The Ecological Patterns of Benthic Invertebrates in a Great Lakes Coastal Wetland

J. Great Lakes Res. 30(1):58–69 Internat. Assoc. Great Lakes Res., 2004 The Ecological Patterns of Benthic Invertebrates in a Great Lakes Coastal Wetland Richard A. MacKenzie*, Jerry L. Kaster, and J. Val Klump Center for Great Lakes Studies University of Wisconsin W.A.T.E.R. Institute 600 E. Greenfield Ave. Milwaukee, Wisconsin 53204 ABSTRACT. Benthic macroinvertebrates were sampled in the summer of 1997 using a standard D frame kick net along a transect across the Peshtigo wetland, a river delta wetland on the coast of Green Bay, Lake Michigan, to describe the spatial and temporal dynamics of the invertebrate community. Various abiotic factors, including sedimentation rates determined from 210Pb and 137Cs as a proxy for delivery of riverine organic matter, were also measured to determine which factors influenced these dynamics. Significant decreasing gradients in dissolved oxygen and pH with distance from the river, coupled with trends in sedimentation rates, chloride, and sum nitrate (nitrate + nitrite), revealed that riverine water was mixing with wetland water up to 100 m from the wetland-river interface. Aboveground primary production and total invertebrate densities exhibited Weibull type distributions, with highest rates and numbers occurring 20 to 100 m from the Peshtigo River. Invertebrate densities were largely represented by Asellus sp. isopods (12–53%) and exhibited highest numbers in September. Invertebrate diversity at the genus level linearly decreased with distance from the river based on the Simpson’s index of diversity (r2 = 0.60, p < 0.05) and the Shannon-Wiener function (r2 = 0.73, p < 0.01). Patterns observed suggest that there is an “optimal” zone for benthic invertebrates in the Peshtigo wetland 20 to 60 m from the Peshtigo River that is protected from high-energy events (e.g., storms, boating) in the Peshtigo wetland by a buffer zone (0 to 20 m) but is close enough to benefit from replenished levels of dissolved oxygen, nutrients, and organic matter delivered via the Peshtigo River. INDEX WORDS: Coastal wetlands, spatial patterns, benthic invertebrates, densities, diversity, Green Bay, Lake Michigan. INTRODUCTION Great Lakes coastal wetlands provide important habitat (i.e., spawning, nursery grounds) for many migratory waterfowl (e.g., Canadian geese, Branta canadensis) (Prince et al. 1992) and for fish found in the open waters of the Great Lakes (e.g., yellow perch, Perca flavescens) (Becker 1983). This is largely due to the benthic macroinvertebrates residing there, a valuable food resource for these organisms (Prince et al. 1992, Brazner and Beals 1997). Recently, invertebrate communities in these systems have received increased attention (i.e., Botts 1999, Gathman et al. 1999, Kashian and Burton 2000). However, little is known of the spatial distri- bution of benthic macroinvertebrates within Great Lakes systems (but see Brady et al. 1995, Cardinale et al.1997, Burton et al. 2003). In coastal wetlands, inundation via spring runoff, seiches, or storm events can lead to fluctuations in water levels (Prince and Burton 1996), creating “high energy” zones at the interface between wetlands and adjacent bodies of water (Krieger 1992). Energy from these events can mix water between wetlands and adjacent bodies of water and result in measurable gradients in various physicochemical parameters in the water column (i.e., dissolved oxygen, turbidity, pH) (e.g., Cardinale et al. 1997). In a littoral wetland in Saginaw Bay, mixing occurred in wetland areas as far as 150 m from the wetland edge-lake interface (Brady et al. 1995, Cardinale et al. 1997). The presence of stalks of dead and live emergent macrophytes (e.g., sedge (Carex sp.), bur *Corresponding author. E-mail: rmackenzie@fs.fed.us Present address: U.S.D.A. Forest Service, Institute for Pacific Islands Forestry, 1151 Punchbowl St. Rm. 323, Honolulu, HI 96813 58 Invertebrates in a Great Lakes Wetland reed (Sparganium sp.)) at the wetland edge in riverine systems can reduce the flow of inundating