The influence of stream age and environmental variables in structuring meiofaunal assemblages in recently deglaciated streams

The influence of stream age and environmental variables on meiofaunal assemblages were examined for 15 streams across a temporal gradient of 200 yr in Glacier Bay National Park, southeast Alaska. Meiofaunal assemblages were found in all streams but varied in diversity and abundance. Copepod species found had a wide range of habitat affinities and good dispersal abilities; we argue that the observed copepod assemblage in a given Glacier Bay stream is determined by habitat availability rather than dispersal constraints. Two-way indicator species analysis (TWINSPAN) classified the meiofaunal assemblages largely according to stream age. Stepwise multiple regressions were performed on environmental variables and meiofaunal species richness or abundance, and predictors differed between taxa. The sorting coefficient, a measure of the diversity of particle sizes in the sediment, best explained the variation in both total harpacticoid copepod abundance and meiofaunal taxon richness (together with stream gradient). Harpacticoids were more abundant in relatively well-sorted streams and declined as the range of particle sizes within the streams increased; the opposite pattern to that found in the marine environment. The abundance of Bryocamptus spp. was best explained by models incorporating stream age and indicators of habitat complexity, whereas abundance of Tardigrada, Chaetogaster, and Moraria affinis increased with increasing substrate stability. A habitat templet was developed for the meiofaunal assemblages in Glacier Bay streams. Stream age (incorporating stream habitat complexity) formed one axis and the second axis was the sorting coefficient. Colonization and succession studies of streams and rivers at the spatial scale of entirely new river channels are limited for macroinvertebrates (Fisher 1990) and very rare for meiofauna (although see Gaviria 1998; Robertson and Milner 1999; Milner et al. 2000). Meiofaunal studies that do exist principally focus either on new channels with upstream sources of potential colonizers (e.g., flood alleviation channels; Gaviria 1998) or have examined the relationship between the environmental gradient occurring with increasing distance from a glacier terminus and the hyporheic assemblage in a single river (Malard 2003; Malard et al. 2003) or the larger scale temporal changes (100s to 1000s yr) in lentic assemblages following deglaciation (e.g., Duigan and Birks 2000). In contrast, studies of meiofaunal colonization at the patch-scale are relatively well represented (e.g., Palmer et al. 1992; SchmidAraya 2000), as are similar studies on macroinvertebrates that may include some members of the temporary meiofauna (young macrofauna that are of meiofaunal size) (e.g., Matthaei et al. 1997; Gjerlov et al. 2003). In this study, we focus on the permanent meiofauna, organisms remaining within the meiobenthic size category throughout their lifespans (retained by a 63-mm sieve but passing through a 1-mm sieve; Giere 1993), and use the spatial gradient of glacial recession in Glacier Bay National Park, Southeast Alaska, to elucidate reach scale comparisons across a temporal gradient of 200 yr. Specifically, we examine large-scale patterns of colonization and primary succession in stream meiofaunal communities following ice recession rather than small-scale patch colonization or rapid colonization from sources within the same watershed (e.g., Boulton et al. 1991; Palmer et al. 1992). As these are new watersheds with no remnants of previous biological communities, colonization involves dispersal from other stream systems and subsequent community development relies on primary successional processes. In contrast with many lotic macroinvertebrates, the permanent meiofauna do not possess winged dispersal stages. There are, however, several ways they could disperse into new stream systems in Glacier Bay, including dormancy and subsequent dispersal by wind, waterfowl, and animals (e.g., Caceres and Soluk 2002; Figuerola and Green 2002). The development of postglacial streams in coastal Alaska involves complex interactions among the fluvial system, riparian vegetation, and geomorphic processes influencing sediment supply (Sidle and Milner 1989). Increasing stream age is thus linked to changes in many other stream characteristics, such as higher temperatures, greater quantities of coarse particulate organic matter Acknowledgments We are grateful to the National Park Service for supplying the MV Stellar and to Captain Dan Foley and crew. We thank the many individuals who were involved in the fieldwork, particularly Chad Soiseth and Ian Phillips, and we acknowledge the support of the National Environment Research Council of the United Kingdom (grant GR9/2913) for funding toward this research. Peter J. A. Shaw was generous with his time in connection with data analysis. We thank two anonymous reviewers whose comments improved the manuscript. Limnol. Oceanogr., 51(3), 2006, 1454–1465 E 2006, by the American Society of Limnology and Oceanography, Inc.