USGS Home
Contact USGS
Search USGS

Herbivores and detritivores that live on wetland substrates feed on fallout from the water column and epipelic algae. Those that occupy the water column consume plankton and suspended organic particulates. Plant-associated invertebrates feed mostly on periphyton. Macroinvertebrates that live on the water surface (neuston) are primarily predators or scavengers.

Prairie-wetland invertebrate species structure is controlled by basin hydrologic functions and associated chemical characteristics. The length of time between spring flooding and summer drawdown determines what fauna can occupy a given wetland basin. Temporarily and seasonally flooded wetlands are dominated by invertebrates that can complete their life cycles before the basins dry out. Invertebrates that neither fly, tolerate extended periods of desiccation, nor produce eggs that tolerate desiccation are eliminated during drought, and require some form of dissemination to facilitate their reintroduction into nonintegrated isolated wetlands (Swanson 1984). Transport mechanisms involved in reintroduction are important to our understanding of the ecology of wetland complexes in the Prairie Pothole Region (Swanson 1984).

Basin morphology, as well as hydrologic function and water quality, influences the fauna that will occupy a prairie wetland (Swanson et al. 1984). Wetland P1 of the Cottonwood Lake area (Swanson 1987a) contains a large, shallow bay dominated by wet meadow and shallow marsh (Figure 15). This wetland (foreground, Figure 16) has no surface outlet, and as a result, responds to surface runoff by flooding and maintaining a large shallow-marsh zone nearly every spring. The invertebrate fauna of this zone is typical of the many isolated seasonally flooded basins in the area. Wetland P8 (background, left, Figure 16) is located in a valley of high relief and also contains a surface outlet (Figure 17). As a result, water levels are stable, and this wetland is dominated by open water and a deep-marsh zone and supports a fauna similar to the deep-marsh zone of wetland P1. The stable water and abruptly sloping shoreline greatly restrict the wet-meadow and shallow-marsh zones and their associated fauna.

Figure 15. Wetland P1 of the Cottonwood Lake, ND, study area showing vegetative zones.

The invertebrate taxa associated with prairie wetlands are typical of the many taxa described for eutrophic and alkaline lakes and temporary and vernal ponds of the United States by Pennak (1978). There is little work, however, on the systematics of aquatic invertebrates that occupy the different wetland classes in the Prairie Pothole Region of North America (Murkin 1989). Some exceptions are the aquatic mollusk (Clarke 1973, 1981; Cvancara 1983), zooplankton (LaBaugh and Swanson 1988), and the aquatic Coleoptera (Hanson and Swanson 1989). Macroinvertebrates associated with semipermanently, seasonally, and temporarily flooded basins have been described in feeding-ecology studies of breeding waterfowl summarized by Swanson and Duebbert (1988). Schultz (1987) described the invertebrate taxa of semipermanently flooded basins in Day County, SD, during the summers of 1984 and 1985.

The aquatic mollusks of North Dakota consist of 44 species, of which 13 are mussels, 9 are pill clams, and 22 are snails (Cvancara 1983). Snails, however, account for a major segment of the fauna of seasonally and semipermanently flooded prairie wetlands. Twenty-one species of snails have been identified in the Hudson Bay drainage of North Dakota and eighteen species in the Mississippi River drainage. Mussels and most pill clams are confined to flowing water and permanent lakes.

Figure 16. Wetland P1 of the Cottonwood Lake, ND, study area in the foreground. Wetland P8 in the background to the left.

Figure 17. Wetland P8 of the Cottonwood Lake, ND, study area showing vegetative zones.

Helisoma trivolvis-Lymnaea stagnalis associations are found in permanent and semipermanently flooded lakes. Molluskan species that are part of this association include Cincinnatia cincinnatiensis, Amnicola limosa, Pisidium nitidum, Valvata tricarinata, Helisoma anceps, Physa gyrina, Gyraulus parvus, and Stagnicola elodes (Cvancara 1983). The first five species are found primarily in permanent lakes. However, permanent lakes that contain large semipermanently and seasonally flooded zones will contain molluskan species that are associated with these zones.

Wetland basins dominated by deepmarsh and shallow-marsh zones support pulmonate snails that produce relatively thin shells and do not have gills. Members of this subclass can occupy habitats that routinely exhibit anaerobic conditions. They respond to low dissolved oxygen by moving to the surface where they can use atmospheric oxygen. Factors influencing gastropod distribution include water permanency, total dissolved salts, dissolved oxygen, and substrate characteristics. Waters with >20 mS/cm conductivity do not support mollusks. Some species, however, such as Stagnicola elodes, Physa jennessi, and Armiger crista, tend to tolerate elevated concentrations of dissolved salts (Cvancara 1983). Mollusks occupy spring seeps associated with saline lakes (Swanson et al. 1984). Mollusk scientific names used in this report follow those used by Clarke (1981). Aquatic mollusks play an important role in the food web of prairie wetlands (Cvancara 1983; Swanson et al. 1974; Pennak 1978).