waters (Meeker 1996) and attenuate the energy from these events. This reduced flow results in the deposition of suspended sediments or seston (Rasmussen and Rowan 1997). Thus, benthic invertebrates at the wetland edge may benefit to a greater degree than organisms in the wetland interior from the continual replenishment of oxygenated water, nutrients, or organic matter (i.e., phytoplankton) from an adjacent river or lake. In Lake Michigan, wetland loss has been greatest along the southern coastline, primarily in urban and industrial areas near Milwaukee and Chicago. Today, wetlands along the northern coast of Lake Michigan and Green Bay continue to be threatened by increased development (Douglas Wilcox USGS, pers. comm.). Few studies have examined the invertebrate communities in Green Bay wetlands, and those that have, focused on insect emergence (McLaughlin and Harris 1990, King and Brazner 1999). Clearly additional information is needed on invertebrate community structure in Green Bay wetlands in order to understand how development and loss of coastal wetlands might impact the bay. The objectives of this study were to describe the spatial and temporal dynamics of the benthic macroinvertebrate community residing in the Peshtigo wetland, and to determine which abiotic factors influenced these dynamics. Sedimentation rates calculated from 210Pb (e.g., Robbins and Edgington 1975) and 137Cs (e.g., Livingston et al. 1988) radionuclide chronologies associated with the stratigraphy of sediment cores were used as a proxy for delivery of riverine organic matter (i.e., phytoplankton, bacterially conditioned detritus). It was hypothesized that: 1) a river-wetland mixing continuum will result in decreasing gradients of physicochemical factors (i.e., nutrients, dissolved oxygen) and sedimentation rates with distance from the Peshtigo River, 2) primary production rates are influenced by the Peshtigo River and will decrease with distance from the river, 3) benthic macroinvertebrate densities and diversity will be greatest at the riverine-wetland interface due to gradients in physicochemical parameter and the increased delivery of riverine organic matter. MATERIALS AND METHODS Study Site This study was conducted in the Peshtigo River wetland, a 4 km2 river delta wetland located at the 59 confluence of the Peshtigo River and Green Bay in northeastern Wisconsin (Fig. 1). The Peshtigo River is a 5th order river draining a watershed of 2,800 km2 that consists largely of wetlands, farmland, and second growth forests. Over the last 20 years, the average river discharge has been 26.2 m3.s–1 with the highest monthly discharge in April-May. The Peshtigo estuary, south of the town of Peshtigo, has not been channelized, and sediment deposition throughout the estuary is the result of natural processes. Study Design Stations sampled for this study were established during the late fall of 1996 when the drained marsh surface could be easily accessed to take sediment cores for radionuclide analysis. Nine wetland stations (P30, P32, P34, P36, P37, P38, P39, P40, P41) were randomly chosen along transects established from the Peshtigo River towards the wetland interior to determine how far the river could deliver sediments to the marsh surface. These stations were then revisited at least once a month from May through October of 1997 to collect biological and physicochemical data. Four stations (P30E, P36E, P38E, P41E) were also created in riverine areas directly adjacent to four wetland stations for collections of physicochemical data from the Peshtigo River. The nine wetland sampling stations were characterized by dense stands of emergent macrophytes that included Typha sp., Carex sp., Sparganium sp., and/or Phalaris arundinacea as well as various species of submergent vegetation (e.g., Potamogeton sp. and Myriophyllum sp.). Average water depth was 0.5 ± 0.04 m and ranged from 0.1 to 0.8 m. The four riverine sampling stations were characterized by open water to sparse emergent macrophytes that included Typha sp., Carex sp., Sagittaria sp., Scirpus sp., and/or Lythrum salicaria. Submergent