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

We quantified production dynamics, determined the trophic basis of production, and assessed the overlap in resource utilization within a diverse assemblage of mayfly larvae from the submerged woody (snag) habitat 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) of channel bottom) in two consecutive years. Relatively little change occurred between years for four out of six families. Biomass turnover rates (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 occurred: (1) concentration in winter (Ephemerella argo, E. dorothea, Eurylophella sp., and Isonychia), (2) concentration in summer (Baetis ephippiatus, Tricorythodes sp., Caenis spp.), or (3) spread throughout the year and peaking in summer (Baetis intercalaris, Stenonema modestum, S. integrum, S. exiguum, and Hexagenia sp.). Temporal overlap of production, using the proportional similarity index, was highest among closely related taxa (>0.60), with the least overlap (<0.15) among taxa in different families. Overlap based upon the amount that various food types contribute to production was high among all species, suggesting even less resource partitioning for food than for time. Of the. annual food consumption (as dry mass) by mayflies per unit snag surface area (423 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 production. We suggest that a fluctuating habitat (snags), a continuously replenished food supply, high biomass turnover, and high drift densities help perpetuate a persistent, resilient, and diverse mayfly assemblage with little resource partitioning.