Ecotonal shifts in diversity and functional traits in zoobenthic communities of karst springs

We examined the potential importance of distance from the spring source (reach), season, and physicochemical variability on zoobenthic biodiversity, biomass, and trait-based metrics [functional feeding groups (FFG) and habit traits] in three karst spring systems in the Ozarks region of Missouri, USA. Density and biomass were measured during summer and winter in each spring source and three downstream reaches of each springbrook. Nearly 20,000 invertebrates in total were collected, identified, and counted. We measured biomass directly or by length–weight relationships. Overall downstream and seasonal patterns of diversity, biomass, and traits were similar among springs, but species dominance and composition varied significantly. Taxonomic richness increased downstream in all springs, but evenness decreased. Amphipods and isopods were the most abundant taxa at the spring source but were progressively replaced downstream by insects (with dominance varying among springs and seasons). Density and biomass for flatworms, snails, and insects increased downstream, while crustacean numbers diminished along this ecotone. FFG increased from 3 at the spring source (mostly shredders) to 4–7 downstream where scrapers, collectors, and/or predators predominated. Physicochemical factors proved minor at best as mechanistic explanations for ecotonal changes. We conclude that structural and functional community changes could be attributed to a matrix of interrelated factors including organic substrate, food availability, and biotic interactions.