Crustaceans are a major component of the invertebrate fauna of prairie wetland complexes. Species structure within a wetland complex is influenced by hydroperiod and salinity. Two species of amphipods, Gammarus lacustris and Hyallela azteca, dominate the macrocrustacea of semipermanently flooded wetlands. The former species occupies wetlands that have an extended wet phase, are low in dissolved salts, and have relatively deep, cool water. Hyallela azteca occupies most semipermanently flooded wetlands. Both species routinely invade seasonally flooded wetlands by attaching themselves to aquatic birds and mammals during the summer months, but are eliminated during drawdown (Swanson 1984). Amphipods are replaced by salt-tolerant Anostraca as salt concentrations increase (Swanson et al. 1984).

Decapods are not found in nonintegrated wetland basins in southcentral North Dakota, but are found in oxbow lakes and integrated wetlands in the headwater areas of tributaries that feed perennial river systems.

Eubranchiopods, such as Anostraca (fairy shrimp), Conchostraca (clam shrimp), and Notostraca (tadpole shrimp), are the dominant macrocrustaceans occupying temporarily and seasonally flooded basins in the Prairie Pothole Region. Fairy shrimp are also found in large wet meadow and shallow-marsh zones of semipermanently flooded wetlands. Species composition of fairy shrimp populations change with increasing salt concentrations of the water column. Amphipods and fairy shrimp are occasionally present in the same wetland. Brine shrimp (Artemia salina) predominate where salt concentrations are >35 mS/cm.

Invertebrates occupying the water column of a prairie wetland complex in the Cottonwood Lake, ND, study area are listed in Table 4. Wetland T8 is seasonally flooded and functions as a ground-water recharge area. Wetland T3 is a seasonally flooded through-flow system. Wetland P1 is a nonintegrated, semipermanent wetland that has no surface outlet and functions as a ground-water discharge area. Wetland P8 is a through-flow, semipermanent wetland that contains a surface outlet. Wetland Pll is a semipermanent wetland that has no surface outlet, functions as a ground-water discharge area, and concentrates salts by loss of water to the atmosphere.

Insects, particularly Coleoptera, Odonata, and Hemiptera, play an important role in the ecology of prairie wetlands by assuming the role of top predators within the water column. Wetland insects differ from the gastropods and crustacea of prairie wetlands in that most are capable of flight and can auickly invade shallow wetlands soon after they receive water. This applies especially to adult Coleoptera; members of this group are found in seasonally flooded wetlands soon after spring snowmelt is complete (Hanson and Swanson 1989).

Descriptions of insects often omit information on habitat. Light traps that attract adults from a relatively large area are routinely used for collecting. This procedure, while efficient for taxonomic studies, cannot be used to associate adults with the types of aquatic habitat that support larvae.

Insects are similar to gastropods and crustacea in that their species structure is influenced by hydroperiod and salinity. A study of the Coleoptera of seasonally and semipermanently flooded wetlands in a Cottonwood Lake wetland complex demonstrated differential use of these wetlands by species (Table 5). Of the 57 species identified, 47 and 38 were collected on seasonally and semipermanently flooded wetlands, respectively (Hanson and Swanson 1987); 18 and 11 species were found only on seasonally and semipermanently flooded wetlands, respectively. Individual species showed preferences for one of the wetland classes even though they occupied both. For example, only one Agabus antennatus was found on seasonally flooded wetlands, compared to 172 specimens collected on semipermanently flooded wetlands. On the other hand, 88 Hydroporous fuscipennis were found on seasonally flooded wetlands, but only 2 on semipermanently flooded wetlands. Of the Coleoptera reported for the State of North Dakota 39% were found on the 97-ha Cottonwood Lake study area (Hanson and Swanson 1989).

Nelson and Butler (1987) examined seasonal abundance of larval chironomids and emergence patterns of adults for two years on two wetland classes on the Cottonwood Lake study area. They found that seasonally flooded wetlands had lower larval densities than semipermanently flooded wetlands, and that densities of emerging adults were lower in the seasonally flooded sites.

Fresh and oligosaline wetlands (Cowardin et al. 1979) in the Prairie Pothole Region support a typical species structure of aquatic invertebrates as described for the United States by Pennak (1978), but as salt concentrations increase, only the most salt-tolerant groups such as the shore flies (Ephydridae) persist.