This study examined the effect of Brazos River wastewater and desalination point source discharge on benthic community structure using paired water quality and benthic macroinvertebrate assessment.  Discharge sites included the Waco Metropolitan Area Regional Sewerage System (primary volumetric discharge) Robinson Desalination Plant (secondary) in McLennan County, Texas.  Benthic artificial substrate Hester-Dendy colonization and concurrent water quality sampling occurred over an optimal five-week period from October to November 2008 across a twenty point grid with two upstream controls and three downstream treatment transects.  The following water quality variables were examined using standard APHA methodology: water temperature; conductivity; dissolved oxygen; total suspended solids; carbonaceous biochemical oxygen demand; inorganic phosphorus: PO₄; and inorganic nitrogen: NO₃, NO₂, and NH₃.  Research objectives included 1) comparing relative taxonomic affinities between upstream and downstream macroinvertebrate communities, 2) examining the correlation between water quality criterion and benthic community assemblages, and 3) after removing bias caused by spatial autocorrelation, examining whether the deterministic effect of wastewater and desalination effluent gradation on water quality variables influenced macroinvertebrate community structure. 

Results/Conclusions

Several water quality variables evinced a marked change directly above and below the wastewater discharge transect (temperature, conductivity, dissolved oxygen, and ammonia) whereas the desalination discharge seemed to have no effect.  Comparing sampling point community simpson’s diversity to ambient water quality using a spearman’s correlation proved inconclusive due to overall community homogeneity.  An nMDS ordination was performed to assess community similarity based on relative taxonomic components, then overlaid with environmental water quality data.  Based on this test, water temperature, dissolved oxygen, and ammonia are the most important water quality factors determining benthos community composition.  These effects were more directly linked to sampling point orientation distance from the wastewater discharge point using a Partial Mantel’s Test.  In conclusion, while wastewater discharge does significantly effect potential community-driving environmental criterion, such changes do not trigger corresponding benthic macroinvertebrate community shifts detectable within a single autumn season five week period trial.