Seasonal analysis of species diversity and functional group organization of aquatic invertebrates in two coastal streams

Seasonal patterns of species diversity and functional organization of benthic invertebrates were studied for a one year period in two coastal streams, Lost Man Creek and Prairie Creek, from March 1986 through March 1987. Invertebrates in Lost Man Creek were most abundant in summer months and least abundant in spring. Invertebrates in Prairie Creek were most abundant in autumn and least abundant in spring. Species diversity values did not change significantly between seasons in both streams. Diversity values for Lost Man Creek samples were significantly lower throughout the year compared to Prairie Creek samples, which may reflect the effects of logging activities in the Lost Man Creek watershed over 20 years earlier. Seasonal changes in functional group had both expected and unexpected results. As predicted by the River Continuum Concept, shredders were most abundant during autumn and least abundant during the spring in both streams. Contrary to the River Continuum Concept, scraper abundance in both streams was lowest in winter. Predators were most abundant during summer in both streams. Collectors exhibited different patterns of seasonal abundance within the two streams. Collector abundance in Lost Man Creek peaked dramatically in summer, then declined throughout the remainder of the study. Prairie Creek collectors exhibited a gradual increase in numbers from spring through autumn and then declined through winter. The unexpected results in the functional organization of stream invertebrates in this study may be due to one or more of the following factors: coastal climate; life history patterns of stream invertebrates; quantity and quality of food resources; past logging activities in the Lost Man Creek watershed; local geology; functional group categorization; and nonrandom sampling. Future study needs in coastal streams are discussed.