Habitat characterization of Texas wild‐rice (Zizania texana Hitchcock), an endangered aquatic macrophyte from the San Marcos River,TX, USA

1. The habitat of Texas wild‐rice (Zizania texana Hitchcock), known only from the spring‐fed upper San Marcos River in Central Texas, was classified in terms of physical and chemical conditions, depth, current velocity, associated aquatic macrophytes, and substrate composition. 2. Randomly selected transects in sites with and without Texas wild‐rice were evaluated during May, August and January 1994–1995. Physical and chemical conditions (temperature, dissolved oxygen, specific conductance, pH) were found to be nearly constant and uniform both among transect types and sampling dates. Chemical analysis of the substrate likewise showed few differences among transect types. Turbidity was found to be significantly different among transect types on only one sampling date (Kα_0.05=0.60; p=0.009), but even non‐significant levels of turbidity may present biologically significant impacts to wild‐rice plants. 3. Differences in substrate particle size were highly significant (Kα_0.05=0.71; p=0.001) with Texas wild‐rice occupying sites having moderately coarse to coarse sandy soils (73%) compared with the moderately fine to fine clay soils found at non‐rice sites (82%). However, no significant differences were found in organic matter content among rice and non‐rice transects ( =2.09 and 2.56%, respectively). 4. Texas wild‐rice was found primarily in shallow areas of the river (<1 m) and at higher current velocities than those of non‐rice sites ( ≥0.46 m s⁻¹ and ≤0.22 m s⁻¹, respectively). 5. Texas wild‐rice appears to be more commonly associated with other native species rather than exotic species. Mean percentage composition of exotic macrophytes did not exceed 29% in areas where Texas wild‐rice grew, but they accounted for nearly 47% of the composition in areas where rice did not grow. 6. The results of this study will allow for reliable and accurate identification of the remaining suitable habitat of Texas wild‐rice in the San Marcos River for possible reintroduction and development of management plans. Copyright © 1999 John Wiley & Sons, Ltd.