AQUATIC CONSERVATION: MARINE AND FRESHWATER ECOSYSTEMS Aquatic Conserv: Mar. Freshw. Ecosyst. 20: 655–667 (2010) Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/aqc.1138 Diversity, abundance, and size structure of bivalve assemblages in the Sipsey River, Alabamay WENDELL R. HAAGà and MELVIN L. WARREN JR US Forest Service, Center for Bottomland Hardwoods Research, Oxford, MS, USA ABSTRACT 1. Patterns of mussel diversity and assemblage structure in the Sipsey River, Alabama, are described. Qualitative data were used to describe river-wide patterns of diversity. Quantitative data were used to describe the structure of mussel assemblages at several sites based on whole-substrate sampling that ensured all size classes were detected. 2. Major human impacts to the stream are limited to apparent effects of coal mining in the headwaters and the impoundment of the lower 9 km of the river by a dam on the Tombigbee River. These impacts resulted in a sharp decline in mussel diversity in the headwaters, and extirpation or decline of populations of several large-river species in the lower river that were probably dependent on colonization from the Tombigbee River. 3. Despite localized impacts, mussel assemblages throughout much of the river appear to be mostly intact and self-sustaining. These assemblages have several attributes that differ substantially from those in more degraded streams: (1) high retention of historical species richness; (2) gradual, longitudinal increase in species richness from upstream to downstream, resulting in distinctive headwater and downstream assemblages; (3) ubiquity of most species within particular river segments; (4) low dominance and high evenness with large populations of many species; and (5) frequent recruitment for most species resulting in occurrence of individuals in many size classes. 4. Few detailed and demographically unbiased descriptions of relatively intact mussel assemblages exist. We propose that characteristics described in the Sipsey River can be used as a baseline comparison for assessing relative degree of assemblage alteration in other streams and can serve as goals for restoration efforts. Published in 2010 by John Wiley and Sons, Ltd. Received 11 March 2010; Revised 9 June 2010; Accepted 16 July 2010 KEY WORDS: freshwater mussels; Unionidae; Corbicula; community; demography; recruitment; viability; conservation; Mobile Basin INTRODUCTION characteristics of mussel assemblages relatively uninfluenced by human impacts is limited. The structure and function of most freshwater mussel Understanding the natural structure of mussel assemblages assemblages is altered by a wide variety of human impacts is important from ecological and conservation perspectives. (Bogan, 1993; Neves et al., 1997; Regnier et al., 2009) in several Elucidation of mechanisms responsible for the origin and characteristic ways: loss of a substantial portion of original maintenance of diverse mussel assemblages can be confounded species diversity, depressed population sizes, altered patterns if observed assemblage structure is largely a legacy of human of species relative abundance, and highly skewed size and age impacts. Similarly, lack of information about unaltered frequency distributions, usually with a preponderance of large, assemblages makes it difficult to specify desired assemblage old individuals (Holland-Bartels, 1990; Layzer et al., 1993; attributes as goals for restoration efforts. The rarity of young Ahlstedt and McDonough, 1993; Poole and Downing, 2004; age classes in many populations portends further, widespread Warren and Haag, 2005). Historical literature documents declines, but demographic attributes of stable populations are original mussel faunas of many regions (Clark and Wilson, unknown. Until population dynamics of mussels are better 1912; Ortmann, 1918; Isely, 1924) but provide only presence/ understood, assessments of population viability in degraded absence data or, at best, qualitative observations on species streams can be aided by using demographic data from relative abundance. Consequently, knowledge of the relatively unaltered assemblages as a comparative baseline. *Correspondence to: Wendell R. Haag, US Forest Service, Center for Bottomland Hardwoods Research, Forest Hydrology Laboratory, 1000 Front Street, Oxford, MS 38655, USA. E-mail: [email protected] yThis article is a US Government work and is in the public domain in the USA. Published in 2010 by John Wiley and Sons, Ltd. 656 W. R. HAAG AND M. L. WARREN JR In addition to the scarcity of unaltered assemblages available METHODS for study, accurately characterizing mussel assemblages is difficult and time consuming due to the burrowing habits of Study area these animals. Most descriptions of mussel assemblage