Aquatic plant species distributions and associations in Arizona's reservoirs.

We surveyed aquatic plants in 38 reservoirs throughout Arizona from 2004 through 2006 to develop an inventory of species and to determine species distribution and composition patterns. We identified 12 submersed, 3 floating, and 18 emergent aquatic plants to species level; some samples were only identified to genera, and none of the filamentous algae were identified to either genera or species. Two nonnative aquatic plant species were found during the surveys: Eurasian watermilfoil ( Myriophyllum spicatum L.) was found in nine reservoirs and curly-leafed pondweed ( Potamogeton crispus L.) was found at two reservoirs. Filamentous algae were found at most reservoirs, and muskgrass ( Chara spp. L.) was found at 53% of the reservoirs surveyed. Among reservoirs the most prevalent vascular plant species were cattails ( Typha spp. L.) and hard-stem bulrush ( Schoenoplectus acutus [Muhl. ex Bigelow] A.&D. Love) followed by coontail ( Ceratophyllum demersum L.), sago pondweed ( Stuckenia pectinatus [L.] Boerner), and northern watermilfoil ( Myriophyllum sibiricum Komarov). Within reservoirs, coontail or sago pondweed dominated the plant community at five reservoirs, northern watermilfoil at eight reservoirs and Eurasian watermilfoil at four reservoirs. Elevation and depth were significant predictors of occurrence for several species, and the number of aquatic plant taxa was positively related to reservoir surface area. Seven taxa (filamentous algae, Eurasian watermilfoil, curly-leafed pondweed, coontail, sago pondweed, spiny naiad [ Najas marina L.], and northern watermilfoil) are probably the best targets for management because they had high prevalence and percent composition in Arizona, and hence are most likely to be considered problematic.