Distribution of golden alga in the Brazos River and Rio Grande basins

Harmful blooms of golden alga (Prymnesium parvum) in North America were first reported in the Pecos River (Texas) in the 1980s, but this species is now present in at least US 23 states. Several surveys have been conducted to determine the distribution of golden alga in surface waters of the USA. Most of these surveys, however, have relied on hemocytometry, an enumeration technique with low detectability. In addition, there are ecologically important or sensitive areas that have not been methodically surveyed before. Consequently, early surveys may have overlooked the association of golden alga abundance with environmental variables at low levels of abundance. Therefore, the use of environmental DNA (eDNA) analysis that is a highly sensitive and specific technique to identify and determine the presence of “hard-to-see” organisms such as golden alga cells in conjunction with hemocytometry will better examine these associations at the landscape scale. Based on combined hemocytometry and eDNA techniques, this study determined presence and abundance of golden alga in selected streams and reservoirs of the Brazos River and Rio Grande basins, and described the association between selected water quality variables and golden alga presence and abundance. A total of 25 sites were sampled in both basins. Samplings were conducted in fall of 2015 and in spring of 2016 to account for seasonal variations. Sampling sites on the Brazos River included the North and South Forks of the Double Mountain Fork (DMF), the DMF, and mainstem locations downstream to Possum Kingdom Reservoir. The majority of these sites are within designated critical habitat for two newly federally listed fishes, Sharpnose Shiner (Notropis oxyrhynchus) and Smalleye Shiner (Notropis buccula). Sites on the Rio Grande Basin include Salt Creek, a Pecos River tributary containing what may be the last genetically pure population of Pecos Pupfish (Cyprinodon pecosensis) in Texas, and the Trans-Pecos Region of Rio Grande, which includes the Big Bend area, one of the most unique desert ecosystems of North America. The combined results of hemocytometry and eDNA analysis suggested the presence of golden alga in much of the Brazos River and Rio Grande including most sites not previously examined. Results of Zero-inflated Poisson (ZIP) regression suggested that the probability of finding golden alga increases with increasing salinity, while golden alga abundance is negatively related with specific conductance. A broad range of salinity examined in this study, from very low (174 μS/cm) to very high (61,000 μS/cm), provides a better understanding of the association between golden alga presence/abundance and salinity at the landscape scale. Information gained from this study may help stakeholders make informed decisions on future land and water management projects to minimize or avoid the development of conditions that favor blooms, especially at the sites newly identified by this study.