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 2010 Annual Report

Author BIO-WEST Inc
Year 2011
Description Annual report for 2010 on biota study of San Marcos Springs/River
Publisher BIO-WEST Inc
Location San Marcos Springs/River
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The purpose of this report is to document the results of all aquatic ecosystem monitoring conducted in 2010 at San Marcos Springs located in San Marcos, Texas. The monitoring and report preparation was performed by BIO-WEST, Inc. Pflugerville, Texas.

[Excerpted from Executive Summary]
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 2010.… Unlike 2009, discharge in the San Marcos River was above the historical average to begin 2010. This was a considerable departure from the drought central Texas experienced over the previous couple of years. Discharge remained above the historical average until rainfall began to decrease in fall, and flows ended the year slightly below average….

Coverage of Texas wild-rice (Zizania texana) increased from fall 2009 to summer 2010 with an increase of nearly 500 meters squared (m2) in the uppermost section of the San Marcos River….

Flow conditions and recreational pressure in 2009 led to drastic changes in aquatic vegetation resulting in the lowest fountain darter (Etheostoma fonticola) population estimate (normalized estimate calculated based on average fountain darter density per vegetation type over the study period multiplied by aquatic vegetation coverage) of the study in fall 2009. A return to above average flow conditions provided a great opportunity to evaluate how the fountain darter population would recover from the extended lowflow period witnessed in 2009. As summarized above, the aquatic vegetation made a strong recovery which translated into a positive recovery for the fountain darter population as well. In fact, using the same methodology for calculating normalized population estimates, the 2010 fountain darter population estimate was the highest since the study’s inception…. Overall, there is little variation in the average density of fountain darters found among vegetation types in the San Marcos River. As a result, shifts in aquatic vegetation don’t result in dramatic changes in fountain darter densities…. Size class distributions indicate that reproduction occurs year-round (most pronounced in Spring Lake), with reproductive peaks in the spring.

San Marcos Salamander (Eurycea nana) densities increased at each site from fall 2009 to spring 2010 as flow increased above the historical average. Fall densities decreased at all sites with the largest drop at the site in the eastern arm of the San Marcos River immediately downstream of Spring Lake.

The recent drought in central Texas provided a unique opportunity to observe the biota and associated habitat in the San Marcos River over an extended period of lower than average flows. Above average flows in 2010 yielded a chance to observe how the biota and associated habitat recovered in areas that had experienced impacts during the drought. Continued monitoring of this system will provide knowledge on the organisms’ interactions with variable flows, and also temporal responses that can only be detected over an extended period of time….