vegetation was also present and consisted of Potamogeton sp. and Myriophyllum sp. Average water depth was 0.4 ± 0.02 m and ranged from 0.3 to 0.8 m. Physicochemical Parameters Physicochemical parameters have previously been used to demonstrate interactions between wetlands and adjacent bodies of water (e.g., Klopatek 1978, Sager et al. 1985, Cardinale et al. 1997). Thus, several physicochemical parameters, includ- 60 MacKenzie et al. FIG. 1. Location of the study site and the nine sampling stations in the Peshtigo wetland on Green Bay, Lake Michigan. Lightly shaded areas indicate the wetland surface. Riverine stations (P30E, P36E, P38E, P41E) are not labeled on the figure but were adjacent to their respective wetland stations (P30, P36, P38, P41). ing the conservative tracer chloride and conductivity (Wetzel and Likens 1991), were measured monthly from riverine and wetland stations. Temperature and pH were measured in situ 1 to 2 cm from the bottom using an Orion pH electrode and a Beckman φ 11 pH meter, and conductivity was measured using a Fisher conductivity probe. Triplicate samples of unfiltered water were collected from each station 1 to 2 cm from the bottom with minimal resuspension of sediments. Samples were collected in 5-mL glass bottles and fixed immediately for dissolved oxygen (DO) measurements using a micro modification of the Chesapeake Bay Institute technique for the Winkler dissolved oxygen method (Carpenter 1965). An additional 25 mL of water was taken from each station, filtered through 0.2 µm membrane Whatman syringe filters, and analyzed for alkalinity using Gran titrations (Wetzel and Likens 1991), chloride using ion chromatography (APHA 1992), ammonium (NH 4 + ) using Koroleff’s technique (Koroleff 1969), total filterable phosphorus (TFP) using the ascorbic acid technique (APHA 1992), and total nitrate plus ni– trite (Σ NO3 ) using a cadmium- reduction flow-in- Invertebrates in a Great Lakes Wetland jection technique (Patrick Anderson, Great Lakes W.A.T.E.R. Institute, pers. comm.). Field equipment was calibrated prior to each sampling date. Nutrient and chloride standards, laboratory duplicates, and intermittent standards were analyzed to ensure accuracy. Sediment cores were taken from each wetland station once during the fall of 1996 by driving a 7.5 cm diameter, acrylic core liner into the peat using a mallet. Cores were extruded and sectioned into 4-cm intervals, dried at 60°C to a constant weight, and ground into a fine powder. Radioactive isotope recovery tracers (either 208Po or 209Po) were added to each sample in 1 mL aliquots. Hot acid digestions were used to extract 210Pb, 208Po, and 209Po from ground sediments (Gin 1992), and concentrations were determined using alpha spectrometry. 137Cs activity was measured through direct gamma emissions from the ground sediment using an automated NaI-detector multichannel analyzer, an EG&G Ortec GMX Series GAMMA-x HPGE detector, or a Canberra 7224 Ge (Li) detector. Biological Samples Aboveground primary production has been shown to be an effective tool to examine general patterns in plant growth (e.g., Doust and Childers 1998, Rejmankova 2001). Emergent wetland plants were sampled with a sharpened spade from duplicate 17 cm × 17 cm plots (0.3 m2) at each station once a month during the 1997 growing season (May–October). Live aboveground plant shoots were clipped at the sediment-plant interface, dried at 60°C until a constant weight was reached, and weighed to the nearest 0.01 g. Aboveground net primary production (NPP) rates of emergent macrophytes were then calculated for each wetland station by summing the monthly change (positive or negative) in average plant biomass over the 6month sampling period. Invertebrate samples were collected once a month during the same period at wetland stations (n = 9) by dragging a standard 30.5 cm D frame, kick net (1-mm mesh) over 1 m of the wetland sediment (sampling area = 0.31 m 2 ). This sampling method is more effective at sampling invertebrates from dense stands of wetland vegetation than Hester-Dendy or core samplers (Turner and Trexler 