Functional processes versus state variables: interstitial organic matter pathways in floodplain habitats

We assessed the potential rates of two microbial processes, cellulose decomposition potential (CDP) and hydrolytic activity, to explore the relationship of these functional measurements with several state variables of organic matter. Samples were collected from two interstitial sites in the main channel of the Rhone River and 10 sites on the alluvial floodplain representing five habitat types defined by sediment size (fine versus coarse sediments) and interstitial water origin (surface water versus groundwater). Although sites with fine sediments had more interstitial total organic matter, refractory and biodegradable dissolved organic carbon concentrations did not differ among habitat types. Unlike the floodplain, the main channel had high CDP and low hydrolytic activity. In the floodplain, functional measurements varied consistently, and both CDP and hydrolytic activity were lowest at the sites with coarse sediments. Our data imply that microbially mediated processes are different in the main channel and the floodplain and that low levels of organic matter in the coarse sediments are probably due to slow rates of accrual rather than rapid rates of decomposition. Furthermore, the lack of correlation between functional variables and dissolved organic carbon concentrations illustrates the dangers of interpreting ecosystem processes or ecological integrity based solely on state variables.