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

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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 48
(Black 2001, Wang 2007b). A lack of water quality standards and toxicity studies for North American freshwater mussels hampers our ability to verify whether our empirical observations of mussel presence and absence meaningfully relate. Other chemical variables like dissolved oxygen, total phosphorus, and average stream temperature produced insignificant differences for some of the mussels analyzed or inconsistent patterns among species analyzed. Many of the physical habitat parameters overlapped substantially between sites of presence and absence. For two more common species, E. complanata and E. fisheriana, habitat metric scores indicated their presence in streams with higher quality habitat as it relates to fish and benthic macroinvertebrates. Even in light of the relationship between various measures of fish communities and freshwater mussels (Watters 1992, Haag and Warren 1998) and the intuitive association between fish assemblages and stream habitat quality, it would be somewhat speculative to conclude that superior fish habitat is conducive to mussels. Metrics with broad overlapping ranges may illustrate inherent characteristics of habitat variability and a species that traverses multiple physiographic and ecoregions, rather than an ecological tolerance. The pattern also illustrates the often confounding nature of mussel-habitat associations as they relates to mussel distribution (Strayer 2008). For example, Nicklin and Balas (2007) reported mussel density was positively correlated with physical habitat parameters similar to parameters used in Maryland; however, their study was conducted in a stream with a more diverse assemblage of mussels at higher densities. Other studies (Strayer 1981, Strayer and Ralley 1993) have found weak associations with stream reach and microhabitat scale variables. Using a density-habitat or richness-habitat relationship as a management tool or predictive technique should be cautioned against until such relationships have been confirmed in Maryland. A common theme illustrated in plots for all four species examined in this study was their presence in streams with increasing width, discharge, and catchment size. This is not surprising considering the species area relationship that unionids generally exhibit (Watters 1992). The hydrologic variability in headwater streams with smaller watersheds also likely plays a role in the absence of mussels from those habitats. For the most part, there was no clear pattern of mussel species distribution as it relates to forested and agricultural land use. This is in part due to the prevalence of records from the predominantly agricultural Coastal Plain. Mussels generally exhibited a pattern of presence in catchments with very low urban land cover and impervious surfaces, which is no surprise given the sensitivity freshwater mussels display towards watershed degradation (Bogan 1993, Brim Box and Mossa 1999, Poole and Downing 2004). Temporal trends between land alteration and mussel abundance should be examined because of the potential threat it presents to the fauna’s future. Sites where mussels and stream fishes were collected concurrently provide a first step towards identifying potential fish hosts or host-limited populations. In general, nonnative centrarchid spp. were frequently collected with mussels. The low frequency of occurrence for migratory species is likely an artifact of the MBSS fish sampling period rather than host availability. Host fish for A. heterodon occurring in Maryland were previously identified as banded killifish, Blue Ridge sculpin, striped bass, shield darter, and tessellated darter (Michaelson and Neves 1995, White 2007); however, Blue Ridge sculpin and A. heterodon do not have overlapping ranges in Maryland. We have rarely collected banded killifish, shield darter, and striped bass at any site throughout the range 49
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Freshwater mussel records collected
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Freshwater mussel records collected
Page 6 and 7: List of Figures Figure Number Page
Page 8 and 9: List of Tables Table Number Page 1.
Page 11 and 12: INTRODUCTION The diversity of fresh
Page 13 and 14: species richness, host abundance, a
Page 15 and 16: Table 1. Number of sites sampled by
Page 17 and 18: Table 2. Frequency of occurrence fo
Page 19 and 20: Figure 3. continued A P A P A P p =
Page 21 and 22: Figure 3. continued A P A P A P 0 5
Page 23 and 24: Since 2007, A. undulata Say 1817 ha
Page 25 and 26: Anodonta implicata Say 1829 was enc
Page 27 and 28: Table 3. Frequency of occurrence fo
Page 29 and 30: Figure 8. continued A P A P A P p =
Page 31 and 32: Figure 8. continued A P A P A P 0 5
Page 33 and 34: In 2009, E. fisheriana was collecte
Page 35 and 36: Figure 10. Box-and-whisker plots of
Page 37 and 38: Figure 10. continued. A P A P A P 0
Page 39 and 40: Figure 10. continued A P A P A P 0
Page 41 and 42: Elliptio producta Conrad 1836 has p
Page 43 and 44: Lampsilis radiata radiata Gmelin 17
Page 45 and 46: Leptodea ochracea Say 1817 has been
Page 47 and 48: Pyganodon cataracta had previously
Page 49 and 50: Figure 19. Box-and-whisker plots of
Page 55: Corbicula fluminea is a non-native
Page 59 and 60: Maryland (Cummings and Watters 2009
Page 61 and 62: REFERENCES Allan, J.D., & A.S. Flec
Page 63 and 64: Kelly, M.W., & J.M. Rhymer. 2005. P
Page 65 and 66: ——. 1999a. Effects of alien spe
Page 67 and 68: Young, D. 1911. The implantation of
Page 70 and 71: 8-digit watershed Number 8-digit wa
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