Humboldt State University, are investigating the interactions among suspended sediments, turbidity, and salmon in north coastal California streams and estuaries.

Both suspended organic matter and inorganic sediment, which is added to the water column from erosion of the surrounding hills, contribute to turbidity in streams and estuaries. Because organic sediments remain in suspension longer than do similarly sized inorganic components, they tend to reduce light in the water column during the recessional limb of hydrographs. Decreased light may lead to decreased primary production, and in turn a loss of invertebrates that feed by scraping algae from rocks and logs. Concurrently, an increased proportion of organic suspended sediments would benefit filter-feeding invertebrates.

Both scrapers and filtering collectors are important components in the diets of salmon and other drift-feeding fishes, but the net effect of organic:inorganic ratios on prey availability for fish is not known. Apart from indirect effects on fish through their food base, the effect of an increased percentage of suspended organic sediments on light attenuation would also directly impact fish through loss of visual capability, leading to reduced feeding efficiency, feeding rate, and depressed growth. Thus, our objective is to establish the relative importance of size-specific, inorganic vs. organic components of the suspended load in influencing turbidity, sediment flux, and stream health, as reflected in the feeding efficiency of juvenile salmon and their invertebrate food base.

PROGRESS

The first phase of the project concentrated on four streams draining redwood-dominated watersheds. An existing stream gaging program with suspended sediment sampling was supplemented by more intensive biological sampling. Parameters assessed include turbidity, fluorescence (as an index of chlorophyll-a), microbial respiration (indexed by measurement of dissolved oxygen in the field), and abundance of macroinvertebrate functional groups collected with D-frame net samples of cobble and large wood. Foraging efficiency of juvenile salmonids was estimated in field feeding trials, using experimental feeding apparatus and underwater visual observations. Fish condition was estimated from length, mass, and age determinations of individuals collected from minnow traps.

Organics played an important role in turbidity readings up to about 60 NTU’s (turbidity units), on both rising and falling limbs of the hydrograph (Figure 1 a and b). At higher turbidity levels and peak discharges, the sediment samples were more influenced by inorganic material. Early season storms tend to have a higher percentage of organic material in the sediment samples. These data are important because many studies use thresholds of 25 to 30 NTU’s as having an effect on fish-feeding behavior. Our results show that in four streams the organic fraction of the sediment load is high at low to moderate turbidities, and should be specifically included in analyses of suspended sediment concentrations.

Preliminary results show that biomass of invertebrate prey sampled from the foreguts of juvenile coho salmon and steelhead trout declines with increasing turbidity. A decline in prey capture rate of juvenile salmonids with increasing turbidity was also observed in the field. These data are unique in that other studies of salmon feeding efficiency in response to turbidity or suspended sediment concentrations have all been flume- rather than field-based. Although the efficiency of prey capture is decreased at higher turbidities, limited fish feeding activity was still observed at the highest turbidities (45 NTU’s) in which underwater observations were made.

So far we have found that the contribution of organic particles to the suspended load differs among streams, and within streams, on a seasonal and within-storm basis. Organic particles in the suspended load contribute measurably to turbidities in the low to moderate ranges of NTU’s, on both the leading and trailing edge of the hydrograph. The leading and trailing edges of the hydrograph are likely the most biologically relevant – even if this may not be the most relevant for estimation of sediment yields from a basin. Methodologies for analyzing suspended sediment concentrations and organic content at very low levels have not been developed, necessitating collection of much larger samples under low flow conditions. Our field studies show that efficiency of fish feeding is affected by turbidities, but that some limited feeding goes on even at moderate turbidities. Links between suspended sediment composition with the invertebrate prey base, and how the dynamics of suspended organic sediment change in the estuarine environment, have yet to be determined.

Figure 1: Suspended organic sediment is less dense than inorganic sediment, so it represents a greater percentage of particulate matter for a given sediment concentration than inorganic matter. The high number of organic particles in a water column can influence turbidity levels in streams.

PRODUCTS

Madej, M. A., M. A. Wilzbach, K. W. Cummins, S. J. Hadden, and C. C. Ellis. 2003. Composition of Suspended Load as a Measure of Stream Health. Report for California Department of Forestry and Fire Protection Contract 1.22-1757. 60 p.

Madej, M. A. 2004. How suspended organic sediment affects turbidity and fish feeding behavior. Sound Waves. Vol. FY2005 No. 67:7-8.

Madej, M.A. 2005. The role of organic matter in sediment budgets in forested terrain. In Sediment Budgets II. Proceedings of the Seventh IAHS Scientific Assembly at Foz do Iguacu, Brazil. IAHS Publ. 292.

Keywords: Turbidity, seston, suspended sediment, particulate organic matter, benthic macroinvertebrates, fish-feeding behavior