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Influence of Agriculture on Aquatic Invertebrate Communities of Temporary Wetlands in the Prairie Pothole Region of North Dakota, USA

Introduction


North Dakota consists of 17.870 million ha of land area of which 16.269 million ha is currently (1997) divided among 30,500 farms (ND Agricultural Statistics Service 1998a). In 1997, over 11 million ha of North Dakota's farmland were planted to small grains and row crops (ND Agricultural Statistics Service 1998b) requiring extensive manipulation of the soils and applications of agrichemicals. Some of these lands have been in crop production since the late 1800s when the region was first opened to homesteading. Wetlands in the prairie pothole region (PPR) of North Dakota have been severely altered by past and current agricultural practices. Dahl (1990) estimates that 49% of the wetland area originally present in North Dakota has been drained or filled, primarily for the production of agricultural crops. Wetland losses have also been severe in other parts of the PPR as well; Iowa has lost nearly 90% of its original wetland area while Minnesota and South Dakota have lost 42% and 35% respectively (Dahl 1990). Most remaining wetlands are degraded; upland catchment areas are often tilled which increases sedimentation rates (Martin and Hartman 1986, Gleason and Euliss 1996), chemical drift from adjacent fields enters wetlands (Grue et al. 1989), and water-level fluctuations are more variable than in wetlands within noncultivated landscapes (Euliss and Mushet 1996a). Moreover, wetland drainage has altered natural hydrologic cycles of remaining wetlands, and drainage of temporary and seasonal wetlands into semipermanent wetlands has altered historic water depths and hydroperiods throughout the PPR.

Of the 7 wetland classes identified by Stewart and Kantrud (1971), temporary wetlands are likely the most vulnerable to agriculture. During years of average precipitation, temporary wetlands contain water for very short periods, typically after spring runoff or other precipitation events. Temporary wetlands are often cultivated and typically produce crops during all but the wettest years (Stewart and Kantrud 1973). Thus, temporary wetlands in agricultural fields are frequently cultivated and receive direct applications of herbicides, pesticides, and fertilizers. Examining agricultural impacts on plant communities of wetlands in the PPR, Kantrud and Newton (1996) reported lower plant species diversity and greater percentages of unvegetated bottom in the wet meadow communities of agricultural wetlands; the wet meadow zone is the dominant zone of temporary wetlands in the PPR (Stewart and Kantrud 1971). Additionally, Kantrud and Stewart (1977, 1984) reported that waterfowl and other wetland birds rarely used intensively tilled wetlands with large areas of unvegetated bottom.

Efforts to assess and monitor condition of aquatic invertebrate communities of wetlands in the PPR have been hampered by high spatial and temporal variability (Euliss and Mushet 1996b). Much of the invertebrate community variability is attributable to the cyclical nature of climate in this region. The PPR routinely goes through periods of drought followed by abundant rainfall, with extremes in climate usually being separated by 10 or more years (Duvick and Blasing 1981, Karl and Koscielny 1982, Diaz 1983, 1986, Karl and Riebsame 1984). In addition to this strong interannual climate cycle, precipitation events can be extremely localized, often with drought and flood conditions occurring simultaneously within short geographic distances, both within and among ecoregions. Wetland biota in the PPR are well-adapted to the naturally dynamic hydrology of the area and are also highly dynamic, both in abundance and community composition. Hence, presence or absence of a particular species at a certain time may not reflect general wetland condition.

Branchiopoda, including water fleas (Cladocera), fairy shrimp (Anostraca), clam shrimp (Conchostraca), and tadpole shrimp (Notostraca), are wide-spread residents of freshwater habitats (Hutchinson 1967), and they produce drought-resistant resting eggs. Resting eggs present in wetland sediments are the faunal equivalent of hydrophytic seed banks (hereafter referred to as "egg banks"). Like seed banks, egg banks remain viable in dried soils for extended periods, possibly decades (Pennak 1989). Other invertebrates of temporary wetlands leave behind evidence of their existence in the form of shells and cases after a wetland dries. Snails, ostracods, and conchostracans have decay-resistant shells that remain in the soils of temporary wetlands after they dry in the summer. Trichopterans construct cases that are an easily identified indicator of their past presence long after the case builder is gone.

We evaluated an invertebrate sampling approach that is less affected by the natural variability of free-living invertebrates in an effort to develop techniques to monitor wetland condition for the U.S. Environmental Protection Agency's (EPA) Environmental Monitoring and Assessment Program (EMAP). As wetlands pass through drought cycles, invertebrate communities respond to changes in hydrology and chemistry. During severe drought, wetlands dry out completely. Upon reflooding, invertebrates recolonize these wetlands using a variety of means, including resting eggs and cysts, diapause, and aestivation (Wiggens et al. 1980, Euliss et al. 1999). Resting eggs, cysts, and nonactive invertebrates remaining in wetland sediments, plus the decay-resistant shells and cases left from previous cohorts provide an integrated signature of the invertebrate community over time. They also provide a reflection of potential invertebrate community structure when wetlands reflood. Dry soil from wetland sediments contains invertebrate eggs, decay-resistant shells, and other body parts that can provide a historical measure of the invertebrate community of a wetland. Long-term impacts of land use on aquatic invertebrates are reflected in these invertebrate signatures.

The primary goal of our study was to determine if taxon richness and abundance of invertebrate resting eggs, shells, and cases remaining in the soils of temporary wetlands were reduced by agricultural activity. Additionally, we compared viability of cladoceran resting eggs collected from temporary wetlands within agricultural and grassland dominated watersheds to assess impacts of intensive agriculture on population viability.


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