Comprehensive and Critical Period Monitoring Program to Evaluate the Effects of Variable Flow on Biological Resources in the San Marcos Springs/River Aquatic Ecosystem Final 2012 Annual Report

Author BIO-WEST Inc
Year 2013
Description Annual report for 2012 on biota study of San Marcos Springs/River
Publisher BIO-WEST Inc
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This annual summary report presents a synopsis of methodology used and an account of sampling activities conducted during two Comprehensive Monitoring sampling efforts on the San Marcos Springs/River ecosystem in 2012. For ease of comparison, the data are reported in an annual report format similar to previous reports (BIO-WEST 2001a, b – 2012a, b).

Flows in the San Marcos River began the year below the historic average, but spring rains bumped flows above 200 cubic feet per second (cfs) from April to July. This was the first time since October 2010 that flows were above 200 cfs. Summer and fall saw little rainfall resulting in flows declining to below 125 cfs to end the year. The minimum discharge in 2012 was 100 cfs (this occurred several times in January). These lower than average flows are a reflection of the ongoing drought in Central Texas.

Close to spring inputs there was little variation in water temperatures even though flows were below average. Water temperatures were highest at Sessom’s Creek (tributary heavily influenced by runoff) and Rio Vista Dam (where water is pooled by the rapids downstream).

Total amounts of aquatic vegetation in each of the three reaches (Spring Lake Dam, City Park, and I-35) were below the fall and spring averages observed in this study. Vegetation at the City Park Reach is
most susceptible to seasonal changes due to varying amounts of recreation. In 2012, vegetation increased over winter, and decreased by the end of summer with large areas in the upper section (where recreation pressure is greatest) denuded of vegetation. This boom/bust cycle is typical of this reach, and
is the result of increased recreation pressure combined with effects of the extended drought. Aquatic vegetation at the Spring Lake Dam reach follows a similar pattern as City Park with plants growing over winter, and decreasing by fall. Recreation pressure here is also significant in summer since it is adjacent to a popular swimming area and high density housing, but recently-constructed educational signs may provide value. The greatest impacts on aquatic vegetation in recent years have been observed within the I-35 Reach. Since Rio Vista Dam was transformed into a flow-through rapid in 2006, sedimentation has apparently increased resulted in decreased depths and higher velocities, leading to much lower coverage of vegetation, especially in the upper section of this reach. Additionally, two large riparian trees have fallen into the river within this reach, diverting flow and scouring out areas that were previously covered in vegetation. By fall 2012, total area of aquatic vegetation dropped below 300 square meters (m2), the lowest ever observed in this study. These impacts are significant because the largest patches of Cabomba are found here which hold higher densities of fountain darters relative to other San Marcos aquatic vegetation. This section will continue to be closely monitored for other major changes in habitat.

Coverage of Texas wild-rice (Zizania texana) increased by 16% since 2011 resulting in the highest amount (4,367.1 m2) observed in the San Marcos River since the inception of the study. The largest increases were observed within the upper-third of the river where most of the wild-rice is located. Although one large stand at Sewell Park continues to shrink due to sedimentation blocking flow from the upstream end; however, other plants in the area continue to flourish. Texas wild-rice from the City Park Reach to Bicentennial Park increased by 35% from 2011. Coverage of wild-rice in areas downstream changed little over the year. Physical observations of vulnerable Texas wild-rice plants documented changes in several individual plants during 2012. More plants were emergent and
flowering in spring than in fall. Vegetation mats covering wild-rice were less prevalent in 2012 compared to previous years, likely a result of slightly higher flows. The mats can inhibit photosynthesis and cause plants to die if they are not pushed off (either manually or during higher flow events).

Population estimates of fountain darters (Etheostoma fonticola) were relatively high in spring 2012, but decreased to the lowest estimate observed in the study (2000 – 2012) by fall. These estimates are based on aquatic vegetation coverage and consequently follow the same pattern. Continued effects of the dam and recreation pressure during the summer both caused decreases in aquatic vegetation coverage in the I-35 Reach. This was exacerbated by a fallen tree which diverted flow and scoured out a previously vegetated area. As discharge increases, the number of fountain darters in each drop net tends to
decrease. This may be a result of clumping of darters into limited habitat under lower flows. Cabomba (native) and Hydrilla (non-native) exhibit the highest densities of fountain darters in the San Marcos River. These densities are typically lower than in the Comal River system. Dip net data reflects the importance of filamentous algae and bryophytes present in Spring Lake to fountain darter reproduction. These two vegetation types hold the highest densities of darters in both the Comal and San Marcos systems, but are not found in the San Marcos River downstream of Spring Lake.

San Marcos salamander (Eurycea nana) densities were typically higher than average in 2012 at all sites. The highest densities were observed at the Hotel Reach where bryophytes are abundant resulting in higher quality habitat. Salamander densities in this area were higher than the study average in both
spring and fall. Densities of salamanders rebounded at the Riverbed Reach in 2012 after construction activities in 2011 led to fewer salamanders. Numbers were slightly above the study average for both spring and fall. Throughout the study, salamander densities have been most variable at Sample Area 21
(~ 5 meters from the dam) because it is below Spring Lake Dam in an area of public access. This site exhibits higher velocities and typically less aquatic vegetation than areas in Spring Lake. Additionally, rocks that salamanders use for cover are often moved due to recreation activities. While spring densities
were similar to previous years, the density in fall was the second highest recorded in the study. Educational signage recently placed in this area is designed to increase public awareness of the sensitive species present, and will hopefully reduce habitat disturbance.

In conclusion, although Central Texas continues to be plagued by a drought, 2012 monitoring activities suggest that populations of threatened and endangered species in the San Marcos Springs/River Ecosystem continue to persist despite the mixed effects of the drought. In fact, coverage of Texas wild-rice reached a new monitoring plan high. Similarly, San Marcos salamander densities were above study averages. In contrast, fountain darter population estimates reached a new low in 2012. However, a spring rebound in population estimates is typically experienced, and is expected by spring 2013. Should
continued low flows persist in summer 2013, fountain darter habitat loss will likely continue due to the combined effects of persistent low flows and recreation within study reaches. Therefore, continued monitoring of aquatic vegetation communities and fountain darter populations is particularly important
in coming years. Additionally, continued monitoring of all study components will allow for assessing the effectiveness of Edward’s Aquifer Habitat Conservation Plan (EAHCP) activities being implemented in 2013 and beyond.