Dispersal of Zebra Mussels, Dreissena polymorpha, Downstream of an Invaded Reservoir
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Zebra mussels have recently invaded Central Texas and more information is needed to predict their spread in this region and inform management decisions. Therefore, I examined riverine zebra mussel dispersal, settlement, and growth downstream of Lake Belton, TX, invaded by zebra mussels in 2013. Veliger samples and settlement of juveniles on artificial substrata was monitored at a site in the lake and six sites in the Leon and Little Rivers, 0.4 to 54.7 river kilometers (rkm) downstream of the lake outlet. Veliger density declined with distance downstream with the greatest densities recorded at sites closest to the lake outlet (0.4 and 2.5 rkm). Veligers were found up to 13 rkm downstream. This decline was represented best with a logarithmic decline in May, Aug, Oct 2015 (R2 = 0.75 to 0.94), and with an inverse power relationship in June and September 2015 (R2 = 0.53 to 0.73). No clear pattern was found in April 2016 (R2=0.32, p = 0.06). In contrast, maximum juvenile settlement (437 ± 75 m-2) occurred 2.5 rkm downstream in August 2016, but not immediately downstream of the lake. Differences in settlement rates between sites could not be explained by differences in physico-chemical parameters such as temperature or turbidity as they did not differ significantly between sites. No mussels were found at 27 and 55 rkm downstream on artificial or natural substrata between May through December of 2015, but juvenile mussels were found there in April 2016. This suggests that zebra mussels were dispersal limited in 2015, and were able to disperse further downstream in 2016 probably facilitated by high discharge from Lake Belton.