Spring-fed habitats are found in river systems ubiquitously, and generally exhibit stable thermal regimes. Also, these habitats formed under geology with low water permeability show not only stable thermal regimes, but also stable flow regimes, owing to the slow groundwater discharge. These unique characteristics of spring-fed habitats increase the heterogeneity of river environments, thereby stream animals show contrasting community structures in between spring-fed and runoff habitats and immigrations into spring-fed habitats in specific seasons. This presentation introduces some ecosystem functions of spring-fed habitats in driving the spatiotemporal dynamics of stream animal communities observed from a clastic lowland river system in northern Japan.

First, a comparison of habitat and macroinvertebrate assemblage structures among lowland spring-fed and runoff streams revealed greater cover of fine sediments and higher density of burrowing detritivores (mainly Chironomidae and Oligochaeta) in the spring-fed tributary than in the runoff streams. Because the flow regime of the spring-fed tributary was highly stable, settlements of fine sediments and detritus might cause the high abundance of burrowing detritivores through provision of attractive habitats.

Second, seasonal field investigations indicated that population density of juvenile masu salmon was always higher in the spring-fed tributary than in the neighboring runoff tributary, and peaked in summer and winter. In these seasons, the water temperatures in neighboring runoff streams were severely high or low, and thus the spring-fed tributary might function as a thermal refuge for the salmonids. Moreover, biomass of macroinvertebrates was significantly higher in the spring-fed tributary throughout the season, probably because of the high streambed stability. The abundant macroinvertebrates might increase the carrying capacity regarding food resources for the carnivorous fishes. The aggregation of juvenile salmonids, observed here, suggests that spring-fed streams can provide both thermal refuges and foraging habitats for mobile fishes.

Other spring-fed habitats such as groundwater discharges occurred from bars also exhibited contrasting water temperatures compared to runoff reaches during cold and hot weathers. These spots found in lower reaches from bars are known to be spawning habitats for chum salmon, and may also contribute to provide thermal refuges for fishes. Such temporal aggregations of fishes in spring-fed habitats may further interact with uniquely formed macroinvertebrate communities in such habitats through interspecific interactions. Integrating the geological and ecological processes in interactions among spring-fed and runoff habitats is thus of important foci for understanding the mechanisms on sustaining species diversity in river ecosystems.