1997). Macroinvertebrates were hand-picked from unpreserved kick net samples using 4× magnifying glasses, counted, identified to the lowest practical taxon using various keys (Hilsenhoff 1975, 61 Brinkhurst and Gelder 1991, Brown 1991, Covich and Thorp 1991, Davies 1991), and measured to the nearest 1 mm for total lengths. Taxon identifications were verified when possible by comparison with a specimen collection. Monthly densities (no·m –2 ) were then determined by dividing the counts by the sampled area (0.31 m2). Generic diversity values were calculated using the ShannonWiener method and the Simpson’s index of diversity (Krebs 1989). Statistical Analysis Linear and nonlinear regression analyses of physicochemical parameters, sedimentation rates, and invertebrate diversity versus distance from the Peshtigo River were performed in Sigma Plot 4.0 (1997. SPSS Inc, Chicago, Illinois). Monthly densities of genera that represented ≥ 10% of the total invertebrate population (Appendix 1) were compared using repeated measures (RM) single factor ANOVAs. Pearson’s product moment correlation analysis was used to determine relationships among dissolved oxygen, sedimentation rates, pH, nutrient concentrations, primary production, and monthly densities of genera that represented ≥ 10% of the total invertebrate population. Statistical significances were Bonferroni adjusted at a p-value of 0.05. All statistical analyses were performed using Systat 8.0 (1998. SPSS Inc., Chicago, Illinois). Because variance was greater then mean monthly densities of invertebrates (Appendix 1), densities were log transformed in order to conform to assumptions of normality (Elliot 1977) prior to statistical analyses. RESULTS Physicochemical Parameters Only dissolved oxygen (both concentration r2 = 0.34, p < 0.01 and percent saturation r 2 = 0.22, p < 0.01) and pH (r2 = 0.36, p < 0.05) exhibited exponential decreases with distance from the Peshtigo River, with the lowest values occurring in station P37 (Fig. 2). Both chloride and sum nitrate (ΣNO3–) (only reported from July due to incomplete data sets for other months) also exhibited decreasing trends with distance from the river, with ΣNO3– decreasing by nearly a factor of 4 within the first 100 m from the river. Chloride, conductivity, and alkalinity exhibited increased levels at station P37. Total filterable phosphorous (TFP) concentrations exhibited elevated concentrations from 20 to 120 m 62 MacKenzie et al. or temperature, which ranged from 15.4 to 21.3°C (average 19.1 ± 0.6°C). Sedimentation rates could not be determined from four of the nine stations (P38, P39, P40, P41) because cores were not long enough to obtain background levels of 210Pb and thus determine inputs of “new” 210Pb via sedimentation (Robbins and Edgington 1975). The five remaining cores (P30, P32, P34, P36, P37) revealed that sedimentation rates were greatest in wetland stations near the river and decreased exponentially with distance from the Peshtigo River (r2 = 0.99, p > 0.05) (Fig. 2). The lack of significance between sedimentation rates and distance was likely a result of the small sample size (n = 5). Primary Production Annual aboveground NPP for emergent macrophytes calculated from monthly values ranged from 330 to 2,040 g dry wt·m–2·yr–1. Aboveground NPP rates exhibited a positively skewed peak with distance from the river, similar to a three-parameter Weibull type distribution (Evans et al. 2000) (Fig. 3). Maximum rates were observed 30 to 100 m from the river with lower rates occurring near the river and in wetland stations > 100 m from the river. Rates of NPP were significantly correlated to NH4+ (r = 0.40, p < 0.01) and TFP (r = 0.46, p < 0.01) concentrations. FIG. 2. Spatial gradients in average monthly measurements of dissolved oxygen concentrations, pH, alkalinity, conductivity, and total filterable phosphorus (TFP) concentrations. Data for chloride and ∑ NO3 concentrations are only from the month of July due to incomplete data sets. Sedimentation rates are based upon 210Pb and 137Cs profiles from a single core taken from each station. Error bars