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Wetland macroinvertebrates at the Sterling Wildlife Management Area in southeast Idaho, USA.

ABSTRACT

The goal of this research was to investigate presence of macroinvertebrates (insects, mites, snails, and worms >500 [micro]m) and their availability for waterfowl consumption in the Sterling Wildlife Management Area (SWMA) by documenting colonization of wetland plant litter. Litter bags containing Typha latifolia (cattail) or Schoenoplectus acutus (bulrush) litter were placed in groups on the wetland surface at five locations within predominant stands of each plant species and removed on two sampling dates during the irrigation season (April-September 2001 and 2002). The wetland received irrigation flows in summer but dried in the winter. Predominant taxonomic macroinvertebreate groups in the SWMA were snails (Gastropoda, Planorbidae), mites (Acariformes, Calyptostomatidae), and midges (Chironomidae: Chironominae and Tanypodinae). Shifts in functional feeding group densities over the irrigation season were detected by shifts in the predominant taxonomic groups. Simpson's Diversity (H') increased significantly between July and August both years but was the only community measure to differ significantly over time. Although shifts in taxa were observed, abundance of macroinvertebrates remained consistent throughout the irrigation season and the availability of macroinvertebrates for waterfowl consumption remains consistent. The purpose of this research was to describe the benthic macroinvertebrate community associated with two dominant macrophytes common to WMAs throughout Idaho. Macroinvertebrates within a WMA in a semi-arid location in Idaho were described to investigate their availability as a food source for waterfowl.

Key Words: wetland, macroinvertebrates, functional feeding groups, diversity, waterfowl

INTRODUCTION

Temperate wetlands are some of the most productive ecosystems on earth (Westlake 1963). As a result, wetlands support complex food webs with the large pools of carbon and macronutrients associated with live and senescent macrophytes (Mitsch and Gosselink 2000). Macrophytes (aquatic vascular plants) affect ecosystem processes in wetlands and aquatic habitats. Rooted aquatic plants link the sediments with the overlying water and air; this living interface strongly influences the strata (the chemical, physical, and biological properties of each medium). Similarly, the annual decomposition of senesced litter has considerable impact on each of these strata (Carpenter and Lodge 1986) and in many wetlands litter exceeds biomass. Upon senescence, macrophytes enter the detrital (fragmented dead litter) food chain as autochthonous litter, are rapidly colonized by bacterial algae, and collectively this microbial/detrital milieu becomes important as a food source for macroinvertebrates (insects, mites, snails, and worms >500 [micro]m) (Wetzel 1975, Smock and Harlowe 1983). Consequently, macroinvertebrates are important processors in marsh ecosystems (Johnson et al. 2000) and provide a link in the food chain by becoming available for consumption by predators.

Because macroinvertebrates play an important role in the food web, aid in detritus processing, and can be used as environmental indicators, researchers have more recently focused on benthic macroinvertebrates to better understand wetland function (Fairchild et al. 1999). Although the role of certain macroinvertebrate species in wetlands has been described (Johnson et al. 2000), little is known about macroinvertebrates in constructed wetlands associated with agricultural land in the arid west.

Based on their ecological importance within the food web, macroinvertebrate functional feeding group analysis and community measures may reveal certain ecological values (Fairchild et al. 1999). Macroinvertebrate productivity is important for waterfowl brood movement and foraging behavior (Cooper and Anderson 1996). A diverse macroinvertebrate fauna may provide greater success in meeting dietary requirements (Magee et al. 1993). In addition, breeding success of waterfowl is known to be influenced by macroinvertebrate abundance (Krapu and Reinecke 1992). Waterfowl Wildlife Management Areas (WMAs) are typically managed with monospecific or stands of few different macrophyte plant species. The purpose of this research was to describe the benthic macroinvertebrate community associated with two dominant macrophytes common to WMAs throughout Idaho. Macroinvertebrates within a WMA in a semi-arid location in Idaho were described to investigate their availability as a food source for waterfowl.

STUDY SITE

The Sterling Wildlife Management Area (SWMA) is north of the Snake River near the American Falls Reservoir and the City of Aberdeen, Idaho (Figure 1). The SWMA was established in 1968 for the management of waterfowl production and provides a wide variety of waterfowl and shorebird species (IDFG 2005). Water flows from irrigated fields into a deep water pond and several deep and shallow wetlands (approximately 1 hectare) containing single species stands of common cattail (Typha latifolia) and hardstem bulrush (Schoenoplectus acutus). This study was conducted within a shallow emergent wetland area that was continually flooded from April-September but dry during the remainder of the year.

