An analysis of the effects of photosynthetically active radiation and recreation induced turbidity in the San Marcos River on the vegetative growth of Texas wild rice (Zizania texana Hitch.)
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Texas wild rice (Zizania texana; Poaceae) is an endangered perennial macrophyte known to occur only in the spring-fed San Marcos River, Hays County, Texas. Historically, Texas wild rice (TWR) was reported to reside in the upper 3 kilometers of the river, into the irrigation ditches, and approximately 300 meters behind Spring Lake dam. The current distribution of TWR is within the upper 5 kilometers with 97% of the population occurring within the upper 2.2 kilometers. Studies on various aquatic macrophytes have demonstrated that the availability of photosynthetically active radiation (PAR) is an important abiotic factor affecting a plant’s biomass. The San Marcos River is impacted on a seasonal, weekly and diel basis by contact water recreation. Recreational activities can cause increases in suspended sediment induced turbidity resulting in a decrease in water clarity and reduction in ambient PAR. Two different studies were conducted to test the effect of a reduction in PAR on the vegetative growth of TWR. In the first study, the results of three ex situ experiments involving a reduction in PAR through shade frames (0%, 10%, 20%, 40%, and 80% PAR reductions) found that with only 20% ambient PAR (80% PAR reduction), above ground biomass, below ground biomass, above/below ground biomass, total biomass, shoot number, root number, and total leaf surface area of TWR plants were significantly reduced in two of the three experimental periods. The second study focused on periods of low and high contact recreation use, the suspended sediment induced turbidity response, and the impact on TWR biomass production. Results showed that differences in TWR biomass production existed along a longitudinal gradient in the river, when the Eastern Spillway (ES), which had very limited upstream recreational activity, was compared to the downstream treatment sites located at Sewell Park (SP), Bicentennial Park (BP), and Ramon Lucio (RL), which all had substantial upstream recreational activity. Greater growth was found in TWR plants at the upstream ES site. Differences in biomass production are likely the result of lower levels of suspended sediment in the water column at the ES site, allowing for higher levels of photosynthetically active radiation (PAR). No difference in the amount of periphyton on the leaf surface of plants at the different study sites under either low or high contact recreation use was found. Therefore, periphyton does not account for observed differences in TWR growth. Additional factors related to habitat suitability requirements for TWR may also have contributed to differences in biomass production found along the longitudinal gradient in the river. Findings from this study suggest that locations in the river receiving more than 20% ambient PAR provide optimum habitat for the reintroduction of TWR. The results of these studies provide useful information for future conservation and management measures in the effort to restore TWR to once historical habitat, as well as continue to increase areal coverage in the San Marcos River.