Climate variability and size-structured interactions among juvenile fish along a lake-stream gradient

We examine the influence of biological interactions and interannual variation in climate an the size structure and trophic interactions between age-0 largemouth bass (Micropterus salmoides) and age-0 creek chubs (Semotilus atromaculatus) along a lake-stream gradient in northern Minnesota, USA. Considerable year-to-year variation in temperature and precipitation occurred during the 10-yr study (1983-1992), with most extreme hut-dry conditions in 1988 during a 3-yr drought. Decreased stream temperature and reduced variation in temperature: occurred downstream from the lake outlet because of increased groundwater inputs. Age-0 bass were captured in the stream in each of the eight years sampled, but bass abundance peaked in downstream reaches in 1988, concurrent with a large increase in the downstream density of age-0 chubs. Median length ratio of bass to chubs increased greatly in the 1988 drought year. The shift in relative length of bass and chubs during the drought was because chub length decreased in downstream areas, and bass shifted from feeding an invertebrates to feeding on chubs in downstream but not in upstream reaches. Comparison of bass and chub growth in the presence and absence of interspecific competition for invertebrate resources in an experimental stream revealed higher growth rates for chubs than for bass and little interspecific competition. Experiments in the artificial stream indicated that the presence or absence of chubs susceptible to bass predation had only marginal effects on bass survival, but growth rates of bass were substantially higher if chubs susceptible to predation were present. Only when chubs were present was the growth of individual bass dependent strongly on size, and this led to rapid divergence in the size distribution of bass in chubs vs. no-chubs treatments. Our results indicate that hot-dry climatic conditions associated with drought have potentially strong effects on size-structured trophic interactions between age-0 fish along lake-stream gradients. Hot-dry climatic conditions alter the spatial distribution of lake fish along temperature gradients in the stream, the reproductive success and synchrony of lake and stream fish development, and the eventual growth of predatory lake species. The response of fish to climatic variation appears to be strongly influenced by their position in the "hydrologic landscape." The most dramatic fish response occurred in stream reaches influenced by groundwater.