USGS Home
Contact USGS
Search USGS

Aquatic Macrophytes and Algae

Although the relative extent of macrophyte herbivory is not well documented in prairie pothole wetlands, it is generally recognized that most macrophyte production eventually ends up as detritus (Davis and van der Valk 1978b). The abundant production of detritus may be the most important source of nutrients and energy for the invertebrates in wetland habitats (Mann 1988, Murkin 1989). Either through direct consumption of decaying macrophyte tissue or the consumption of associated microbial fauna, this litter is generally thought to be extremely important to wetland invertebrates (Mann 1988). However, recent stable isotope studies have indicated that algae may be a more important food resource for invertebrates than macrophyte litter. Neill and Cornwell (1992) present evidence, based on stble isotope signatures, that aquatic macrophytes were not important sources of carbon for aquatic invertebrates in the Delta Marsh. This has been confirmed by a recent study in wetlands at the Woodworth Study Area (Wrubleski and Detenbeck, in prep.). δ¹³C signatures for most aquatic invertebrates closely matched algae, but not macrophytes (Fig. 21.3). This has been confirmed by a recent study by Wrubleski and Detenbeck (in prep.). Except for epipelon, δ¹³C signatures of algae were more deplete (more negative) than the aquatic macrophytes (Fig. 21.3). Most aquatic invertebrates also had δ¹³C signatures that were more deplete than the macrophytes and closely matched the algal δ¹³C signatures, indicating their reliance upon algae as their principal source of carbon.

Fig. 21.3. Stable carbon and nitrogen isotope ratios for the algae, macrophytes, and aquatic invertebrates sampled in 10 wetlands at the Woodworth Study Area, North Dakota, 1994-95. Shaded boxes for the algae (phytoplankton, periphyton, epipelon, and metaphyton) and macrophytes (floating, emergents, and submersed) represent the complete range of values obtained. Mean and ranges are presented for the following aquatic invertebrates: 1, Cladocera; 2, copeppods; 3, Glyptotendipes; 4, Tanypodinae; 5, Hydracarina; 6, Ephemeroptera; 7, Chaoboridae; 8, Chironomus; 9, Zygoptera; 10, Corixidae; 11, Notonectidae; 12, Dytiscidae; 13, Hydrophilidae; 14, Anisoptera; and 15, Gastropoda.

Algal communities in prairie wetlands have generally been ignored (Crumpton 1989, Murkin 1989, Goldsborough and Robinson 1996), and consequently the effects of herbivory on algae have not been studied. In other wetland habitats invertebrate grazing has been found to be important in structuring algal communities and overall productivity (Cattaneo 1983, Hann 1991, Botts 1993). The relative importance of the different algal communities is unknown. Metaphyton has been reported to be an important habitat for many aquatic invertebrates (Ross and Murkin 1993, Olson et al. 1995), but does not appear to be a food resource (Goldsborough and Robinson 1996). Evidence from stable isotopes suggests that phytoplankton and periphyton are important food resources to aquatic invertebrates during the summer period (Wrubleski and Detenbeck, in prep.). More effort is needed to determine the relative importance of each algal community, and how these values change over seasonal and longer-term wetland cycles.

Trophic Functions of Invertebrates in Prairie Wetlands

Aside from waterfowl, aquatic invertebrates are important food resources for passerines (Willson 1966, Mott et al. 1972, Voigts 1973, Twedt et al. 1991), shorebirds (Eldridge 1987), grebes, and other wetland birds. Adult aquatic insects (e.g., chironomids, dragonflies, mayflies) originating from wetland habitats provide an important food resource for many nonwetland birds as well (Busby and Sealy 1979, Sealy 1980, Guinan and Sealy 1987).

