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Wetlands Ecology and Management 6: 59–68, 1998. 59 © 1998 Kluwer Academic Publishers. Printed in the Netherlands. Changes in the abundance and diversity of coastal wetland fauna from the open water/macrophyte edge towards shore B.J. Cardinale1,2, V.J. Brady3,4 & T.M. Burton3 1Department of Fisheries & Wildlife, Michigan State University, East Lansing, MI, 48824, U.S.A. 2Present address: Department of Biology, University of Maryland, College Park, MD 20782, U.S.A. 3Department of Zoology, Michigan State University, East Lansing, MI 48824, U.S.A. 4Present address: National Health & Environmental Effects Research Lab, US EPA, Duluth, MN 55804, USA Received 15 December 1997; accepted in revised form 28 June 1998 Key words: benthic invertebrates, community structure, fish, Laurentian Great Lakes, marsh, microcrustacea, physical and chemical gradients, zooplankton Abstract Great Lakes coastal wetlands are widely recognized as areas of concentrated biodiversity and productivity, but the factors that influence diversity and productivity within these systems are largely unknown. Several recent studies have suggested that the abundance and diversity of flora and fauna in coastal wetlands may be related to distance from the open water/macrophyte edge. We examined this possibility for three faunal groups inhabiting a coastal wetland in Saginaw Bay, Lake Huron. We sampled crustacean zooplankton and benthic macro-invertebrates at five distances from open water in the summer 1994, and fish at three distances from open water in 1994 and 1995. We found significant spatial trends in the total abundance and diversity of zooplankton and fish, as well as the diversity of benthic macro-invertebrates. Zooplankton abundance and taxa richness were highest at intermediate distances from open water in a transition zone between the well-mixed bayward portion of the wetland, and the non-circulating nearshore area. Benthic macro-invertebrate taxa richness increased linearly with distance from open water. In contrast, fish abundance and species richness declined linearly and substantially (abundance by 78%, species richness by 40%) with distance from open water. Of the 40 taxa examined in this study, 21 had significant horizontal trends in abundance. This led to notable differences in community composition throughout the wetland. Our results suggest that distance from open water may be a primary determinant of the spatial distributions of numerous organismal groups inhabiting this coastal wetland. Several possible reasons for these distributions are discussed. Introduction degraded as a result of human influences (Krieger et al., 1992). Despite their diminishing quality and extent, the There are approximately 1,200 km2 of coastal wet- Great Lakes coastal marshes are still widely recog- lands in the United States that fringe the Laurentian nized as areas of concentrated biodiversity and pro- Great Lakes (Mitsch and Gosselink, 1993). Along ductivity (Stuckey, 1989; Smith et al., 1991; Krieger, many shorelines these wetlands are quite impressive in 1992; Randall et al., 1996; Brazner and Beals, 1997). extent; yet, they represent only a small portion of the However, our knowledge of the factors that influence coastal marshes that were present before Europeans ar- the diversity and productivity of flora and fauna within rived in the US. An estimated 60–80% of Great Lakes coastal wetlands is still rudimentary, at best. If we wetlands have been lost to agricultural, residential, or are to predict the consequences of habitat loss, or di- industrial development since settlement (Comer et al., rect efforts to conserve or restore coastal wetlands in 1995). Of those that remain, many wetlands are highly the Great Lakes, it is imperative that we uncover the Article: wetlmitsch19 GSB: 9 Pips nr. 184695 (wetlkap:bio2fam) v.1.1 wetmit19.tex; 19/01/1999; 16:57; p.1 60 Figure 1. The position of benthic macro-invertebrate and zooplankton sampling stations relative to distance from the open water/wetland edge, and the location of fyke nets within the wetland study site in Saginaw Bay, Lake Huron, Michigan (USA). environmental factors that influence the structure of grazers and filter feeders decline, often by several or- communities inhabiting these systems. ders of magnitude, with increasing distance from open Several recent studies performed in coastal wet- water (Brady et al., 1995; Cardinale et al., 1997). lands of Saginaw Bay, Lake Huron, have described an Given these previous results, we wondered if the axis of environmental variation that may have wide- spatial distributions of other coastal wetland fauna spread biological importance. Suzuki et al. (1995) might be related to distance from the open wa- documented physical and chemical discontinuities that ter/macrophyte edge. Concurrent with our study of formed horizontally from the open water/macrophyte epiphytic invertebrates (Cardinale et al., 1997), we had edge towards the shore. As wind-induced surface two other ongoing projects at our