Five distinct macroinvertebrate assemblages were identified using cluster analysis of the rank-order abundances of 13 orders in 15 freshwater springs of central Pennsylvania, U.S.A. A principal components analysis of 20 environmental factors indicated that an insect-dominated assemblage occurred in low pH, softwater sandstone springs and an amphipod-dominated assemblage was associated with medium to hardwater springs with a silt to gravel substratum. Three other assemblages were found in hardwater limestone springs: the peracaridan-hydrobiid assemblage was characterized by dense macrophyte beds and a silt to cobble substratum, the peracaridan-triclad-glossosomatid assemblage by a rubbly, gravel-cobble substratum and relatively high discharge, and the peracaridan-hydrobiid-oligochaete assemblage by a silty substratum and dense mats of green algae.Most of the common macroinvertebrate species were associated with specific physical, chemical, and (or) vegetational factors, but abundance associations between species pairs were generally lacking. We infer that assemblages are primarily determined by the match between the environmental mosaic of individual springs and the ecological requirements of the available species, rather than by strong interspecific interactions.Species-environment associations were assessed in an additional study of five closely neighboring springs in which some environmental variables of the wider study were nearly constant.
The major topographic features and river courses of the mid-Appalachian Mountains are geologically ancient. Small rheocrenes are numerous in carbonate valleys with macroinvertebrate assemblages typically dominated by peracaridans and sometimes gastropods, with subordinate abundances of bivalves, triclads, and insects. Springs were approximately rank ordered by temporal persistence, using size, catchment area, proximity to base level, and bedrock permeability factors as criteria. A 38-m2 rheocrene, Ell Spring, was sampled seasonally over a 2-year period for distribution and abundances of taxa. Physicochemical factors and rank order of ordinal abundances were stable the 1st year, but less so the 2nd year after a watercress cover was removed. Ell Spring is divided into nine distinct habitat patches. Some species distributions are strongly associated with patches and others are broader. Regionally, heterozygosity and allele frequency patterns of Gammarus minus (Amphipoda) are conditioned by latitude, indicative of the effects of Pleistocene glaciation, and by distance to regional master streams. These factors do not detectably influence the ordinal composition of macroinvertebrate assemblages. However overall invertebrate abundances and the ratio of non-insect orders (which are presumably less rapid colonists) to insect orders are greater in long-persisting than in frequently disturbed springs. The species assemblages of disturbed springs may be influenced by recent history as well as by water chemistry, substratum, and other equilibrium factors.
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