Survival, Growth and Condition of Freshwater Mussels: Effects of Municipal Wastewater Effluent

<div><p>Freshwater mussels (Family Unionidae) are among the most imperiled group of organisms in the world, with nearly 65% of North American species considered endangered. Anthropogenic disturbances, including altered flow regimes, habitat alteration, and pollution, are the major driver of this group's decline. We investigated the effects of tertiary treated municipal wastewater effluent on survivorship, growth, and condition of freshwater mussels in experimental cages in a small Central Texas stream. We tested the effluent effects by measuring basic physical parameters of native three ridge mussels (<i>Amblema plicata</i>) and of non-native Asian clams (<i>Corbicula fluminea</i>), before and after 72-day exposure at four sites above and below a municipal wastewater treatment plant outfall. Survivorship and growth of the non-native Asian clams and growth and condition indices of the native three ridge mussels were significantly higher at the reference site above the outfall than in downstream sites. We attribute this reduction in fitness below the outfall to elevated nutrient and heavy metal concentrations, and the potential presence of other untested-for compounds commonly found in municipal effluent. These results, along with an absence of native mussels below the discharge, indicate a significant negative impact of wastewater effluent on both native and non-native mussels in the stream.</p></div

Preliminary water quality test results used to determine site locations.

<p>Preliminary water quality test results used to determine site locations.</p

Growth and survival of the non-native Asian clam (<i>Corbicula fluminea</i>) after 72 days of exposure to effluent at four sites in Wilbarger Creek (Texas, USA).

<p>(A) Mean whole wet mass in grams at day 0 (black bars) and day 72 (gray bars) of the study; (B) mean total length in millimeters at day 0 (black bars) and day 72 (gray bars) of the study; (C) mean percent survivorship at day 72. Error bars represent ± 1 standard error. Columns with the same letter were not statistically different from each other at p = 0.05. An asterisk next to a column denotes significant change in that parameter between day 0 and day 72. Site 1, the reference site, was located upstream of the discharge, Site 2 just below the discharge, and Sites 3 and 4 downstream of the discharge.</p

Discharge limitations of effluent from the Wilbarger Wastewater Treatment Facility at 0.5 million gallons per day (MGD) discharge stage.

<p>Discharge limitations of effluent from the Wilbarger Wastewater Treatment Facility at 0.5 million gallons per day (MGD) discharge stage.</p

Map showing locations of the four study sites in relation to the Wilbarger wastewater treatment plant (WWTP) discharge (City of Manor, Travis County, Texas, 30°20'47.23"N, 97°32'56.74"W).

<p>Inset shows the location of Travis County (shown in green) in the state of Texas. Discharge indicates the discharge of the WWTP effluent.</p

Mean mass and body condition index (BCI) values of pre- and post-exposure measurements of <i>A</i>. <i>plicata</i> and percent change (± Standard Error) for the parameters measured in each site.

<p>Mean tissue dry mass and mean BCI-dry values were only measured at day 72.</p><p>Significant results of t-tests comparing pre- and post-exposure data indicated by asterisks:</p><p>* Indicates p-value of < 0.05.</p><p>** indicates p-value of < 0.01.</p><p>Mean mass and body condition index (BCI) values of pre- and post-exposure measurements of <i>A</i>. <i>plicata</i> and percent change (± Standard Error) for the parameters measured in each site.</p

Physical responses of the native threeridge mussel (<i>Amblema plicata</i>) to effluent in the 72-day exposure experiment at four sites in Wilbarger Creek (Texas, USA).

<p>Error bars represent ± 1 standard error. Columns with the same letter were not statistically different from each other at p = 0.05. (A) and (B) represent change in pre-exposure (day 0) and post-exposure (day 72) wet mass and condition index respectively. An asterisk next to a column denotes significant (p<0.05) change in that parameter between day 0 and day 72. (C) and (D) represent post-exposure data only. Site 1, the reference site, was located upstream of the discharge, Site 2 just below the discharge, and Sites 3 and 4 downstream of the discharge.</p

Results of MANCOVA and ANCOVA tests on <i>A</i>. <i>plicata</i> data.

<p>Tests on whole wet mass and BCI-wet were performed on the percent change from pre- to post-exposure measurements. Tests on BCI-dry and dry tissue mass were performed on data collected post-mortem. MANCOVA was analyzed using all four measured parameters as dependent variables. Both MANCOVA and ANCOVA tests were run with the average pre- and post-exposure mussel whole wet mass as covariate.</p><p>Results of MANCOVA and ANCOVA tests on <i>A</i>. <i>plicata</i> data.</p

Mean <i>C</i>. <i>fluminea</i> survivorship and pre- and post-exposure measurements and percent change (± Standard Error) for the physical parameters measured in our study for each site.

<p>Post-exposure data for Sites 2 and 3 are unavailable due to complete mortality of <i>C</i>. <i>fluminea</i> at those sites.</p><p>Significant results of t-tests comparing pre- and post-exposure data within sites indicated by asterisks:</p><p>*** Indicates p-value of < 0.001.</p><p>Mean <i>C</i>. <i>fluminea</i> survivorship and pre- and post-exposure measurements and percent change (± Standard Error) for the physical parameters measured in our study for each site.</p