Conducting the Environmental Impact Analysis and Assessment - Environmental Impact Analysis: Process and Methods - page 207

Thus, the expansion and stimulation of invasive mussel populations in
the Great Lakes from DCR management practices was determined to be a
potentially serious impact and thus under the requirements of NEPA war-
ranted a “hard look” by the USCG as they developed regulations for the
management of DCR.
In order to investigate and predict the potential for invasive mussel
impacts, a series of scientific investigations were designed and conducted
(U.S. Coast Guard 2008). The investigations included literature review to
document whether mussels were already established in the Great Lakes, fac-
tors limiting their presence and density, their life history, and environmental
requirements at each life-history stage. The investigation also included an
extensive series of laboratory tests.
The first laboratory test conducted was Phase I Adult Mussel Attachment.
This experiment consisted of introducing both zebra and quagga mussels
to aquaria with each type of DCR and native sediments. The percentage of
mussels attaching to each sediment type was determined and attachment
was found to be much more prevalent in aquaria with DCR compared to
those with just native sediment.
After reviewing the Phase I results that showed a very strong mussel attach-
ment preference for DCR compared with native sediments, the project team sus-
pected that the study design may have been too conservative (i.e., potentially
over predicting impacts). To address this concern, a Phase II adult mussel attach-
ment study was designed to more closely simulate in situ conditions. On the lake
floor, the DCR would generally be covered by a thin layer of natural sediment
due to the heavy nature of DCR sinking into the sediment surface. Also, biotur-
bation (i.e., burrowing and feeding activity of animals living in the sediment
or at the sediment-water interface) working and resuspending the sediment
tends to cover the DCR with a thin layer of native sediment. The attachment
experiment was repeated to address this issue by covering the DCR with a 0.5
to 1.0 mm layer of Lake Superior sediment. The Phase II results (Figure 5.15)
strongly indicated a preference for adult mussel attachment to DCR, even when
covered with a thin layer of sediment, compared with the natural soft sediment
substrate. Thus, the adult mussel attachment studies indicated that discharge
of DCR could result in an impact by increasing mussel attachment and thus a
potential increase in mussel population density and range.
Attachment to DCR by adult mussels was identified as a concern; how-
ever, the potential for impact from increased colonization by larval mussels,
or veligers, could be an even greater concern because the veligers are more
mobile than the adult mussels. As one would imagine, the heavy shells of
the adults limit their mobility and ability to spread and colonize new areas.
Their evolutionary development has accommodated this limitation and both
zebra and quagga mussels have mobile, free-floating larval forms which
swim and drift with the current and can travel dozens of kilometers. This
enables a rapid spread of the population, if there is suitable substrate for
veliger attachment once they reach maturity.