Larval habitat associations of Progomphus obscurus at two spatial scales (Odonata: Gomphidae)


Progomphus obscurus is one of the most abundant dragonflies in South Carolina, USA. We collected dragonfly larvae from 127 sites in the Enoree River and nine of its tributaries, and correlated the abundance of P. obscurus larvae with physical and chemical characteristics of these streams. As expected for this burrowing species, larval abundance varied among streams and was significantly correlated with mean silica concentrations and the proportion of sandy-bottom sites in these streams. We also examined habitat associations on a smaller spatial scale. We sampled P. obscurus larvae by kick-seine from fine sand, coarse sand, and pebble sediment types in five sites in the Enoree River basin. Larvae were collected, preserved in 75% EtOH, and their body lengths were measured. Sediment samples were collected and Ro-tapped, and mean particle size was calculated. Larvae were present in a greater fraction of the ‘fine sand’ samples than ‘pebble’ samples. In addition, abundance was inversely correlated with mean particle size. Mean larvae size was weakly correlated with mean particle size, and inversely correlated with larval abundance. This species is associated with sandy-bottom streams and fine sandy sediments within streams. Changes in sediment characteristics resulting from channelization, increased flooding, and increased stream flow could change sediment composition and impact the abundance of this common dragonfly species.

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