Year-to-Year Variation in the Aquatic Macroinvertebrate Fauna of a Northern California Stream

The benthic macroinvertebrate community of a third-order coastal stream in northern California was examined in mid May (end of the wet season) and late August (near the end of the dry season in the prevailing Mediterranean climate of the region) over a 7-yr period in which there was substantial year-to-year variability in precipitation and, consequently, stream discharge. Ephemeroptera, Trichoptera, and Diptera were the dominant components of the macrobenthic community, accounting for 93% of total individuals and 62% of the 81 taxa collected in mid May, and 96% of total individuals and 64% of the 69 taxa collected in late August. In mid May, significant reductions in species richness and Simpson's diversity were observed during a year of extreme drought and in years with above-average wet season rainfall. Macroinvertebrate density decreased and relative abundance of Chironomidae increased as wet season rainfall increased. In late August, year-to-year variability in community parameters measured was substantially reduced. Drought conditions favored proliferation of a few tolerant taxa (e.g., the caddisfly Gumaga nigricula). An understanding of the influence of abiotic conditions on biotic patterns can be useful in separating the effects of perturbation from natural variability.

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