Influence of fine particulate matter composition on the distribution of macroinvertebrates in a Sierra Foothills watershed

Anthropogenic land use transformations impact watershed hydrology and increase fine sediment delivery to stream systems. These impacts are known to alter lotic macroinvertebrate assemblages, however much less is known about the mechanisms causing these changes. Fine sediment composition was investigated as a causal factor influencing macroinvertebrate distribution in the upper Fresno River watershed. Fine sediment composition was characterized as the total amount (mg/cm2), size distribution, and organic matter content of particles from 1.5 to 250 microns in the upper streambed substrate. Macroinvertebrate assemblages differed among 7 sites spaced throughout 169 stream kilometers. Six quantitative substrate variables: Dmean, Pareto c, specific surface area, sediment dry weight, % FPOM, and pebble count were used to construct fine sediment specific tolerance values for 21 common macroinvertebrate genera, identifying taxa that may be sensitive to development in the watershed. Sampling sites were scored using macroinvertebrate tolerance values based on water quality from literature and newly generated fine sediment tolerance values from the results. Fine sediment composition scores distinguished sites considerably more than water quality scores providing evidence that fine sediment composition was more important than water quality for assessing the biological difference between sites. Results demonstrate further links between fine sediment composition and macroinvertebrate assemblages and suggest a mechanism for how watershed development may change measures of ecological integrity.