Freshwater Mussel Monitoring and an Alternate Thermal Standard

QCNS discharges its thermal effluent into the Mississippi River at river mile 506.4 via a dual diffuser pipe.  They are investigating an application for an adjusted thermal standard.  An Essential Habitat Area (EHA) for Lampsilis higginsii, a federally endangered freshwater mussel (unionid) species, occurs a few miles downstream of the diffuser pipe between river miles 503.0 and 505.5.  Exelon wanted to ensure that an adjusted standard would not affect the indigenous shellfish community under 316(a) of the Clean Water Act, and identify and mitigate impacts to L. higginsii under Section 10 of the Endangered Species Act.  The unionid life cycle (spawning, glochidia release, fish host activity, fish host infestation period) and behavior (siphoning/feeding rates, burrowing) is temperature dependent, however the thermal triggers and critical thermal maximums for most species are unknown.  Temperature changes were predicted with computer modeling, and a unionid monitoring program was established to satisfy Section 316(a) and Section 10 requirements.  The area between river mile 495.4 and 515.0 was investigated for unionid communities.  The three beds closest to the diffuser (Upstream, Steamboat Slough, and Cordova EHA) were monitored in 2004, 2005, 2006, 2007, and 2008.  Temperature probes were placed in the substrate at these three beds to determine actual exposure temperatures.  In 2007, fish were sampled between May and August to detect fish host availability and glochidial infestation with different water temperatures.  Unionid communities were sampled using quantitative and qualitative techniques to determine density, age structure, mortality, and species composition, and metrics were statistically compared among years.  Some community differences were apparent, however differences could have been attributed to habitat and zebra mussel infestation rather than or in addition to temperature.  Three additional beds were added to the monitoring program in 2007 and 2008 to facilitate impact assessment.  Differences in all unionid communities were apparent, but were likely due to habitat differences rather than existing thermal standards.  Ambient temperatures in one monitoring year (2006) exceeded water temperature expected with an adjusted thermal standard.  Some mortality occurred that could be attributed to increased water temperatures, but effects were greater upstream than downstream of the power plant’s diffuser.  Data from the mussel monitoring program were used to demonstrate that QCNS would not affect the integrity of the indigenous freshwater mussel community and develop the first freshwater mussel Habitat Conservation Plan under Section 10 of the Endangered Species Act.