ABSTRACT: Black Cypress CreekSpecial Study
Black Cypress Bayou is listed in Appendix D of the Texas Surface Water Quality Standards (TSWQS) as a perennial stream with an intermediate aquatic life use designation and a corresponding average 24-hour dissolved oxygen (D.O.) criteria of 4.0 mg/L. It is on the draft 2004 303(d) list as a water body not meeting its D.O. standard and in need of standards review. The objective of this study is to provide information for use in assigning appropriate segment specific criteria to this water body. The Black Cypress Bayou subwatershed was sampled at four sites from near the headwaters at Hughes Springs to the confluence with Big Cypress Bayou near Jefferson. Biological, physical habitat, and 24-hour D.O. data were collected during the summer months of 2000-2001. Additional 24-hour DO samples were collected during 2000-2001 during different seasons and at a variety of flow conditions. Water chemistry samples were collected during 1998-2001. Black Cypress Bayou did not meet the D.O. criteria during July and August 2000 or August 2001. Dissolved oxygen samples collected during other months supported the criteria. August 2000 was an exceptional low-flow period (<7Q2) and Black Cypress Bayou flowed intermittently with perennial pools throughout its length. Biological samples collected during the summers of 2000-2001 suggested a higher aquatic life use than the designated intermediate, especially for the fish community. Apparently, the fish assemblage in the Black Cypress watershed has the ability to withstand periodic low summertime DO conditions of short duration.
A regression equation was developed based on water temperature, stream flow, and watershed size. The model was able to explain 95.5% (adjusted R²) of the observed D.O. readings. The regression equation should be considered as a site-specific seasonal criterion for dissolved oxygen in Black Cypress Bayou in the water quality standards and should be used by the TCEQ for assessment purposes and wastewater permitting decisions.
Based on habitat, fish, and water chemistry data, the Black Cypress Bayou subwatershed should be considered as a candidate for a least-impacted reference stream within the South Central Plains Ecoregion. Fish survey scores were in the high to exceptional subcategories. Benthic macroinvertebrate scores were in the intermediate to high subcategories. Low nutrient concentrations suggest minimal contributions from point or non-point sources.
Testing the Black Cypress D.O. Model
The Black Cypress Creek D.O. model was developed from 38 24-hour data points collected during 1998-2005. Data were collected during all seasons, but primarily during the Index Period (March 15-October 15). Using three variables: temperature, flow and watershed size, the model was able to explain over 95% of the variability in observed D.O. values.
Black Cypress Creek was determined to be a least-impacted reference site based on chemical, biological, and habitat data. Data from fish surveys showed the Black Cypress to support high to exceptional aquatic life use. Since the Cypress Basin is a relatively small and homogeneous geographical area, it is logical to assume that the Black Cypress D.O. model could serve the entire basin.
To test this hypothesis, 100 24-hour D.O. data points that had corresponding values for temperature, flow and watershed size were compared to the model. Data were used from multiple locations on 18 different streams that had a geographical coverage over the entire basin. The period of record was 1995-2005. These years included the full spectrum of hydrological conditions from wet to dry years. Eighty five percent of the data met or exceeded the Black Cypress Creek D.O. model. If the model were used as a seasonal D.O. criteria throughout the basin, one of the 18 streams surveyed would be placed on the 305(b) list for possible TMDL implementation. If however, the same 100 data points were compared to current Water Quality Standards for high aquatic life use, then six of 18 streams would be on the 305(b) list. This is a larger percentage of impaired streams than previously collected biological data would indicate. A randomly designed biological survey of 19 streams from throughout the Cypress Basin during 1995-1996 indicated that all streams surveyed either fell within the intermediate (N=7) or high (N=12) aquatic life use category (Crowe and Hambleton, 1998).