represent ± 1 SE. from the river with lower concentrations observed in the river and in the wetland interior. No patterns were observed in NH 4 + concentrations, which ranged from 0.25 to 2.3 µM (average 1.4 ± 0.2 µM), Macroinvertebrate Patterns One hundred and ten different taxa were identified from the Peshtigo wetland. Only those organisms whose average monthly densities were ≥ 1% of the total invertebrate densities in one of the wetland stations are reported (Appendix 1). For a complete species list, see MacKenzie (2001). Average monthly densities of total invertebrates ranged from 1,562 ± 445 (station P41) to 344 ± 60 no·m–2·yr–1 (station P37). Like NPP, total invertebrate densities exhibited a Weibull type distribution with distance from the river (Fig. 3). Densities initially increased with distance from the river with maximum densities occurring in wetland stations 20 to 60 m from the river. Invertebrate densities were lower in wetland stations > 60 m from the river with the exception of P32, which was marked by a high density of snails (Gyraulus sp., Aplexa sp.) and isopods (Asellus sp.). Of the taxa listed in Appendix 1, the seven most abundant organisms in the nine wetland stations were: Aplexa sp., Asellus sp., Invertebrates in a Great Lakes Wetland 63 FIG. 3. The spatial distribution of average monthly densities of total invertebrates, seven major invertebrate taxonomic groups (Aplexa sp. snails, Asellus sp. isopods, Chironomus sp. midge flies, Crangonyx sp. amphipods, Dicrotendipes sp. midge flies, Gammarus sp. amphipods, Gyraulus sp. snails), net primary production, and diversity indices from the Peshtigo wetland. Diversity regressions do not include values reported from P32. Error bars represent + 1 SE. Gyraulus sp., Chironomus sp., Crangonyx sp., Dicrotendipes sp., and Gammarus sp. All but Crangonyx sp. exhibited spatial patterns in density similar to total invertebrate densities, with greater numbers in wetland stations 20 to 60 m from the river (Fig. 3). Crangonyx sp. densities decreased with distance from the river. Of these seven organisms, Asellus sp. was generally the most abundant invertebrate throughout the wetland (84 to 795 no·m–2·mo–1), representing 12–56% of the invertebrates sampled. Invertebrate densities were highest from June through September, with the highest densities in September (Fig. 4). Asellus sp. was typically the most abundant invertebrate in all samples over time, representing 20–50% of the average monthly densities of all invertebrates, which did not significantly change over time. The high densities in September were a result of increased Chironomus sp., Asellus sp., and Gyraulus sp. abundances. The significant seasonal differences in Chironomus sp. abundances (p < 0.05) may have been due in part to the mesh size of the sampling net which would have missed the smaller instars that occur in the spring and early summer. Invertebrate diversity at the genus level decreased with distance from the river based on the Shannon-Wiener function and Simpson’s index of diversity. Excluding station P39 which was shown to be an outlier, both measures of heterogeneity 64 MacKenzie et al. FIG. 4. Temporal distribution of average densities of seven major taxonomic groups of benthic macroinvertebrates in the Peshtigo wetland. Invertebrate abundance was dominated by large numbers of Asellus sp. isopods. Other invertebrates represent the sum of remaining taxa ≤ 10% of the total density. tracked a decreasing linear trend, which was significant for both abundant weighted (Simpson’s index) (r2 = 0.60, p < 0.05) and rare-weighted (ShannonWiener) (r2 = 0.73, p < 0.01) measures of generic diversity. Dissolved oxygen was significantly and positively correlated with average monthly densities of Crangonyx sp. (r = 0.31, p < 0.05) and Gammarus sp. (r = 0.37, p < 0.01). Aboveground NPP was also positively correlated to average monthly densities of Crangonyx sp. (r = 0.33, p < 0.05) and Gammarus sp. (r = 0.37, p < 0.01). Sedimentation rates were positively correlated to average monthly densities Dicrotendipes sp. (r = 0.34, p < 0.05) and