2007 NORTHEASTERN NATURALIST 14(3):481–491 The Macroinvertebrates of Ruppia (Widgeon Grass) Beds in a Small Maine Estuary Rachel A. Keats1 and Laurie J. Osher2,* Abstract - Little information exists on macroinvertebrate community composition in small, micro-tidal, Ruppia maritima (widgeon grass)-dominated Maine estuaries. Qualitative and quantitative assessments of the macroinvertebrate fauna of widgeon grass beds in Northeast Creek estuary (Acadia National Park, ME) are presented here. The community was dominated by euryhaline freshwater invertebrates including midge larvae (Chironomidae: Dicrotendipes, Cricotopus, Chironomus), oligochaetes, damselfly larvae (Coenagrionidae: Enallagma), amphipods (Gammaridae: Gammarus), gastropods (Hydrobiidae: Hydrobia), ostracods (Cytheridae: Cyprideis), and water boatmen (Corixidae: Trichocorixa). Macroinvertebrate abundances at the sampled sites were 35,100 individuals/m2 in both August and September, and 22,200 individuals/m2 in October. This study provides baseline faunal-community data that can be used in future monitoring studies. Introduction Estuaries and coastal ecosystems worldwide are continually faced with increased coastal development and its associated human impacts. Estuarine resources in coastal Maine are threatened by nutrient enrichment associated with atmospheric deposition (Miller 1999) and increased development in contributing watersheds. Though much work has been done to investigate the effects of anthropogenic impacts on estuarine systems, the majority of these studies have focused on large estuaries and coastal embayments. Little information exists on the faunal communities of smaller, high-latitude, Ruppia maritima L. (widgeon grass)-dominated estuaries. Without prior knowledge of the natural communities, it will be impossible to detect changes in these communities and to design and direct restoration projects should environmental conditions deteriorate. Faunal communities of estuaries are highly variable, due to the varying physical conditions of these systems. The distribution of species is driven mainly by salinity and salinity fluctuations (Diaz 1989, Lopez 1988, Remane and Schlieper 1971, Ristich et al. 1977, Sanders et al. 1965, Verhoeven 1980), although size of the estuary, substrate size and stability, and past and present connections to other water bodies for colonization are also important in determining distributions (Verhoeven 1980, Williams and Hamm 2002). 1Woodard and Curran, 41 Hutchins Drive, Portland, ME 04102. 2Department of Plant, Soil and Environmental Sciences, 5722 Deering Hall, University of Maine, Orono ME 04469-5722. *Corresponding author - laurie@maine.edu. 482 Northeastern Naturalist Vol. 14, No. 3 Submerged vascular plant beds are important for species diversity and productivity in estuarine systems (Fredette et al. 1990, Heck et al. 1995, Mattila et al. 1999, Orth et al. 1984). These aquatic macrophytes provide habitat complexity, abundant food sources, sediment stability, and refuge from predation compared to surrounding soft substrates. Widgeon grass is a submerged vascular plant that is common in estuaries and not restricted to areas of high salinity. To date, several studies of the fauna of widgeon grass beds have appeared in the literature (Fredette et al. 1990, Heck et al. 1995, Knowles and Bell 1998, Verhoeven 1980, Wenner and Beatty 1988). These researchers observed a range of euryhaline marine and freshwater species as well as brackish-water specialists in widgeon grass beds. This study was conducted in the Northeast Creek (NEC) estuary of Acadia National Park in Maine. NEC is a relatively pristine system that receives low amounts of nitrogen from the surrounding watershed (Nielsen 2002). NEC estuary is currently threatened by nutrient inputs from atmospheric pollution and increased residential development. It is located in an area where nearby estuarine systems are already moving towards eutrophic conditions (Doering and Roman 1994, Doering et al. 1995, Kinney and Roman 1998). Little baseline research has been done in small, micro-tidal, high-latitude, widgeon grass-dominated estuaries. We completed qualitative and quantitative assessments of the faunal community in the widgeon grass beds of NEC estuary from May to October of 2001 in order to describe and document the community for future monitoring. Methods Study site Northeast Creek estuary is located in Acadia National Park, Mount Desert Island, ME (Fig. 1). This small micro-tidal estuary occupies a drowned river valley (Gehrels et al. 2002) fed