INFLUENCE OF GROUND AND SURFACE WATER INTERACTIONS ON ALGAL COMMUNITIES IN THE RIO GRANDE

Algal community structure and process were investigated along a 450-km segment of the Rio Grande, from below the Rio Conchos confluence to the IBWC river gage near Langtry, Texas, during a period of low-flow conditions in 2010-11. The abundance and species composition of algal communities and estimates of river metabolism were used to assess trophic conditions, organic enrichment, and longitudinal changes in water quality. The Rio Grande exhibits a gradient of salinity, nutrient, and organic enrichment extending from the Rio Conchos confluence down river through the Wild & Scenic River segment. During low-flow conditions, the upper 110-km segment of the river (from Presidio to Castolon) was characterized by dense growths of benthic algae, large abundances of algal seston, and high rates of metabolism. Algal communities in the upper segment were dominated by brackish-water species, notably centric diatoms that are indicative of eutrophic to hypereutrophic conditions. The middle 155-km segment of the river, from Castolon to La Linda, was a transition zone where the abundance of algae and rates of metabolism were moderate and water-quality conditions were changing, as indicated by decreases of specific conductance and pH and increases in water temperature. Algal communities in the middle segment were dominated by planktonic green, blue-green, and diatom taxa indicative of mesotrophic or eutrophic conditions, and halophilic, freshwater benthic diatoms. The lower 180-km segment of the Rio Grande, from La Linda to Foster Ranch, was a zone of improving water quality resulting from considerable discharges of relatively higher-quality ground water from the Edwards-Trinity (Plateau) aquifer in the U.S. and similar aquifers associated with the Del Carmen and Glen Rose Formations in Mexico. The abundance of algae was average compared with streams and rivers in the U.S., generally, and rates of metabolism were low. Benthic algal communities were dominated by filamentous red algae, benthic diatoms, and nitrogen-fixing algae indicative of mesotrophic conditions and relatively low concentrations of dissolved nitrogen.