Crangonyx sp. (r = 0.34, p < 0.05). The only significant correlation between nutrient concentrations and invertebrate densities was NH4+ and Crangonyx sp. (r = 0.30, p < 0.05). DISCUSSION Wetland Physicochemical and Primary Production Patterns The significantly decreasing gradients in dissolved oxygen concentrations and pH with distance from the Peshtigo River, coupled with trends observed in chloride, ∑NO3–, and sedimentation rates, suggest the presence of a riverine-wetland mixing continuum between the Peshtigo River (or Green Bay) and the Peshtigo wetland. This was most apparent within the first 100 m away from the riverine-wetland interface (Fig. 2). The lower flow rates in wetland stations compared to the adjacent river (pers. obs.) suggest that these gradients were established as the river inundated the wetland and water velocity was slowed by emergent vegetation. The elevated levels of alkalinity, conductivity, and chloride at station P37 (Fig. 2) may have been Invertebrates in a Great Lakes Wetland due to a localized input of groundwater high in CaCO3, as the Peshtigo wetland is located above a large dolomite deposit (Ostrum 1981). These parameters were thought to be responsible for the large population of Pisidium sp. fingernail clams that were also found there (Appendix 1) and can tolerate the low levels of dissolved oxygen (McMahon 1991) that occurred there. Aboveground NPP may have been higher 30 to 100 m from the river because of delivery of nutrients to plants via the Peshtigo River. This was supported by the significant correlations between aboveground NPP and NH4+ and TFP. Few studies have examined spatial patterns in plant growth in riverine wetlands and those that have, focused on standing crop or biomass. Although these are not rate measurements, the gradients described from these studies are similar to the gradient in NPP observed in the Peshtigo River wetland. In a riverine wetland in Poland, standing crop of emergent plants decreased with distance from the river, which was attributed to inundation of the river, and nutrient release from sediments (Wassen et al. 2003). Metaphyton biomass and cover also decreased with distance from inflows in a riparian freshwater marsh because of the removal of nutrients by dense mats of metaphyton at the wetland-river interface (Wu and Mitsch 1998). Benthic Invertebrate Ecological Parameters The most abundant invertebrates in the Peshtigo wetland were the Asellus sp. isopods, which were the densest invertebrates at most stations both spatially and temporally (Figs. 3 and 4), as in other wetlands (e.g., Pickard and Benke 1996, Smock 1999). Their abundance in the May samples was likely due to their ability to over-winter in wetland sediments (pers. obs.) coupled with their high spring production rates (Duffy and LeBar 1994) allowing them to rapidly re-colonize and exploit detrital food sources, an important component of their diet (Smock and Harlowe 1983). The lack of shredder organisms in this system (Appendix 1) suggests that isopods likely play an important role in the mechanical breakdown of the large amounts of aboveground NPP that occurs there. Trends in invertebrate density and diversity patterns, physicochemical parameters, and aboveground NPP from the Peshtigo wetland suggest that invertebrate communities in delta river wetland systems may be laterally influenced by abiotic and biotic factors from the riverine and wetland interior 65 (or upland) sides. Similar patterns have been described for salt marsh plant communities (e.g., Bertness 1991a, b) and intertidal invertebrate communities (e.g., Menge and Sutherland 1987, Beukema and Flach 1995) where community structure is influenced by abiotic (e.g., wave action, inundation, nutrient availability) or biotic (e.g., competition, predation) factors occurring on the oceanic or estuarine and upland sides, respectively. In the Peshtigo wetland, the lower invertebrate densities in stations 0 to 20 m from the river may have been influenced by high-energy events that occur in the river such as spring run-off, seiches, or storm events (Krieger 1992). Other