study site. Neither waves were gradually reduced by macrophytes, turbid- was specifically designed to examine spatial distri- ity, dissolved oxygen, and pH decreased with distance butions; however, we were able to recast the data from open water while alkalinity, conductivity, and in a manner that allowed comparison of crustacean most dissolved ions increased. Changes in the abiotic zooplankton, benthic macro-invertebrate, and fish as- environment were accompanied by rather substantial semblages at several distances from open water over a changes in the biomass of phytoplanktonic and epi- five month study period. Here, we present data show- phytic algae, both of which declined by 80–97% from ing how (1) abundance, (2) taxonomic richness, and open water towards shore. (3) community composition of these three organis- We found nearly identical abiotic gradients in sev- mal groups changed with increasing distance from the eral coastal wetlands throughout Saginaw Bay (Cardi- open water/wetland edge. nale, 1996). In our most studied site, we have shown Methods that the biomass and net primary production of epi- phytic algae decline substantially from open water Study site towards shore in correspondence with the abiotic gra- Our study site was located in a coastal emergent dients (Cardinale et al., 1997). We have also found that wetland typical of those extending around the south- the biomass, diversity, and survivorship of epiphytic eastern shore of Saginaw Bay, Lake Huron, USA (43◦ wetmit19.tex; 19/01/1999; 16:57; p.2 61 Table 1. The relative abundance of zooplankton taxa. Numbers Collection of invertebrates shown for the major taxonomic groups are the percentage of the ∗∗ total number collected from that station. Due to the disparity in taxonomic resolution, the relative abundance of individual clado- From June to October of 1994, we took monthly ceran taxa is based only on the total number of cladocera, not the samples of crustacean zooplankton and benthic macro- total number of zooplankton. Thus, columns do not sum to 100%. invertebrates at five stations along a transect that Dots are shown where a taxon was not present on any sampling bisected the S. americanus stand from open water to- date. DFOW = distance from open water into the macrophyte bed. wards shore (Figure 1). Zooplankton were sampled at ◦ TAXA DFOW = 20 m 100 m 180 m 280 m 360 m a random distance (1–10 m) and direction (0–360 ) from each station using a modified Gerking sampler Copepoda (21.5 cm i.d. with 250 (µm mesh, Mittelbach, 1981) Cyclopoida 19 18 15 25 41 which encircled the entire water column from surface Calanoida 2 11 6 1 < 1 to bottom along with any macrophyte stems present. Ostrocoda 6 21 32 10 11 Water depth was concurrently measured so that zoo- Cladocera** 73 50 47 64 48 plankton abundance could be reported per volume of Acroperus harpae < 12 6 1221 Alona quadrangularis 12 28 18 4 9 water. At the same location, a sediment corer (4.5 cm Bosmina longirostris 20 5 2 3 3 i.d.) was used to collect benthic macro-invertebrates Ceriodaphnia sp(p) 32 23 11 22 16 to a soil depth of 10 cm. Two cores were taken from Chydorus sphaericus 33 1 18 each location and pooled into a composite sample. All Diaphanosoma birgei < 11 1 1011 samples were immediately placed in a cooler with ice Eurycercus lamellatus < 11 2 11 and transported to the laboratory within 3–4 hours for Ilyocryptus spinifer < 1 3 13 30 2 processing. Leydigia sp(p) 1 < 1 In the laboratory, zooplankton and benthic macro- Macrothrix sp(p). < 1 17 1 invertebrate samples were rinsed through a 250 µm Monospilus dispar 12<1 sieve with any plant stems rubbed by hand to dis- Pleuroxus sp(p) < 11 2 49 lodge the animals. The 250 µm mesh was suffi- Side crystallina 30 29 25 7 8 cient to capture most crustacean zooplankton in our Simocephalus sp(p). 1 < 1 < 1512 study site, but was too large for most rotifers, which were subsequently ignored by our study. Specimens were preserved in 95% ethanol with rose bengal dye. Prior to counting and identification, each sediment 370N83◦380W, Figure 1). Vegetation in this area sample was sugar floated (Anderson, 1959) and sub- was unprotected from prevailing northwesterly winds sampled. One-sixth of the total was examined under × that swept across the shallow open water of the bay. 10 magnification and benthic macro-invertebrates As a result, pelagic surface waves generally arrived were counted and identified to an operational taxo- perpendicular to the stand, and during our study were nomic unit. A minimum of 50 specimens, but typically observed to penetrate 160–200 m into the vegetation. far more, was identified from each sample. Simi- The macrophyte community was almost mono- larly, zooplankton were identified and enumerated at × typic being dominated by three-square bulrush, Scir- 40–400 from one-sixth of each sample (generally pus americanus, extending 500 m from the shore. 100 specimens). Small, isolated patches of Scirpus acutus, Typha an- Collection of fish gustifolia,andSagittaria sp.

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