Mean annual precipitation at the site is approximately 30 cm, with summer air temperatures frequently over 37[degrees] C (NCDC 2001). The area is composed primarily of Quaternary basalt with sediments deposited by the Bonneville flood about 15,000 years ago. A series of lakebeds, deposited after intermittent basalt eruptions diverted and dammed the surface runoff, underlie the American Falls Reservoir and much of the surrounding area (Low and Mullins 1990). Historically, the soils on the sites are Declo loam, a non-hydric well-drained soil. However, over time hydric soil characteristics have developed within the SWMA and organic soils were present from the surface to 20 cm deep.

[FIGURE 1 OMITTED]

METHODS

Field studies were conducted during the irrigation season (April-September) over a two-year period (2001-2002). Senesced leaves and shoots of T. latifolia and S. acutuswere harvested from the wetland in late March 2001 and 2002 and provided as a substrate and possible food source for macroinvertebrate colonization. On April 15, two litter bags containing T. latifolia and one empty bag were placed in five groups within a stand of T. latifolia and two litter bags containing S. acutus along with one empty bag were placed in five groups within a stand of S. acutus at the SWMA. One randomly-selected litter bag of each plant species was removed from each of the five groups and an empty bag was removed from within each area at each of the two sampling dates: June 15 and August 15, respectively. A 50 [micro]m sieve was placed under the bag as it was removed and the contents of the bag and sieve were immediately placed in a Ziploc[C] bag on ice for transport to the laboratory. The experimental design was replicated in 2002 except an additional 10 bags (five containing S. acutus and five containing T. latifolia) were placed in both stands in April 2002 and removed in August 2002. In the laboratory, litter from the 5-mm mesh bags was gently rinsed over 250 [micro]m and 50 [micro]m mesh sieves to capture fine sediments and macroinvertebrates. The contents of the 50 [micro]m sieve, representing the fine sediment, was dried to constant weight at 60[degrees] C and weighed. In 2002 the fine sediment from the bags also was ashed in a muffle furnace at 550[degrees] C to determine the percent organic: inorganic of the sediment. Macroinvertebrates were sorted under a microscope (10x magnification), preserved in 70% ethanol, and later identified to family or genus.

Temperature has been linked to leaf breakdown rates in aquatic systems (Robinson and Jolidon 2005) therefore, air and water temperatures were recorded at two-hour intervals over the duration of this study, using HOBO temperature data loggers (Onset Computer Corporation, Bourne, Massachusetts, USA). Two data loggers were placed on the wetland surface adjacent to litter bags at opposite ends of the wetland. Degree-days were calculated by summing the average daily temperature over the sampling period for each wetland. This represents an integrated measure of the wetland's total heat budget (Royer 1999).

[FIGURE 2 OMITTED]

Macroinvertebrate communities are complex so functional feeding group (FFG) analysis compresses the richness and rareness issues and shift the focus from individual species to that of guilds of organisms and their links between food resources (plant litter and periphyton) and morphological and behavioral feeding adaptations (Smock and Harlowe 1983). FFG guilds can be a valuable tool for general description and analysis of wetlands (Resh and Rosenberg 1984, Mitsch and Gosselink 2000); however, they ignore the importance of taxa variability and richness within FFGs (Wissinger 1999). For example, taxonomic richness and diversity provide and an indirect measure of foodweb structure and secondary productivity for vertebrates (Fairchild et al. 1999), which provide better insight to ecosystem stability and function than FFG analysis alone (Wissinger 1999). Therefore, differences in community measures (i.e. diversity and taxa richness) as well as FFG guilds were evaluated (Merritt and Cummins 1996).

The relative abundance, Simpson's Diversity Index (H'=1 [SIGMA][p.sub.i.sup.2]) taxa richness, and density of functional feeding groups were calculated for all macroinvertebrates colonizing the leaf litter. Differences in the density of functional feeding groups (FFG) and community measures (abundance, density, diversity, and richness) of macroinvertebrates between sampling dates (June 15-August 15) were compared with a t-test (SPSS 1999). Relationships between macroinvertebrate community measures and sediment and degree-days were compared with a Pearson correlation (SPSS 1999).

