## 4 Discussion  
The overarching goals of this study were to describe relationships between landscape-level and microhabitat environmental variables and the species richness and relative abundance of freshwater mussels in southeastern coastal plain streams. Overall, data analyses suggested two conclusions. First, mussels are generally uncommon in upper tributaries of the Pearl River watershed. Patchy distributions (53% of sites had mussels) and low abundance (26% of sites yielded >10 mussels) characterized mussel assemblages across the study area, despite this region's central location in an overall area of high mussel diversity [7; 44; 79]. Further, analyses suggested that sampling was more likely to fail to capture mussels than to capture mussels (76%) or a species (56%), i.e., among randomly selected stream sites in this watershed, only approximately one in four sites would yield mussels. Thus, presence/absence of the mussels accounted for considerably more variation in the data than measured microhabitat, land cover, or geology variables. Second, mussel relative abundance and richness follows a pattern of greater relative abundance and richness in the upper watershed, with both declining in tributaries closer to the Gulf of Mexico, concomitant with changes in water quality, land cover, geology, and substrate. Interestingly, similar influences of longitudinal position in the watershed and landscape variables were reported for mussels in the Kalamazoo River in Michigan [80]. Where mussels were present, decreasing CPUE and richness appeared to be largely driven by an upper to lower watershed gradient. Microhabitat and water quality (dissolved oxygen, pH, temperature, % less than 0.250 mm grain size, and % 0.125 mm), land cover (% agriculture, % wetland, % deciduous forest, % evergreen forest, and % mixed forest), and geology (% Alluvium, % Citronelle, % Deweyville, % Pascagoula, % coastal geology, and % high terrace geology) modified this gradient to locally influence CPUE and richness. This pattern has not been previously reported in the southern coastal plain and is strikingly similar to patterns of higher habitat quality and mussels in the upper watershed of River Raisin, also in Michigan [15]. Taken together, these results suggest opportunities and challenges to mussel conservation and restoration projects in the southeastern coastal plain. In this study, CPUE and richness were typically much lower compared to earlier studies employing timed-search methods in lower reaches of streams this region [80; 81] but were comparable to sites sampled in the Bogue Chitto River watershed by [56] (6.3 individuals per effort; average 1.5 species per site). Importantly, the author of [56] trained this sampling team to ensure comparable methods and effort, and we are confident that surveys accurately reflected mussel assemblages at the study sites. However, sparseness of the assemblages in these streams does not imply unimportance to regional biodiversity. Our collections included _Anodontooides radiatus_ and _Uniomerus declivis_, neither of which were found in recent (2012-2014) mainstream Pearl River mussel surveys ([82]; [PERSON], Baton Rouge U.S. Fish and Wildlife Conservation Office, personal communication), emphasizing the contributions of tributary streams to overall mussel diversity of larger river systems. Microhabitat data at the study sites were representative of coastal plain systems, i.e., low gradients, extensive woody debris, and silt/sand substrate mix [49; 83; 84; 85]. Mussels exhibited positive and negative associations with pH, but pH was likely less important than the correlated land cover and habitat variables in the PCA, as pH was below 6.6 in only 3 of the 27 streams and was not found to be an important structuring variable for mussel assemblages in Gulf of Mexico coastal plain streams and rivers [84].