high energy events in the Peshtigo River include recreational boating or ice scour in the early spring when portions of plants, sediments, or invertebrates can be broken off the wetland edge as rafts of frozen material (Geis 1985). Predation by transient fish from the river or Green Bay may also have affected invertebrate densities in wetland stations 0 to 20 m from the river. In a separate study, juvenile yellow perch (Perca flavescens) and largemouth bass (Micropterus salmoides) were found in wetland stations closer to the Peshtigo River (unpublished data) and are known to feed on invertebrates during this life stage (Becker 1983). The lower dissolved oxygen concentrations in wetland stations further from the Peshtigo River may be important in causing the lower invertebrate densities and diversity there. Microbial decomposition of plant material in wetlands can lead to limiting oxygen levels, which reduces invertebrate numbers (Gabor et al. 1994) and diversity (Thorp and Covich 1991). Dissolved oxygen levels of 85% saturation can increase mortality rates of amphipods (Pilgrim and Burt 1993). Thus, the low levels of dissolved oxygen occurring in stations further from the Peshtigo River at the wetland interior (20–40% saturation) may have resulted in the lower invertebrate densities, particularly Gammarus sp. and Crangonyx sp., that occurred there. Plant production in the Peshtigo wetland suggests that consumers were not limited by detrital input (e.g., Batzer 1998). The quality, rather than the quantity, of food may have been an important factor contributing to patterns observed in invertebrate densities. The significant correlation between sedimentation rates and densities of Crangonyx sp. and Dicrotendipes sp., coupled with the higher sedimentation rates in stations near the river, suggests that certain organisms may benefit from the deliv- MacKenzie et al. 66 ery of riverine phytoplankton and particulate organic matter. Thus, there appears to be an “optimal” zone for benthic invertebrates in the Peshtigo wetland 20 to 60 m from the Peshtigo River established by biotic and abiotic factors influenced by mixing of riverine and wetland water. This optimal zone is close enough to the Peshtigo River to benefit from inputs of nutrients, dissolved oxygen, or organic matter, but appears to be protected by a buffer zone (0 to 20 m) from high energy events such as storms, seiches, or boating activity. ACKNOWLEDGMENTS This research was supported in part by the Wisconsin Sea Grant Institute under grants from the National Sea Grant College Program, National Oceanic and Atmospheric Administration, U.S. Department of Commerce, and the State of Wisconsin. Federal grant number R/EC-2, project number NA446RG0481. Support was also provided by the National Science Foundation, NSF OCE-9727151, and the University of Wisconsin—Milwaukee (Mortimer Award). We thank Tracy Sykes, Jeremy Solin, Jim Waples, and Don Szmania for their invaluable assistance in the field and the lab. Constructive review of this paper was provided by Michele Dionne, Katherine C. Ewel, and two anonymous reviewers. REFERENCES APHA. 1992. Standard Methods for the Examination of Water and Wastewater, 16th Ed. 1992. Washington D.C.: American Public Health Assoc. Batzer, D.P. 1998. Trophic interactions among detritus, benthic midges, and predatory fish in a freshwater marsh. Ecology 79:1688–1698. Becker, G.C. 1983. 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Wetlands 18:9–20. Submitted: 31 January 2003 Accepted: 16 December 2003 Editorial handling: Lynda D. Corkum APPENDIX 1. Average monthly densities of macroinvertebrates (± 1 SE) that represented ≥ 1% of the total invertebrate population sampled from the nine stations in the Peshtigo wetland from May through October of 1997. Bold values represent major taxa groups and are equal to the sum of all species collected from that group. Trophic guilds are indicated in bold parentheses after genera and are based upon descriptions of food preference and functional feeding mechanisms (Pennak 1978, Brown 1991, Davies 1991, Covich and Thorpe 1991, Merritt