## 4 Discussion
No adverse DO conditions were present at the time of sampling, though hypoxic (<2 mg/L) events and their associated effects on mussels and fishes [40; 41] could have occurred noturally or seasonally due to stream eutrophication and high BODs [86]. Although relatively high across study sites, DO levels were still positively associated with mussel CPUE and richness, suggesting higher DO or conditions that enhance and maintain higher dissolved oxygen (e.g., reaeration, higher flows, and shading) were important for mussels. The association of higher mussel richness and higher DO was also noted in [84], although they also recognized the association of higher DO with a suite of other characteristics associated with higher mussel abundances, including forested land cover. Low DO is common in Louisiana streams [87], and despite negative associations between low DO levels and mussel abundance (e.g., [41]), other studies also have sampled mussels in coastal streams characterized by very low DO (e.g., [40; 88]). In these Pearl River tributaries, it is likely that, similar to pH, associations with DO actually reflect associations with position in the watershed (i.e., upper watershed sites had higher DO and were where most of the mussels were found). Although stream temperatures were not highly variable and were lower than critical thermal limits [89; 90], analyses indicated mussels were negatively associated with higher temperatures, or the lack of shading in more open downstream channels that would promote increased temperatures. Higher temperatures increase mussel energetic expense, including oxygen consumption and regulation [34]. Although higher temperatures and lower DO could both have negatively impacted mussels, associations with temperature may also have reflected position in the watershed. Substrate size classes also influenced CPUE and richness, with CPUE and richness decreasing with increasing % less than 0.250 mm and % less than 0.125 mm. Although juvenile mussels have been positively associated with larger substrate particle sizes [15; 22; 43], adults are often associated with finer sediments, based on the idea that reduced habitat disturbance (e.g., velocity and turbulence) favors mostly fine substrates, which in turn is positively associated with mussel presence, species richness, and abundance [24; 35]. However, in this study, adult mussels were more abundant with higher richness in sites with relatively coarser substrate sizes. Generally, sites with higher fine sediment were associated with coastal deposits and geology lower in the watershed. However, a few sites with fine sediment (Adam's Creek, Silver Springs Creek, and Big Creek Tributary) were associated with more evergreen forest cover further from the Gulf of Mexico. These finer sediment sizes have been reported to reduce aquatic insect richness as well [86; 91; 92], and these results further support efforts to control excess fine sediment inputs in these coastal streams. Although not assessed in this study, it is likely the fine sediment in these sites was the result of past timber harvesting activities in these pine forests, which has been shown to contribute significant amounts of fine materials to coastal plain streams, particularly in the absence of best management practices [93]. Geology and land cover are intrinsically tied with position in the watershed [83] and may be affecting substrate composition and indirectly playing a role in mussel abundance and distribution. High areal coverages of developed land, primary agriculture, and low land cover percentages of evergreen forest suggested increased runoff may be limiting habitat suitability in some of these streams. High Terrace watersheds are often characterized by elevated soil erosion, bank failures, channel instability, flashy stream flow, and high rates of sediment displacement [86]. Increased urban/disturbed land area is associated with increased runoff and more erratic hydrology in small streams [94].