Aquatic Macrophytes and Algae at Old Woman Creek Estuary and Other Great Lakes Coastal Wetlands

J. Great Lakes Res. 18(4):622-633 Internal. Assoc. Great Lakes Res., 1992

AQUATIC MACROPHYTES AND ALGAE AT OLD WOMAN CREEK ESTUARY AND OTHER GREAT LAKES COASTAL WETLANDS

David M. Klarer1 and David F. Millie2 IOld Woman Creek State Nature Preserve & National Estuarine Research Reserve Ohio Department of Natural Resources 2514 Cleveland Road, East Huron, Ohio 44839

2Southern Regional Research Center U.S. Department of Agriculture Agricultural Research Service P.O. Box 19687 New Orleans, Louisiana 70179 and Department of Biological Sciences Loyola University New Orleans, Louisiana 70118 ABSTRA CT. Studies on aquatic macrophyte and algal floras of Old Woman Creek estuary (0 WC) are examined in light of work conducted in other Great Lakes coastal wetlands. Since the last detailed inventory of aquatic macrophytes at OWC in 1973, many emergent and floating leaved species have become very restricted in their distribution, or have disappeared altogether. Possible causes for this vegetation shift are discussed. The algal flora of OWC is distinct from that in adjacent Lake Erie. Storm events are proposed to be a major factor in regulating phytoplankton species composition and dynamics in OWe. Primary productivity in OWC appears to be dominated by the algal communities rather than by macrophytes. The botanical research on Great Lakes coastal wetlands conducted to date has provided only cursory insights into these complex and dynamic communities. Knowledge of factors regulating these communities along with the roles these communities play in coastal wetland dynamics is needed. The impact of changing watershed use patterns on these wetland communities has not yet been determined. Finally, symbiotic and competitive interactions between the macrophyte and algal communities need to be elucidated. INDEX WORDS: Algae, aquatic macrophytes, estuary, Lake Erie, Great Lakes, wetland.

INTRODUCTION

Keddy and Reznicek (1985, 1986) examined the role of changing lake water levels and seed banks in determining the vascular flora. Fewless (1986) hypothesized that factors affecting seed germination may explain the distribution of Scirpus validus Vahl. in wetlands of Green Bay, Lake Michigan. Historically, botanical research in wetlands has concentrated on the aquatic macrophyte component. The algae have either been ignored or only rarely studied (cf. Mitsch and Gosselink 1986). This certainly has been true for research conducted in wetlands of the southern Great Lakes. Frederick (1975) and Millie (1979, Millie and Klarer 1980, Millie and Lowe 1981, 1983) were among the first

The aquatic vascular flora of Great Lakes wetlands has been the focus of extensive research (see Geis and Lee 1977; Harris et al. 1977, 1981; Keddy and Reznicek 1985, 1986; Stuckey 1989). Although most research conducted to date has concentrated on documenting changes in species composition, density, and area of cover, researchers recently have begun to examine specific environmental and physiological processes affecting these changes. For example, Geis (1985) examined the impact of ice formation and breakup on perennial macrophyte distribution in Lake Ontario wetlands.