and Cummins 1996). These categories include collector filterers (CF), collector gatherers (CG), detritivores (D), filterers (F), predators (P), and scrapers (S). Average Monthly Density (no⋅m–2⋅mo–1) Station distance from river (m) Taxon Phylum Mollusca Class Gastropoda Order Pulmonata Family Planorbidae Gyraulus sp. (D) Promenetus sp. (D) Planorbula sp. (D) Family Physidae Aplexa sp. (D) Physa sp. (S) Physella sp. (S) Class Pelecypoda Family Sphaeriidae Pisidium sp. (F) Phylum Annelida Class Oligochaeta Family Lumbriculidae Family Naididae Class Hirudinea Family Erpobdellidae Mooreobdella sp. (P) Family Glossiphoniidae Alboglossiphonia sp. (P) Batrocobdella sp. (P) Glossiphonia sp. (P) Helobdella sp. (P) P36 5 P30 6 P41 19 P38 28 P40 56 P39 86 P37 117 P32 277 P34 551 174 ± 700 210 ± 260 230 ± 980 274 ± 830 91 ± 29 154 ± 790 45 ± 12 475 ± 256 175 ± 118 126 ± 490 11 ± 10 5±3 91 ± 30 40 ± 29 14 ± 10 163 ± 105 59 ± 30 2±2 172 ± 590 72 ± 32 7±3 36 ± 12 32 ± 13 — 105 ± 700 43 ± 29 — 18 ± 13 — 5±4 185 ± 113 3±2 13 ± 70 18 ± 14 — — — 31 ± 25 — — 17 ± 900 — — 6±2 — 24 ± 14 — — 16 ± 70 1±1 — 5±5 — — 9±5 9±7 91 ± 67 42 ± 20 37 ± 36 60 ± 37 59 ± 49 21 ± 21 — — — — — — 68 ± 37 17 ± 12 1±1 23 ± 90 27 ± 13 111 ± 620 48 ± 23 111 ± 620 7±3 21 ± 13 70 ± 43 42 ± 25 13 ± 60 — 7±3 — 12 ± 70 14 ± 14 1±1 7±4 12 ± 70 14 ± 14 — — 18 ± 12 1±1 68 ± 43 — 36 ± 26 — — — — 1±1 — — — 1±1 — 1±1 — — — — — — — 1±1 8±7 — — 1±1 18 ± 10 1±1 — — 5±4 — 8±6 67 ± 49 18 ± 13 4±2 — 16 ± 80 5±4 1±1 8±6 67 ± 49 1±1 2±1 — 4±3 1±1 — — 1±1 1±1 1±1 — 1±1 4±3 — 1±1 1±1 Invertebrates in a Great Lakes Wetland APPENDIX 1. 69 Continued. Average Monthly Density (no⋅m–2⋅mo–1) Station distance from river (m) P36 5 P30 6 P41 19 P38 28 P40 56 P39 86 P37 117 P32 277 P34 551 Phylum Arthropoda Class Crustacea Order Isopoda Asellus sp. (D) 333 ± 178 382 ± 137 333 ± 178 382 ± 137 170 ± 770 170 ± 770 592 ± 279 592 ± 279 Order Amphipoda Crangonyx sp. (D) Gammarus sp. (D) Hyallela sp. (D) 146 ± 530 69 ± 37 16 ± 10 — 67 ± 16 58 ± 17 6±4 3±1 349 ± 870 26 ± 17 266 ± 960 57 ± 24 111 ± 390 69 ± 37 32 ± 21 9±6 77 ± 23 66 ± 22 2±1 9±8 116 ± 350 34 ± 70 64 ± 27 17 ± 80 2±1 2±1 — — 22 ± 80 22 ± 80 — — 4±2 4±2 — — 2±2 1±1 2±2 2±2 5±4 — 5±3 1±1 12 ± 10 181 ± 900 226 ± 136 506 ± 222 238 ± 930 254 ± 140 42 ± 16 43 ± 16 53 ± 45 17 ± 50 177 ± 890 221 ± 134 503 ± 221 234 ± 920 247 ± 137 38 ± 15 25 ± 12 47 ± 44 15 ± 50 Class Insecta Order Coleoptera Order Diptera Family Chironomidae 822 ± 226 107 ± 280 126 ± 400 373 ± 150 297 ± 169 822 ± 226 107 ± 280 126 ± 400 373 ± 150 297 ± 169 Subfamily Chironominae Tribe Chironomini Chironomus sp. (CG) Dicrotendipes sp. (CG) Einfeldia sp. (CG) Endochironomus sp. (S) Polypedilum sp. 14 ± 11 27 ± 27 17 ± 17 14 ± 14 9±6 53 ± 48 97 ± 97 9±9 1±1 6±4 171 ± 156 11 ± 7 82 ± 33 89 ± 57 10 ± 6 52 ± 38 9±7 70 ± 45 21 ± 15 — 137 ± 88 2±2 37 ± 23 — 1±1 9±8 1±1 4±3 — — 1±1 — — 1±1 9±9 28 ± 26 9±9 1±1 — — 5±3 1±1 — — 3±3 Tribe Tanytarsini Micropsectra sp. (CF) Paratanytarsus sp. (CF) Tanytarsus sp. (CF) Stempellinella sp. (CF) 11 ± 8 42 ±30 24 ± 16 7±5 1±1 31 ± 26 19 ± 12 1±1 — 93 ± 61 4±3 — 1±1 43 ± 30 3±3 1±1 — 37 ± 30 16 ± 16 1±1 — 14 ± 13 2±1 3±3 — — — 9±5 — 4±4 1±1 1±1 — 1±1 2±1 — 5±5 46 ± 40 29 ± 16 9± 6 2±2 9+9 1±1 22 ± 22 — — — 19 ± 16 3±2 — — — 1±1 — 5±5 11 ± 70 7±7 — — 5±5 — — — — — — 35 ± 35 3+3 — 6±6 — 2±2 — 4±4 — — — — 21 ± 21 — — 3±3 3±3 2±2 2±2 124 ± 100 124 ± 100 57 ± 25 56 ± 24 35 ± 22 35 ± 22 3±2 2±2 — — 14 ± 90 14 ± 90 3±2 1±1 12 ± 40 4±3 32 ± 19 40 ± 27 26 ± 23 1±1 18 + 11 7±4 18 ± 13 9±5 — — 6±5 8±6 1±1 14 ± 12 3±1 11 ± 60 Suborder Zygoptera Family Coenagrionidae Enallagma sp. (P) Nehalennia sp. (P) — 1±1 — 3±3 21 ± 16 4±4 — 29 ± 28 1±1 16 ± 16 — — — 3±3 — 4±4 4±4 1±1 Order Trichoptera 1±1 1±1 — 9±5 1±1 3±2 1±1 — 1±1 Order Ephemeroptera Family Caenidae Caenis sp. (CG) Family Siphlonuridae Siphlonurus sp. (CG) Family Leptophlebiidae Habrophlebia sp. (CG) Leptophlebia sp. (CG) Order Hemiptera Trichocorixa sp. (P) Order Odonata Suborder Anisoptera Family Libellulidae Libellula sp. (P) Total 917 ± 186 881 ± 300 1,562 ± 44501,371 ± 379 1,423 ± 409 441 ± 122 344 ± 600 1,057 ± 3160 570 ± 238