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Sedimentation of Prairie Wetlands

Effects on Primary Production

The production of aquatic macrophytes and algae is an important component of prairie wetland food chains. Aquatic macrophytes provide structural habitat for invertebrate and vertebrate life and also provide substrates for colonization by epiphytic algae and microbes that are important foods of aquatic invertebrates (Murkin 1989). Once macrophytes senescence, they contribute litter for colonization by microbes which provide additional food resources for aquatic invertebrates (Mann 1988). In addition to epiphytic algae, phytoplankton and epibenthic algae are also major sources of carbon in prairie wetlands and are important food resources of aquatic invertebrates (Murkin 1989; Neill and Cornwell 1992). Anthropogenic sedimentation has potential to suppress primary production and alter natural food chain interactions. Increased sediment in the water column generally reduces the depth of the photic zone and hence reduces the light available for primary production by aquatic macrophytes and algae (Ellis 1936; Robel 1961; Dieter 1991). As sediment falls out of suspension, deposition may be adequate to bury epibenthic algae, macrophytic photosynthetic substrates, and seed banks (Rybicki and Carter 1986; Hartleb et al. 1993; Jurik et al. 1994; Wang et al. 1994). Jurik et al. (1994) and Wang et al. (1994) demonstrated that sediment depths of 0.25 cm can significantly reduce species richnesss, emergence, and germination of wetland macrophytes. Jurik et al. (1994) also found that the greatest decreases in germination occured for species with the smallest seeds. Hartleb et al. (1993) showed that seed germination of water milfoil (Myriophyllum spicatum L.) was significantly reduced when buried by more than 2 cm of sediment, and Rybicki and Carter (1986) found that survival of water-celery (Vallisneria americana MICHX) tubers declined 90% when buried by 10 cm, and by 100% when buried by 25 cm of sediment. Although, these studies demonstrated the relationship between sedimentation and germination, the causative agent that inhibits germination or survival is poorly understood. For example, covering of seeds with varying depths of sediment may alter light and/or redox conditions that inhibit seed germination or the sediment may create a physical barrier to emergence.

The magnitude and timing of anthropogenically enhanced sedimentation may influence structure and recolonization of plant communities in prairie wetlands. Under natural conditions, plant communities in prairie wetlands are dynamic and undergo cyclic changes in response to short- and long-term water-level fluctuations and salinity. Four prairie wetland cyclic conditions were identified by van der Valk and Davis (1976): dry marsh, regenerating marsh, degenerating marsh, and lake. During the dry marsh or drawdown phase, sediments and seed banks are exposed and mudflat annuals and emergent plant species germinate and recolonize the wetland (van der Valk and Davis 1976). Since recolonization is dependent on viable seed banks, the covering of seed banks with sediment has potential to impede the recolonization process (Jurik et al. 1994). Additionally, loss of wetland volume from acclerated sedimentation makes wetlands shallower, which allows monodominant stands of cattails, normally restricted to water depths <60 cm (Bellrose and Brown 1941) to expand. Such stands of vegetation contribute little to biological diversity and exacerbate problems with agricultural interests because they provide roost sites for blackbirds that depredate cereal crops (Linz et al. 1996).

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