A comparison of the effects of regulated and non-regulated hydrologic regimes on fine sediment deposition and benthic macroinvertebrate distributions Min L. Choy Abstract This study examines how fine sediments deposited in streams in the Santa Clara Valley of California affect the density and composition of benthic macroinvertebrates commonly used as water quality bioindicators. The focus is on the three insect orders Epheroptera, Plecoptera and Trichoptera (EPT) because they are considered generally intolerant of stream pollution. Excess fine sediments (<2mm) alter the substrate composition by filling in interstitial spaces and coating surfaces, thereby changing habitat suitability for aquatic organisms including salmonids and macroinvertebrates. Dams may affect fine sediment levels in streams because regulated flows have less capacity and competence to move fine sediment. The present study on six streams addresses whether regulated flow and non-regulated flow sites have different amounts of fine sediment and if EPT metrics differ among the two flow regimes. Five collections were made at each site using a Hess sampler with a 500 µm mesh net. A nested analysis of variance showed regulated flow sites to have significantly (p< 0.05) higher amounts of fine sediment. EPT richness was also significantly different at regulated flow sites. EPT richness and density were negatively correlated with the amount of fine sediment present. This polygonal distribution shows that fine sediment is a limiting factor in EPT richness and density. In addition, due to the difficulty of measuring fine bed sediments, the study examined whether turbidimeters can substitute as a quick field method for estimating fine bed sediment. Results show a curvilinear relationship between fine bed sediment and turbidity (r = 0.77, p<0.01).Introduction Rivers play a paramount role in shaping the landscape by selectively eroding, transporting and depositing sediments on the land in their journey towards the ocean (Lemly 1982). These sediments range from fine clays up to large boulders. Fine sediments are a category of sediments consisting of fine sand (<2000 to >62 µm), silts (<62 to >4 µm), and clays (<4 µm). Fine sediments occur naturally in streams but are considered a pollutant when they are in excess of natural levels. Sediments are considered one of the top stream pollutants by the U.S. Environmental Protection Agency (EPA 2004). Excess fine sediments are primarily human-induced and can result from disturbances such as dams, surface mines and construction activities (Wood and Armitage 1997) and land use relating to agriculture, forestry, and residential development. Dams strongly influence sediment transport because they often modify the natural discharge regime (Poff et al. 1997). Dams disrupt the often natural highly variable flow regime, replacing it with a less variable low discharge (Poff and Ward 1989, Mount 1996, Ligon et al. 1995). Reduced peak flows hinder sediment transport capacity and competence (Wilcock et al. 1996). As a result, reaches downstream of dams can accumulate fine sediment deposits without natural scouring. Fine sediment introduced from downstream tributaries may also accumulate on the bed if reservoir storage has sufficiently reduced the river’s transport capacity (Kondolf and Wilcock 1996). Bankfull discharge with a recurrence interval of 1.5 yr is often considered the channel forming flow (Dunne and Leopold 1978). Due to flow regulation, bankfull discharge may seldom occur. Resulting fine sediment deposition may change channel morphology and the physical aquatic habitat with deleterious effects on macroinvertebrates, algae, macrophytes, and fish (Wood and Armitage 1997). Macroinvertebrate health is very important to the aquatic ecosystem because these ubiquitous organisms provide a vital food source for many fish, birds, and insects. Reductions in macroinvertebrate densities can negatively influence fish populations (Wood and Armitage 1997). The addition of fine sediment to a stream alters the substrate composition by filling in interstitial spaces and coating surfaces which affects substrate suitability for aquatic organisms (Culp and Davies 1985, Erman and Ligon 1988, Wilcock et al. 1996). Fine sediments can 2increase insect drift, deposit on respiratory structures, and reduce dissolved oxygen availability (Culp et al. 1985, Brittain and Eikeland 1988, Lemly 1982, Wood and Armitage 1997). The effects of fine sediments on macroinvertebrates in Mediterranean climate streams have not been extensively studied. Hubert et al (1996) studied macroinvertebrate density and substrate relationships in a small, high plains stream in Montana and indicated changes to the macroinvertebrate assemblage from an increase in fine sediment. Erman and Ligon (1988) conducted a study below a water-filtration facility in the San Francisco Bay area, but it was an atypical system in which the pulses of sediment addition to the stream were very high for brief periods of time, whereas my study involved fine sediment deposits that resulted from nearly constant regulated flow year around. Studies of dam-associated fine sediments and macroinvertebrates in the Mediterranean climate region would contribute to better understanding of the impacts of dams. In addition to the deleterious effects of excess fine sediments on macroinvertebrates, other ecological impacts result from fine sediment deposition and distribution. Fine sediment is an issue of importance in assessing potential habitat for threatened and endangered fish such as steelhead trout and coho salmon on the Pacific Coast. Fine sediments can greatly impact fish spawning grounds by filling in the interstitial spaces of the substrate, which reduces the availability of dissolved oxygen to incubating eggs (Kondolf and Wilcock 1996). Fine sediments can also be important in the transport and fate of many contaminants. In the Bay area, mercury is likely transported by fine sediments from abandoned mercury mining sites in South San Jose to the Bay. Many studies support the widely accepted view that as grain size decreases, the concentration of sediment-associated contaminants increases (Old et al. 2003). These are only a few reasons to study the distribution of fine sediments in local south Bay streams. Nationwide, sediments are recognized as a severe problem, yet no national program exists to study this
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