RESULTS

No significant differences in macroinvertebrate abundance, diversity, and richness were found between the two vegetation types in June and August 2001 and 2002 at SWMA. Therefore, all the data from the cattail and bulrush areas were combined for each sample date. Overall, the predominant taxonomic groups were snails (Gastropoda, Planorbidae) and mites (Acariformes, Calyptostomatidae). However, in June 2002, the predominant taxonomic groups in the SWMA were snails, mites, and midges (Chironomidae: Chironominae and Tanypodinae). Overall, the aquatic functional feeding groups with the highest average proportional densities were scrapers (40%; snails), parasites (18%; mites), and collector-gatherers (24%; midges). A shift in the density occurred between June and August during both years from higher scrapers (60% in 2001 and 38% in 2002) and parasites (30% in 2001 and 24% in 2002) to collector gatherers (20% in 2001 and 46% in 2002). These shifts corresponded with the predominant taxa present at the time.

Between June and August 2001 average macroinvertebrate richness (p=0.041) and Simpson's diversity (H') (p=0.017) increased significantly (richness increased from 3.6 to 6.0 and diversity from 0.4 to 0.6) but abundance and density did not. In 2002, only Simpson's diversity increased significantly (p<0.001) from June to August (from 0.3 to 0.6, Figure 2).

Sediment accumulation on litter bags did not differ significantly between the plant species or sampling dates in or 2001 and 2002. The organic fraction was >85% for all the sampling dates. Using all the bags from both species, there were no relationships found between macroinvertebrate community measures and sediment mass or degree days during any of the sampling dates in 2001 or 2002.

DISCUSSION

Macroinvertebrates are important processors in marsh ecosystems (Johnson et al. 2000) and then become available for predatory consumption. Predominant macroinvertebrate taxa in the SWMA were mites, gastropods, and midges. These taxa are well adapted to colonizing new habitats, quickly reestablish after flooding (Wiggins et al. 1980, Brooks 2000), and are important for waterbird diet (Magee et al. 1993). Dabbling ducks prefer shallow, ephemeral wetlands with emergent vegetation like the SWMA and several species of dabblers often use the SWMA (IDFG 2005).

Temporary temporal shifts in richness and diversity are common within aquatic habitats and a significant increase in Simspon's diversity of macroinvertebrates, typical of temporarily flooded wetlands, was observed in the SWMA from June to August 2001 and 2002. This increase indicates that waterfowl at SWMA may have a greater opportunity to meet dietary requirements later in the irrigation season (Magee et al. 1993). Seasonal shifts in wetland macroinvertebrate FFG are also common (Nelson et al. 1990, deSzalay and Resh 1996) and can influence waterfowl feeding behavior (Cooper and Anderson 1996). Although shifts in FFG were observed within the SWMA in 2001 and 2002 they generally corresponded to predominant taxa present at that sampling date. Abundance of macroinvertebrates didn't change significantly over the irrigation season, indicating that the availability of macroinvertebrates for waterfowl consumption may also remain consistent.

The SWMA was established to preserve wetland vales and provide benefit to waterfowl species of the area and is known to attract and provide habitat for a variety of waterfowl and shorebird species (IDFG 2005). With this new information, local land managers will have the baseline data needed to design additional waterfowl diet studies. Additional research on brood density and behavior should be conducted to investigate the relationship between the macroinvertebrates found in the SWMA and the waterfowl diet.

ACKNOWLEDGMENTS

The Idaho Department of Fish and Game gave permission to conduct experiments at the Sterling Wildlife Management Area (SWMA). This research was funded by: the Idaho State Board of Education, the Center for Ecological Research and Education (CERE), the Society of Wetland Scientists Student Research Grants Program, the Graduate Student Research and Scholarship Committee (GSRSC), and the Idaho State University Department of Biological Sciences. The author thanks MS committee members Dr. G. Wayne Minshall, Dr. Richard Inouye, and Dr. Glen Thackray. Thanks to Dr. Andrew M. Ray, Christina Relyea, Amanda Rugenski, Cecily Nelson, and Melanie Ovard for assistance with field/laboratory work, data analysis, and/or manuscript review.

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Author:Ray, Heather
Publication:Journal of the Idaho Academy of Science
Geographic Code:1U8ID
Date:Jun 1, 2007
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