by a number of freshwater streams. It is approximately 4 km long within a 2400-ha watershed (Nielsen 2002). The estuary averages about 1 m in depth with a narrow tidal range (less than 0.5 m). An old rock dam impedes tidal exchange so that the estuary is generally poorly flushed. In 2001, top and bottom salinities in NEC estuary increased from 0‰ in May and June to around 30‰ in October, indicating that the system was dominated by freshwater inputs in the spring and became increasingly more marine throughout the summer (Fig. 2; B. Kopp, US Geological Survey [USGS], Augusta, ME, unpubl. data). While 2001 was a year of lower-than-average precipitation (NADP 2006), this salinity pattern is typical for the estuary (similar patterns in 2000; J.M. Caldwell and C.W. Culbertson, USGS, Augusta, ME, pers. comm.). The estuarine system is vegetated with widgeon grass along half of its length. This study was performed in the dense beds of widgeon grass approximately halfway up NEC, just downstream of the mouth of Aunt Betsy’s Creek (Figs. 1 and 3; 2007 R.A. Keats and L.J. Osher 483 44.4181°N, 68.3133°W WGS84/NAD83). The substrate of the estuary in this location is a silt-loam soil containing organic matter of detrital origin. Figure 1. Map of Northeast Creek watershed and study site. Northeast Creek is located on Mt. Desert Island, ME. Study-site location is indicated by the arrow. 484 Northeastern Naturalist Vol. 14, No. 3 Sample collection The widgeon grass beds sampled in this study were both uniform and dense. Because the estuary is shallow, micro-tidal, and mixed, there was Figure 3. Photo of Ruppia maritima (widgeon grass) beds at the study site (photo courtesy of H. Neckles, USGS, Augusta, ME). Figure 2. Top and bottom water salinities (PPT) in Northeast Creek estuary (unpubl. data) from May to mid-November 2001 (B. Kopp, USGS, Augusta, ME). 2007 R.A. Keats and L.J. Osher 485 very little variation in the habitat at the study site. Locations for both qualitative and quantitative samples were chosen based on a visual inspection of the area via canoe. Sample location-selection criteria included a water depth of approximately 20–30 cm (average for the study site), and a widgeon grass bed that was dense (no substrate visible through the vegetation) and undisturbed (epiphytes were visible on the blades). Sampling techniques were designed to sample the macroinvertebrate fauna closely associated with widgeon grass. All samples were taken from a canoe. Qualitative samples of the macroinvertebrates associated with widgeon grass were collected using several methods on May 25, June 28, July 12, and July 27 in 2001. To sample the epifauna and mobile fauna associated with widgeon grass, two methods were used. First, one-gallon plastic bags were placed over the macrophyte beds, and garden shears were used to cut the macrophytes as close to the sediment as possible so that the bags could be sealed and removed. Second, a net was used to sweep the macrophyte beds, and net contents were placed in plastic bags. To sample the infauna down to a depth of 10 cm, a 10-cm diameter core sampler was used (using the same method described below, except for the process for including macrophytes in the sample). Quantitative samples of the infauna, epifauna, and mobile invertebrates associated with widgeon grass beds were collected on August 24, September 13, and October 19 in 2001. A 10-cm diameter clear acrylic core sampler was used to sample the entire water column, the macrophytes, and the benthos down to a 10-cm depth. The core sampler was inserted into the substrate down to the 10-cm line on the sampler. Then a rubber stopper was used to seal the top of the sampler, macrophytes not associated with the sample were separated from those in the sample using garden shears, and the sample was removed from the estuary and placed in a large plastic bag. This bag was stored in a cooler until all samples were collected and then brought to the laboratory for further processing. Each core sample was processed as a single sample. Ten replicate samples were collected per sampling date. All qualitative and quantitative samples were rinsed through a 500-mm sieve and preserved in 70% ethanol with Rose Bengal dye. Macroinvertebrates were removed from the samples and identified to genus when possible using keys by Epler (2001), Merritt and Cummins (1996), Peckarsky et al. (1990), and Weiderholm (1983). Information on the presence of taxa was used to qualitatively compare samples throughout the seasons. Mean abundances of the invertebrates