Freshwater mussel records collected by the Maryland Department of ...

DISCUSSION
When combined with previously collected records, the data from 2009 have
produced distributional accounts for 14 species of mussels, four of which are state
endangered. As in previous years, new distributional records or encounters of rare
species continued. Plots of environmental variables provide a valuable context to
describe the conditions of sites and their catchments where mussel species were collected
and potential reasons why they were absent from other sites. Some of the variability
evident in the range of environmental conditions can likely be attributed to the relatively
low sample sizes for A. heterodon and P. cataracta. This further emphasizes the need for
a larger data set of mussel presence and potentially different environmental variables
before more rigorous species level analyses are prudent. Consequently, large sample
sizes can create a situation where statistical significance may not equate to ecological
significance (Strayer and Smith 2003) and is likely evident in the analyses of some
variables for E. complanata. Finally, we caution that the relationships we report are
based on presence-absence data, which typically has weak power to relate a decline in a
species to a condition (Strayer 1999b). Given the short period of time our data analyzes
and lack of historical mussel records at sites with environmental data, it would
inappropriate to use these results to determine population declines without confirmation
of the statistical power to detect a change in rates of mussel encounters at multiple scales.
Chemical variables (pH, specific conductance, and ANC) that relate to water
acidity and ionic concentrations were generally higher at sites where each of the four
species investigated were present compared to sites where they were absent. In the
context of their range (i.e. predominantly Coastal Plain streams), this likely indicates an
avoidance of streams with acidic, blackwater conditions and soft water, which would not
be favorable for shell deposition (Strayer et al. 1981). Another re-occurring pattern
observed was the narrow range of total nitrogen and nitrate concentrations at sites where
mussels were present. It is generally assumed that increased landscape alterations and
nutrient loads impair aquatic organisms, such as unionids (Richter et al. 1997, Strayer et
al. 2004), which are among the more sensitive to ammonia (Augspurger et al. 2003). In
general, nutrient concentrations observed at sites of mussel presence were below median
lethal levels for A. heterodon, L. subviridis, and congenerics of L. cariosa and P.
cataracta (Black 2001, Augspurger et al. 2003, Wang et al. 2007a); however, chronic
ammonia exposure to juvenile mussels may have far more serious consequences on
mussel survival (Black 2001, Wang et al. 2007b). While lower nutrient concentrations
were not correlated with mussel distribution in the Upper Susquehanna and Allegheny
rivers (Strayer and Fetterman 1999, Nicklin and Balas 2007), those studies examined
nutrients at sites within a single watershed versus sites within many watersheds across a
broad region as examined in this study. The narrow range of nutrient concentrations
observed at sites with mussels compared to the broad range at sites without may provide
further evidence for their role as nutrient regulators in stream ecosystems (Vaughn and
Hakenkamp 2001, Strayer 2008, Vaughn et al. 2008). Nutrient deficient waters may also
pose limiting conditions to mussels because many of the items they consume are
primarily and secondarily produced (Strayer et al. 1981, Nichols and Garling 2000,
Raikow and Hamilton 2001). The mean and range of chloride concentrations where A.
heterodon was present in Maryland was also consistent with recent toxicological studies
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