Factors Structuring Fish Assemblages Along a Bog Lake Successional Gradient

Three alternate and distinct fish assemblages were identified in 43 northern Wisconsin lakes encompassing a gradient from bog ponds to small, oligotrophic lakes having little bog character. Multivariate analysis of physical/chemical variables identified three general environmental factors that influence community type and species richness. These factors are: (1) habitat size and heterogeneity; (2) lake productivity— pH; and (3) winter oxygen concentrations. Ordination of lake fish assemblages along these environmental gradients revealed that two factors, lake pH and winter anoxia, were especially crucial in determining assemblage type. Maintenance of these discrete assemblages is also the result of biotic interactions (exclusion of cyprinids due to predation or competition from centrarchids). A centrarchid assemblage dominated lakes across a wide range of habitat types and pH, providing winter anoxia does not occur. Lakes with low winter oxygen concentrations harbor a cyprinid assemblage if the pH is above 5.2—5.4. Lakes that are acidic and also have low winter oxygen contain only two physiologically hardy species comprising the Umbra—Perca assemblage: central mudminnow (Umbra limi) and yellow perch (Perca flavescens). Five fishless lakes had physical and chemical characteristics similar to lakes with an Umbra—Perca assemblage. Within the fish assemblages, no subgroups of closely associated species were evident. For Umbra—Perca lakes, few environmental differences were evident between lakes having (1) no fish, (2) central mudminnows only or, (3) yellow perch and central mudminnows. The sequential loss of species along a gradient from oligotrophic seepage lakes to bog ponds reflected the increased harshness of the chemical environment and the loss of near—shore habitat zones. Species that are specialized zooplanktivores, that utilize hard—bottom substrates for spawning for feeding activities, or that are intolerant of low pH were limited to lakes with little bog character. Lakes in intermediate stages of succession contain acid—tolerant species that are food and habitat generalists. Lakes near the final stages of senescence contained only yellow perch and central mudminnows. The location of bog lakes at the lower end of several environmental gradients makes them valuable sites for studying the influence of environmental factors and biotic interactions on the structure of aquatic communities.