were calculated for the quantitative samples based on the surface area of the core sampler (78.5 cm2). Results A total of 14 macroinvertebrate taxa, including nine insect taxa and two crustacean taxa, were found in the widgeon grass beds of NEC estuary between May and October 2001 (Table 1). The most common insect taxa found were three genera of non-biting midge larvae (Chironomidae: Dicrotendipes, 486 Northeastern Naturalist Vol. 14, No. 3 Cricotopus, Chironomus), damselfly larvae (Coenagrionidae: Enallagma), and water boatmen (Corixidae: Trichocorixa). Less common insect taxa found were Diptera of the families Ceratopogonidae and Tabanidae, and Odonata of the family Anisoptera. Other macroinvertebrates commonly found were water mites (Acari), amphipods (Gammaridae: Gammarus), ostracods (Cytheridae: Cyprideis), and oligochaetes. Average total macroinvertebrate abundances were 35,100 individuals/m2 in both August and September, and 22,200 individuals/m2 in October. Abundances of each taxa for August, September, and October are presented in Figure 4. Total numbers of insect taxa declined between August and October, with the majority of this decline associated with decreased numbers of Cricotopus, Chironomus, Enallagma, and Trichocorixa. Dicrotendipes abundance increased in August, but decreased in October. Gammarus also decreased between August and October. Cyprideis and Hydrobia were both abundant throughout the sampling period. Oligochaetes were found at low levels on all sampling dates. Discussion Northeast Creek estuary was dominated by euryhaline freshwater invertebrates from May to November 2001 (Table 1). The most common Table 1. Presence of macroinvertebrate taxa in Ruppia maritima (widgeon grass) beds in Northeast Creek estuary from May to October 2001. Presence of taxa is indicated by an X. 2001 Taxa May 25 June 28 July 12 July 27 Aug. 24 Sept. 13 Oct. 19 Insecta Chironomidae Dicrotendipes X X X X X X X Cricotopus X X X X X X X Chironomus X X X X X X X Ceratopogonidae X X Tipulidae X Other Diptera X X X X X Coenagrionidae Enallagma X X X X X X Other Odonata X Corixidae Trichocorixa X X X X X X Acari X X X X X X Crustacea Malacostraca Gammaridae Gammarus X X X X X X X Ostracoda Cyprideis X X X X X X X Gastropoda Hydrobia X X X X Oligochaeta X X X X X X X 2007 R.A. Keats and L.J. Osher 487 invertebrates were non-biting midge larvae (Chironomidae: Dicrotendipes, Cricotopus, and Chironomus), damselflies (Coenagrionidae: Enallagma), gastropods and ostracods. Less common invertebrates were oligochaetes, water boatmen (Corixidae: Trichocorixa), water mites (Acari), and amphipods (Gammaridae: Gammarus). Several studies of Ruppia-dominated estuarine systems outside New England (Heck et al. 1995, Knowles and Bell 1998, Wenner and Beatty 1988) observed faunal communities dominated by mollusks, polychaetes, and crustaceans, with few insects and oligochaetes. However, these estuaries have a more constant marine influence than NEC, and the faunal communities are very different from the one found in NEC estuary. The fauna of NEC estuary is most similar to a site in the Baltic Sea sampled by Verhoeven (1980). Like NEC, this Baltic estuary was found to have sparse euryhaline freshwater fauna, widgeon grass, and organic-rich mud. All of the invertebrates found have been previously observed in brackish waters. Non-biting midge larvae (Chironomidae) have been found in Figure 4. Macroinvertebrate abundances in Ruppia maritima (widgeon grass) beds in Northeast Creek estuary in August, September, and October 2001. Abundances are in individuals per square meter, and error bars represent 1 standard error. 488 Northeastern Naturalist Vol. 14, No. 3 freshwater, brackish water, and marine systems (Cheng 1976, Colbo 1996, Remane and Schlieper 1971). The three chironomid genera—Dicrotendipes, Cricotopus, and Chironomus—are typically found in euryhaline freshwater systems. Dicrotendipes has been documented in New Brunswick estuaries (Williams and Hamm 2002) and Maine brackish salt-marsh pools (MacKenzie 2005), and Cricotopus and Chironomus have been documented in many saline ecosystems (Colbo 1996, Frid and James 1989, MacKenzie 2005, Menzie 1981, Sutcliffe 1961, Williams and Hamm 2002, Williams and Williams 1998). Several Chironomus species are actually brackish-water specialists (Remane and Schlieper 1971, Verhoeven 1980). Enallagma and Trichocorixa are freshwater insect genera known to be tolerant of brackish water (Merritt and Cummins 1996). Related species have been observed in Maine salt-marsh pools (MacKenzie 