Freshwater Meiofauna and Surface Water-Sediment Linkages: A Conceptual Framework for Cross-System Comparisons

Meiofauna are ubiquitous and taxonomically diverse in the sediments of most freshwaters, from gravel bed streams to the muddy bottoms of lakes and ponds (Pennak, 1988; Hakenkamp & Palmer, 2000; chapters in this book). Densities of active meiofauna are especially high in the region of maximum biological and chemical activity in many aquatic habitats - the sediment-water interface (Wetzel, 1990). This raises functional questions about the likely roles of meiofauna in modifying many of the ecological processes that occur at this interface (Hakenkamp & Morin, 2000; Palmer et al., 2000). Such influences could occur via active means (e.g. burrowing activities) or passively, through changes to sediment permeability and structure. In turn, we can ask the converse question: how do these environmental gradients and interactions between surface and subsurface habitats influence the abundance and diversity of meiofauna in running and standing waters? Many ecological processes occurring in the 'dynamic ecotone' (cf. Gibert et al., 1990) of the upper sediments are mediated by hydrological exchanges between surface and pore-water, even in standing waters. The direction and magnitude of these exchanges of water dictate environmental conditions such as redox potential, food quality and availability, movement of sediments, and transport of dissolved nutrients and organic carbon (reviews in Brunke & Gonser, 1997; Duff & Triska, 2000) . Directly and indirectly, these variables and their interactions are likely to govern the distribution and abundance of meiofauna (e.g Strayer, 1994; Dole-Olivier & Marmonier, 1992a; Brunke & Gonser, 1997; Hakenkamp & Palmer, 2000). Conversely, activities of the meiofauna may influence these same environmental variables (e.g. via bioturbation) as well as the directions and magnitude of the water exchanges. Although freshwater benthic and interstitial macrofauna (i.e. invertebrates > 500/lm) burrowing and moving in the surface sediments have been implicated in altering subsurface environmental conditions (Boulton, 2000), there is little evidence for substantial bioturbative activity of meiofauna, even in fine sediments.