Biological responses in off -channel habitat to hydrologic gradients and river management practices in Mississippi River Pool 25

Few studies have examined the biological responses to water-level management (WLM) in the Upper Mississippi River (UMR). The WLM in UMR Pool 25 creates extended dry periods whereby lower-pool backwaters dry extensively (∼60 days/yr). Macroinvertebrate and zooplankton communities and benthic organic matter (BOM) was compared in replicated paired plots, one vegetated and one experimentally de-vegetated, during 1999--2001. Differences in BOM between plot types were most pronounced in years following strong vegetation responses. Oligochaeta biomass was significantly higher in vegetated plots, whereas Chironomidae biomass was significantly higher in devegetated plots. Zooplankton density and community metrics indicated positive responses to vegetated plots. Responses varied with the degree of vegetation response, suggesting hydrologic variability can enhance invertebrate diversity in floodplain wetland habitats.;During 2001--2003, I examined the influence of WLM between mid and lower pool sites on BOM, macroinvertebrates, and fishes. Hydrologic models predict lower pool off-channel habitats were less stable and dry with greater frequency and duration. The BOM values were stable at mid-pool but lower pool values were variable. Multivoltine taxa had high biomass and dominated lower-pool habitats while longer-lived taxa had higher biomass in mid-pool sites with longer hydroperiods. Fish communities were dominated by cyprinids, but unique fish taxa were collected in each pool reach, with primarily rheophilic forms in mid-pool and limnophilic forms in lower pool sites. Results indicate that hydrologic gradients associated with a mid-pool control point directly and indirectly influences biological communities in off channel habitats.;During spring flooding in 2002, macroinvertebrate communities were examined in off-channel habitats representing four site types: island sloughs, island fringes, backwaters, and artificial substrates. Nonmetric multidimensional scaling analysis identified significant spatial structuring in physical and biological variables and vector analysis indicated substrates were linked to differences among sites. During the rising limb, island sloughs were different from island fringes (P<0.01). Between sampling dates, island sloughs were significantly different from island fringes and backwaters (all P<0.01). Post-flooding, no significant differences were evident among sites. Results underscore the importance of habitat diversity on fine spatial and temporal scales in this highly managed system, and suggest that management and restoration efforts should target island sloughs.