2005), European Ruppia beds (Verhoeven 1980), and other saline habitats (Cheng 1976, Hart and Lovvorn 2002, Remane and Schlieper 1971). Acari are known to be euryhaline freshwater, while Gammarus, ostracods, oligochaetes, and gastropods may be euryhaline freshwater, euryhaline marine, or brackish-water specialists (Remane and Schlieper 1971, Verhoeven 1980) and have been found in intertidal marine habitats in the Boston Harbor Islands National Park area (Bell et al. 2005). NEC estuary was dominated by freshwater inputs in the spring and became increasingly more marine throughout the summer (Fig. 2). Under freshwater conditions, freshwater species would be expected to replace marine species, because most marine species require high salinities, while freshwater species are tolerant of a wider range of salinities and anoxia (Lopez 1988). Because NEC estuary is dominated almost entirely by freshwater inputs during the spring, it is unlikely that even euryhaline marine species could tolerate these conditions. However, during the more marine conditions in the fall, freshwater species may then be replaced by marine species. While the pattern of greater freshwater inputs in the spring due to increased precipitation (NADP 2006) and melting snow and ice is typical of the area, precipitation in 2001 was below average (NADP 2006) and thus NEC may typically have lower salinities for a greater part of the year, and freshwater species may be expected to dominate for a longer period of time in the estuary than was the case for 2001. The shift in salinity in NEC estuary was accompanied by changes in the community of organisms. Water mites were scarcer in the later summer and early fall samples. Gastropods increased in importance in the late summer and early fall under more marine conditions. In addition, during the period of high and increasing salinity in the estuary from August to mid-October, insects declined from 70% to 50% of the total community. Total insect abundance also decreased during this time period. Possible reasons for the shifts observed include increased salinity over an extended time (Williams and Williams 1998), increased larval emergence, reductions in egg-laying, and other seasonal and life-cycle factors. Freshwater insects were 50–70% of the macroinvertebrate community in NEC estuary between August and October. Very few estuarine studies have 2007 R.A. Keats and L.J. Osher 489 documented such dominance by insect taxa. Insects were found to constitute only 17–54% of the macroinvertebrate fauna of New Brunswick estuaries (Williams and Hamm 2002) and 32% of this fauna in Aber estuary in North Wales (Williams and Williams 1998). However, many researchers that have documented the presence of insects in estuaries do not identify them past the taxonomic level of Order (e.g., Sanders et al. 1965, Wenner and Beatty 1988, discussed by Williams and Hamm 2002). Classically, insects have been thought to be restricted to the upstream freshwater sections of estuaries based entirely on salinity (Remane and Schlieper 1971, Ristich et al. 1977, Sanders et al. 1965). Recent studies have shown that this view is too simplistic and that insects may be significant throughout some estuaries (Williams and Hamm 2002, Williams and Williams 1998). The faunal abundances found in this study in NEC estuary (35,100–22,200 individuals/m2) is on the upper end of the range of abundances found by Verhoeven (1980) in European Ruppia beds (2000–44,000 individuals/m2). Conclusions This study documents the fauna of the widgeon grass beds of a Maine estuary. The macroinvertebrate community of Notheast Creek estuary was dominated by tolerant euryhaline freshwater invertebrates from May to November 2001. The community was composed largely of insect taxa. Our study provides baseline data on the macroinvertebrate community of NEC estuary. The results can be used in future monitoring studies both in NEC and in similar estuarine systems in the Gulf of Maine. Acknowledgments We would like to thank Les Watling and Tom Woodcock for assistance with taxonomic identifications; Blaine Kopp and Hilary Neckles for help with sampler design; David Manski at the National Park Service in Acadia National Park for permission to sample in NEC; Leigh Pendergast, Jack Cromie, Jessica Stone, Roy and Susan Keats, and Bill Gawley for their help with field work; and Bryan Dail and Jean MacRae for microscope and laboratory support. Literature Cited Bell, R., R. Buchsbaum, C. Roman, and M. Chandler. 2005. 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