622

BOTANICAL RESEARCH IN GREAT LAKES WETLANDS researchers to focus on the algae of Lake Erie coastal wetlands. The aquatic macrophyte community is generally believed to dominate primary production in wetland areas (cf. Mitsch and Gosselink 1986). However, several researchers have demonstrated that the attached algal component may make a significant contribution to wetlands production. Allen (1971) attributed 30070 of the total littoral production in a small Michigan Lake to attached algae. Brock (1970) reported that epiphytic algae, rather than macrophytes, were responsible for the majority of primary production of Utricularia communities. LeCren and Lowe-McConnell (1980) reported epiphytic production to be four times that of the macrophytes to which they were attached in a British river system. The relative contribution of phytoplankton to the total wetland production would be expected to increase as the proportion of open water in a wetland increases. Elevated water levels on Lake Erie and the other Great Lakes during the past two decades have caused many former wetland areas to become open-water, shallow embayments with only fringes of aquatic macrophytes. In these areas, phytoplankton would be the dominant primary producers (Reeder and Mitsch 1989). The objective of this paper is to review the vascular plant and algal research which has been conducted at Old Woman Creek (OWC) State Nature Preserve and National Estuarine Research Reserve. This work will be related to research conducted in other Great Lakes coastal wetlands, particularly those in western Lake Erie. Finally, similarities and differences between the botanical communities in Great Lakes coastal wetlands such as OWC and their marine counterparts will be briefly discussed. SITE DESCRIPTION The general physical and chemical characteristics of OWC have previously been described (Krieger 1984, 1989b, Klarer 1988, Klarer and Millie 1989). The OWC coastal wetland has been classified as a "freshwater estuary," along with the drowned portions of other river mouths entering the Great Lakes (Herdendorf 1990). The unique physical and chemical parameters of this coastal wetland type have been discussed in Krieger (1989a). However, several characteristics of the OWC freshwater estuary and its watershed which affect the algae and aquatic macrophytes need to be stressed. The creek

623

LAKE ERIE

KILOMETERS

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UPPER

ESTUARY

FIG. 1. Vegetated sites in Old Woman Creek estuary. Refer to Table 1 for dominant aquatic macrophytes at each site.

is a 2nd-order stream draining an agricultural watershed of approximately 79 km 2 • The estuarine portion of OWC extends 2.1 km southward from the creek mouth. The estuary is divided into distinct upper and lower reaches by a railroad rightof-way (Fig. 1) which have characteristics of riverine and lacustrine wetlands, respectively (after Cowardin et al. 1979). While the upper estuary is largely confined to a narrow, deep channel, the lower estuary extends over 0.3 km 2 with a mean depth of less than 1 m. A small island bifurcates water flow through the lower estuary. The large surface area to depth ratio in the lower estuary ensures that bottom sediments are easily resuspended by moderate winds (Krieger 1984). The 10-

624

KLARER and MILLIE

cation and size of the creek mouth are modified by a shifting-sand barrier beach which isolates owe from the lake for portions of each year. When the mouth is open, seiches and storm surges in Lake Erie can rapidly change the water level in the estuary. AQUATIC MACROPHYTES

The aquatic macrophytes in wetlands along the southern shore of western Lake Erie have been studied since the late 1800s. The first comprehensive inventory of the aquatic plants was undertaken in 1898 by Pieters (1901). About the same time, Moseley (1899) published a list of plant taxa found in the Sandusky Bay region. Studies undertaken on the vascular flora of western Lake Erie have been facilitated by the proximity of Stone Laboratory (Ohio State University). From its founding in 1903 through the present, researchers from Stone Laboratory have studied vegetation dynamics in nearby wetlands. Stuckey (1989) presented a comprehensive review of aquatic macrophyte research and the changes in macrophyte composition in western Lake Erie over the previous 80 years. The aquatic macrophytes of owe estuary were first systematically inventoried in 1973 by Marshall (1977, Marshall and Stuckey 1974). He recognized five distinct vegetation sites in the estuary proper. To aid in comparing changing vegetation patterns from his benchmark study in 1973 with our work through 1989, we have divided two of his sites and have added one site (Table 1, Fig. 1). Distinct changes in the vascular flora of owe estuary have occurred during this 16-year period (Table 1). The most apparent change is the disappearance by the early 1980s of many of the dominant plant species reported by Marshall and Stuckey (1974). In 1973, the flora of the estuary was dominated by Peltandra virginica (L.) Kunth., Nelumbo lutea (Willd.) Pers., and Polygonum coccineum Muhl. Nuphar advena Ait., Nymphaea tuberosa Paine, Potamogeton pectinatus L., and Potamogeton nodosus Poir. also were very common. By the early 1980s, several of the vegetated sites originally described by Marshall and Stuckey (1974) no longer supported aquatic macrophytes. Nelumbo lutea had assumed or retained dominance at virtually all remaining vegetated sites. Nuphar advena and N. tuberosa, although still present, had a more restricted distribution than in 1973. During the mid-1980s, P. pectinatus was not

