Life Histories and Abundance Patterns of Snag-Dwelling Mayflies in a Blackwater Coastal Plain River

Abstract

A diverse mayfly community (28 species) was found in the Ogeechee River, a blackwater river in the Georgia Coastal Plain. Quantitative samples of snag habitat, where most species (22) were found, were collected at 2-4 wk intervals in 1982 to investigate life history characteristics, and at monthly intervals in 1983 to determine consistency of seasonal and annual abundance patterns. Baetidae and Heptageniidae were abundant throughout each year with average densities of 1553-5981 / m² (1982-1983) and 1498-1828 / m² of snag surface, respectively. Field data for the most common species of Baetidae (Baetis ephippiatus, B. intercalaris) and Heptageniidae (Stenonema exiguum, S. integrum, S. modestum) showed no recognizable pattern of cohort development. This developmental asynchrony and long emergence periods for S. exiguum and S. modestum are consistent with previously published growth studies indicating multivoltinism. Ephemerellidae (551-719/m²), Oligoneuriidae (206-469/m²), Tricorythidae (721-1599/m²) and Caenidae (114-110/m²) were also commonly found on snags, but were seasonal in abundance. The major species of Ephemerellidae (Ephemerella argo, E. dorothea, Eurylophella sp.) showed distinct single cohorts developing primarily during winter/spring. Isonychia spp. (Oligoneuriidae) also showed a well-defined winter cohort, but had as many as two weakly defined cohorts during warm months. Caenis spp. (Caenidae) and Tricorythodes sp. (Tricorythidae) occurred primarily in summer months, during which time they were able to pass through multiple generations, as substantiated by either long emergence periods or independent growth studies. Major differences in life histories occurred between rather than within families. Relative abundance and seasonality at the family level were very consistent between years. The high degree of life history overlap in this diverse mayfly assemblage raises questions about the relationship between phenology and community structure, as well as latitudinal patterns of species richness.