structure are based on visual sampling methods, which are The Sipsey River is a tributary of the Tombigbee River biased against small species and size classes or species that (Mobile Basin), draining an area of 2044 km2 (McGregor and burrow deeply (Miller and Payne, 1993; Hornbach and O’Neil, 1992). The headwaters, including the New and Little Deneka, 1996). The few studies that used methods to reduce New Rivers, arise on uplands of the Cumberland Plateau this bias were conducted in highly modified habitats (Payne physiographic province and flow through narrow floodplains and Miller, 1989, 2000; Haag and Warren, 2007). (o1 km) in predominantly sandstone lithology. A short In this study, river-wide patterns of mussel diversity are distance downstream of the confluence of the New and Little evaluated in a relatively undisturbed river and related to major New Rivers, the Sipsey River enters the East Gulf Coastal human impacts and geomorphic factors. In addition, Plain physiographic province and flows about 184 km to the assemblage structure is quantitatively described at several Tombigbee River (Figure 1, McCullagh et al., 2002). Within sites using data from whole-substrate samples to ensure all size the East Gulf Coastal Plain, the river flows mostly through classes were sampled. From this, differences in local Cretaceous gravels of the Fall Line Hills district but enters the assemblage structure are assessed, local population sizes are Black Prairie district about 25 km upstream from its mouth estimated, and patterns of size structure among sites and (Boschung and Mayden, 2004). In the Coastal Plain, the river species are evaluated. occupies an increasingly broad alluvial floodplain averaging Figure 1. Map of Sipsey River, Alabama. Roman numerals indicate river reaches and numbered circles indicate location of quantitative study sites. Inset map shows location of the Sipsey River in Alabama, USA. Published in 2010 by John Wiley and Sons, Ltd. Aquatic Conserv: Mar. Freshw. Ecosyst. 20: 655–667 (2010) FRESHWATER BIVALVE ASSEMBLAGE STRUCTURE 657 3–9 km wide in the middle and lower reaches (measured from contemporary sites; no pre-1981 data exist for the lower river 1:24,000 US Geological Survey topographic maps) and (McCullagh et al., 2002, J. Williams personal communication, characterized by extensive, marginal wetlands including 7 June 2010). To examine diversity patterns unrelated to many secondary channels and oxbow lakes. human impacts, a matrix of species presence/absence in each Much of the Sipsey River is relatively unmodified by human reach was created for both time periods combined. To examine activities, and major impacts are restricted to the upper and temporal changes in diversity in the upper river, species extreme lower sections. The headwaters within the Cumberland occurrences were also compiled separately for the two time Plateau are affected by coal extraction (Pierson, 1991a; see also periods. Assemblage differences among reaches (both time Warren and Haag, 2005), and the greater relief and narrow periods combined) were tested with the multi-response floodplain results in increased sedimentation and channel permutation procedure (MRPP; PC-ORD, McCune and instability from streambank clearing and other sources of Mefford, 1999) (presence/absence, Euclidean distance, 34 erosion (Pierson, 1991a). Because these impacts are restricted to sample sites  41 species, sites grouped by seven reaches). a small percentage of the river, their effects apparently are Because the data matrix contained many zero entries, Beal’s largely attenuated in the middle and lower river. Human smoothing transformation was applied to the matrix to reduce impacts in the middle and lower river are associated mostly problems of zero-inflated data (Beal, 1984; McCune, 1994). with timber harvest. Although commercial pine plantations and The transformation replaced each cell in the matrix with the timber harvest occur throughout the basin, extensive marginal probability of the target species occurring in that particular wetlands preclude most logging within much of the riparian reach based on the joint occurrences of the target species with zone and serve as a buffer from effects of more intensive timber species actually occurring in the reach (McCune and Grace, harvest further from the stream. The lower ca 9 km of the river 2002). For pairwise comparisons among reaches, P-values was impounded in 1976 by Howell Heflin Lock and Dam were adjusted to Po0.05 using the sequential Bonferroni (Tennessee–Tombigbee Waterway) on the Tombigbee River. procedure for multiple comparisons (Holm, 1979). The Effects of impoundment extend to about 1.5 km downstream of chance-corrected within-group agreement statistic (A) was the Alabama Highway