Biological communities vary naturally with elevation (mountains compared to plains) and corresponding changes in the physical habitat. Land use can affect biological communities directly by introduction of contaminants and indirectly by altering the physical habitat. Humans also can alter biological communities by stocking non-native fish. Evaluation of invertebrate and fish communities provides the opportunity to assess responses to changes in environmental conditions using organisms with different life spans. Collection of these data can provide some of the multiple lines of evidence needed to assess water-quality conditions.
An indicator of how land use can contribute contaminants to streams is the measurement of organochlorine pesticides and PCBs in bed sediment, and fish tissue at a site offers a clearer picture of the persistence of these compounds in the environment and their relation to land-use settings (Tate and Heiny, 1996). Although most of these compounds have been banned since the 1970's, they can persist in the environment for decades, which is important because of the multiple pathways for these contaminants to become part of the natural food chain. Assessing the effects of organochlorines on fish and wildlife is becoming increasingly important because many organochlorine pesticides and PCBs have been linked to hormone disruption and reproductive problems in aquatic invertebrates, fish, birds, and mammals (Colborn and others, 1993; Goodbred and others, 1997).
Organochlorine pesticides and PCBs in bed sediment and fish tissue were related to land use. Few organochlorine pesticides and PCBs were detected, and concentrations were low in bed sediment or fish tissue in forested or rangeland areas. Concentrations of organochlorine pesticides and PCBs in whole fish tissue did not exceed U.S. Food and Drug Administration (U.S. Food and Drug Administration, 1989) standards for human consumption of fish fillets. However, concentrations of several organochlorine pesticides and PCBs in whole fish exceeded National Academy of Sciences (NAS) and National Academy of Engineering (NAE) (1973) guidelines for the protection of aquatic life and wildlife. Concentrations of PCBs and chlordane exceeded guidelines primarily in urban areas, whereas total DDT exceeded guidelines in mixed and agricultural land-use areas (Tate and Heiny, 1996).
The status of biological integrity, habitat, and suspended-sediment concentrations in different land-use areas in the South Platte River Basin.
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The highest number of compounds and highest concentrations were detected in the urban and mixed (urban/agriculture) land-use areas.
The number and diversity of aquatic biological communities are directly related to the available stream habitat and concentrations of suspended sediment. A habitat degradation index (HDI) was developed by the NAWQA Program to indicate habitat conditions at a site (Gilliom and others, in press). An HDI score for a site was calculated by summing the ranks of three parameters; the amount of stream modification, bank erosion, and bank vegetative stability. Site scores were then grouped into three degradation classes; significantly degraded, moderately degraded, and minimally degraded. HDI scores varied throughout the basin and showed the effect of human activities on a site-by-site basis. Most forested sites in the mountain area had HDI scores indicating moderately degraded conditions with the exception of one site which was minimally degraded. Forested mountain sites had suspended-sediment concentrations indicating minimally degraded conditions, whereas, the mining-affected site had concentrations indicating moderately degraded conditions. Most plains sites had HDI scores and suspended-sediment concentrations indicating moderately degraded conditions; however, the rangeland site had HDI scores and suspended-sediment concentrations indicating minimally degraded conditions. Two sites in the agricultural area had HDI scores and suspended-sediment concentrations indicating significantly degraded conditions and only one urban site had suspended-sediment concentrations indicating significantly degraded conditions. These results indicate that human activities local to a site has a greater affect on habitat characteristics and suspended-sediment concentrations than basin-scale characteristics such as land use.
An index of biotic integrity (IBI) developed for fish in the South Platte River Basin (Schrader, 1989) can be used as a biological indicator of habitat and water-quality conditions. An IBI for a site was calculated by summing the ranks of three categories of fish-community characteristics; the number and types of fish, fish feeding habits, and fish abundance and health. The ranks were determined by comparing a fish-community characteristic at a site to those expected under natural conditions. The site scores were then grouped into three classes; significantly degraded, moderately degraded, and minimally degraded. IBI scores varied throughout the basin and showed the effect of human activities on a site-by-site basis. Forested sites in the mountain areas had IBI scores indicating moderately degraded conditions, whereas a mining site in the mountains had an IBI indicating a significantly degraded condition. Most plains sites had IBI scores indicating moderately degraded conditions; however, two sites in the plains were significantly degraded: one immediately downstream from Denver and one farther downstream in an agricultural area. Only two sites in the plains had IBI scores indicating minimally degraded conditions: the rangeland site and one agricultural site. These results indicate that local site characteristics had a greater effect on fish communities than basin-scale characteristics such as land use.
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Minnows are the dominant fish family present at the rangeland site and a majority of the agricultural sites in the plains areas of the basin.
The relative abundance of dominant families of fish was affected by natural environmental factors (elevation) and human activities (land use). Trout were present only in mountain streams; however, the relative abundance of trout was lower in areas affected by mining than in minimally affected forest settings. Suckers, which are tolerant of contaminants, were dominant in urban environments. Mixed (urban/agriculture) land-use settings had a similar composition of fish families to urban and agricultural settings but had smaller relative abundance for each family.
Invertebrate taxa also vary naturally with elevation and in response to human activities (land use). The decreased numbers of invertebrate taxa in mining, agricultural, urban, and mixed land-use areas compared to minimally affected areas (forest and rangeland) in the mountains and plains, showed the effect of human activity on the invertebrate communities. The decline in the number of taxa was not a result of natural variability (Tate and Heiny, 1995).
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The highest number of invertebrate taxa are present in the minimally degraded land-use (forest and rangeland) area.
The composition and health of biological communities are the integrated result of the physical and chemical environment at a site. Human activities associated with land use directly (introduction of chemicals) and indirectly (alteration of physical habitat) affect local invertebrate and fish communities. |