Tiger salamanders are a common amphibian found in prairie wetlands (Larson 1968, Buchli 1969, Deutschman 1984, Pterka 1987). Olenick and Gee (1981) reported that tiger salamanders were benthic and fed primarily on Gammarus. Deutschman (1984) reports that tiger salamanders mostly consumed Cladocera, chironomids, amphipods, ephemeropterans, and hemipterans and that larger prey (i.e., large amphipods and chironomids) were preferred over smaller prey such as cladocerans and copepods. While the consumption of large prey maximizes growth (Deutschman 1984), it is likely that most large invertebrates are consumed when they are available in prairie wetlands. At night, tiger salamanders float up in the water column to feed on invertebrates (Anderson and Graham 1967, Branch and Altig 1981). They use deep portions of wetlands as refugia from avian predators during the day. Much of the nocturnal movement of tiger salamanders is apparently from the center and deeper portion of wetlands towards the shoreline. Lannoo (unpublished data) found that funnel traps with openings oriented towards the deeper wetland center caught twice the number of salamanders as traps with openings oriented parallel to the shoreline. Interestingly, Corkum (1984) notes that over 60 percent of the movements of aquatic invertebrates also occurred at night and towards the deeper water of the wetland center.

Conditions in most prairie wetlands are not favorable for fish (Peterka 1989). Frequent drying, nonintegrated watersheds, and harsh winter conditions generally prevent fish from establishing permanent populations. However, they can become established through deliberate introductions. For example, fathead minnows (Pimphales promealas) are released in wetlands for rearing by the baitfish industry (Hanson and Riggs 1995), and rainbow trout (Salmo gairdneri) are released for sport and commercial harvest (Peterka 1989). As in other aquatic habitats, fish can be very important predators of aquatic invertebrates and potentially compete with waterfowl and other marsh birds for food (Swanson and Nelson 1970). Hanson and Riggs (1995) report marked reductions in invertebrate abundance, biomass, and taxa richness in wetlands stocked with fathead minnows. Recently, Bouffard and Hanson (1997) have concluded that fish in wetlands were incompatible with objectives established for waterfowl management, primarily due to the negative impact of fish on invertebrate communities.

Leeches, dragonflies, beetles, and other predaceous invertebrates are abundant in prairie wetlands, but there have been few studies of predator-prey relationships and competition among aquatic invertebrates within these habitats. The importance of these interactions in other aquatic habitats is widely recognized (e.g., Bay 1974, Kerfoot and Sih 1987). In prairie wetlands vertebrate predators such as fish are often absent, making predaceous invertebrates the top aquatic predators. Anderson and Raasveldt (1974) reported that Gammarus and Chaoborus were important predators of zooplankton in prairie lakes and ponds. Rasmussen and Downing (1988) observed that the spatial distributions of benthic dwelling chironomids were determined by the presence of the predatory leech, Nephelopsis obscura. Clearly, invertebrate predators may play an important role in structuring wetland invertebrate communities, but further research is needed to determine their relative importance.

Ecological Functions of Invertebrates in Prairie Wetlands

Aquatic invertebrates are often assumed to play a major role in decompositional pathways in wetlands (Polunin 1984, Mann 1988). However, there is relatively little evidence to support this assumption other than some generalized knowledge of feeding habits for some groups. In a study of macrophyte litter decomposition in the Delta Marsh, Bicknese (1987) reports that aquatic invertebrates had little influence on litter decomposition dynamics. This was further corroborated with stable isotope evidence which indicates that most invertebrates, are not feeding on wetland macrophyte detritus (Neill and Cornwell 1992; Wrubleski and Detenbeck, in prep.). Mining and burrowing activities by invertebrates within litter undoubtedly contribute to increased litter decomposition, but direct consumption does not appear to be important.

Wetlands are often regarded as sources of insect pests such as mosquitoes, horseflies, and deerflies. Most mosquito production in the PPR actually originates from temporary snowmelt and rain pools. For example, Aedes spp. lay their eggs in shallow dry depressions, and the eggs which hatch only when flooded by snowmelt or rainwater (Wood et al. 1979). One mosquito that does occur in more permanent waters and is of concern is Culex tarsalis. This species is the principal vector of western equine encephalitis, a serious viral disease for horses and humans (Wood et al. 1979).

Leeches are common ectoparasites on waterfowl (Trauger and Bartonek 1977). Amphipods, snails, leeches, and other wetland invertebrates act as intermediate hosts for a variety of bird intestinal parasites (e.g., LaBerge and McLaughlin 1989). Biting flies are also vectors of a variety of blood parasites for marsh birds (e.g., Meyer et al. 1974, Bennett et al. 1982). The relative impact of these parasites on wetland bird mortality is not well known (Sargeant and Raveling 1992).