1. We investigated the seasonal variation of biological traits and the influence of interannual rainfall variability on this pattern. Using long-term survey data (6–19 years) from an intermittent and a perennial stream in the Mediterranean-climate region of northern California, we examined 16 fuzzy-coded biological traits (e.g. maximum size, life cycle duration, and mode of respiration).
2. Seasonal habitat variability is higher in the intermittent stream than in the perennial stream. During the winter and spring wet-season both streams flood; however, during the summer dry-season, the intermittent stream forms isolated pools in (occasionally drying completely).
3. Seasonal habitat variability influenced both taxonomic and biological trait composition. Distinct taxonomic communities were present in each season, particularly in the intermittent stream. The intermittent stream also exhibited more seasonal variation in biological traits than the perennial stream.
4. Despite statistically significant seasonal variation, trait composition was relatively stable among seasons in comparison with taxonomic composition and abundance. Taxonomic composition varied considerably between seasons, because of high seasonal and interannual replacement of taxa resulting from seasonal habitat changes.
5. The seasonality of taxonomic composition and abundance was sensitive to interannual rainfall variability. In dry years, the taxonomic composition of communities was more similar between seasons than in wet years, while trait composition was relatively insensitive to rainfall variability.
6. Despite high seasonal variation in abundance and taxonomic composition, biological traits of aquatic macroinvertebrates varied less and exhibited seasonal stability, which may be a result of the unpredictability and harshness of stream environments.