Steichen, J.L.; Denby, A.; Windham, R.; Brinkmeyer, R., and Quigg, A., 2015. A tale of two ports: Dinoflagellate and diatom communities found in the high ship traffic region of Galveston Bay, Texas (USA).Ballast water (BW) discharge by shipping vessels is a known transport vector of harmful species of dinoflagellates and diatoms. With a steady growth in global commerce, ship traffic to ports worldwide has intensified, increasing the risk of invasion by nonindigenous species. From 2008–12, >140 million metric tons of BW was discharged into Galveston Bay, Texas, much more than reported in other highly invaded Bays: San Francisco (96 × 106 mt) and Chesapeake (25 × 106 mt) during the same period. Studies conducted specifically on the dinoflagellate and diatom communities within Galveston Bay have been lacking until the present effort, which used both microscopic and genetic methods. Within one year of sampling, 35 genera of dinoflagellates and diatoms were identified from the two deepwater ports of Houston and Galveston. Seven of the genera are known toxin producers, three of which have formed harmful algal blooms within the Bay: Alexandrium, Gymnodinium, and Prorocentrum. Two genera identified from the ports (Takayama and Woloszynskia) have not been previously reported. This study provides a baseline of the phytoplankton community within these major ports in Galveston Bay before foreign shipping traffic increases due to the expansion of the Panama Canal.