PRODUCTION DYNAMICS AND RESOURCE UTILIZATION OF SNAG-DWELLING MAYFLIES IN A BLACKWATER RIVER

We quantified production dynamics, determined the trophic basis of pro duction, and assessed the overlap in resource utilization within a div erse assemblage of mayfly larvae from the submerged woody (snag) habit at of a Coastal Plain blackwater river (Ogeechee River). Total annual production (as dry mass) was among the highest reported for mayflies, ranging from 20.8 to 42.2 g/m(2) of snag surface (7.4 to 12.3 g/m(2) o f channel bottom) in two consecutive years. Relatively little change o ccurred between years for four out of six families. Biomass turnover r ates (annual production/biomass) were generally high, ranging from 8 ( single generation per year) to 96 (multiple generations per year) for individual taxa. Distinct types of temporal production patterns occurr ed: (1) concentration in winter (Ephemerella argo, E. dorothea, Eurylo phella sp., and Isonychia), (2) concentration in summer (Baetis ephipp iatus, Tricorythodes sp., Caenis spp.), or (3) spread throughout the y ear and peaking in summer (Baetis intercalaris, Stenonema modestum, S. integrum, S. exiguum, and Hexagenia sp.). Temporal overlap of product ion, using the proportional similarity index, was highest among closel y related taxa (>0.60), with the least overlap (<0.15) among taxa in d ifferent families. Overlap based upon the amount that various food typ es contribute to production was high among all species, suggesting eve n less resource partitioning for food than for time. Of the. annual fo od consumption (as dry mass) by mayflies per unit snag surface area (4 23 g.m(-2).yr(-1)), approximate to 87% consisted of amorphous detritus that is rapidly replenished on snags from floodplain-derived seston. This food source accounted for approximate to 70% of total mayfly prod uction. We suggest that a fluctuating habitat (snags), a continuously replenished food supply, high biomass turnover, and high drift densiti es help perpetuate a persistent, resilient, and diverse mayfly assembl age with little resource partitioning.