observed. However, in 1989 several small stands were observed in the lower estuary. Additionally, Marshall and Stuckey (1974) did not report any rooted macrophytes south of the island. During a 5-year period (1981-1985) an extensive bed of N. lutea became established and then disappeared in this area (site 8, Fig. 1). During 1984 to 1985 and 1988 to 1989 aerial photographs were used to map changing vegetation distribution patterns. The open water vegetation during these periods was almost exclusively N. lutea. From 1984 to 1985, large N. lutea beds north and west of the island (sites 2 & 7 respectively, Fig. 1) became fragmented (Fig. 2). Therefore, vegetation cover was less extensive in 1985 than in the previous year. A dense N. lutea bed south of the island (site 8, Fig. 1) was observed from 1981 to 1985, with maximum growth during 1982 and 1983. By 1985, only small remnants of this bed remained (Fig. 2). The N. lutea beds directly north and south of the railroad tracks (sites 4-6, Fig. 1) expanded from 1984 to 1985 (Fig. 2). Vegetation changes observed from 1988 to 1989 were equally dynamic. The N. lutea beds north of the island and railroad right-of-way (sites 2 and 4 respectively, Fig. 1) and west. of the island (site 7, Fig. 1) diminished in size (Fig. 2). The leading edge of the bed north of the island advanced toward the estuary mouth. The small beds at the estuary mouth and south of the railroad (sites 1 and 5 respectively, Fig. 1) did not change (Fig. 2). Rising water levels may have had a significant impact on the vegetation of owe estuary. Changes in water level in Lake Erie clearly affect water levels in the estuary during the periods when the creek mouth is open. Water levels during the growing seasons of 1985 and 1989 were greater than those observed in 1984 and 1988 respectively (Fig. 3). Decreased aquatic macrophyte cover coincided with these increased water levels (see Fig. 2). The disappearance of many dominant species reported by Marshall and Stuckey (1974) also appears to be related to elevated water levels in Lake Erie since the late 1960s (Fig 4). Similar reductions in diversity and area of vegetation cover in other Great Lakes macrophyte communities have been attributed to higher water levels (Farney and Bookhout 1982, Burton 1985). In owe estuary, it has not yet been possible to identify the specific environmental factors which regulate vegetation dynamics. If changes in environmental parameters associated with fluctuating water levels are the driving force in vegetation dynamics, then popula-

BOTANICAL RESEARCH IN GREAT LAKES WETLANDS

625

TABLE 1. Vegetation dynamics in Old Woman Creek Estuary from 1973 to 1989. Asterisk indicates vegetation not present at site. Where more than one taxa are listed, boldface indicates dominant taxon. Refer to Figure 1 for site location in the estuary. Site

2

3

4 5 6 7 8

1973 a

1984

1989

Peltandra virginica Potamogeton pectinatus Potamogeton nodosus Nymphaea tuberosa N.lutea N. tuberosa P. pectinatus P. nodosus Polygonum coccineum Hibiscus palustris Cornus drummondi N.lutea Nuphar advena P. coccineum Nuphar advena N. tuberosa Peltandra virginica P. pectinatus

Nelumbo lutea

N.lutea P. pectinatus

N. lutea

N. lutea

*

*

*

N. lutea

N. lutea

N. lutea

N.lutea

N. tuberosa

N. tuberosa

N. lutea

N. lutea

N. lutea

*

adata from Marshall and Stuckey (1974)

FIG. 2. Vegetation dynamics in the estuary during 1984-1985 and 1988-1989. Blackened areas represent stands of the dominant rooted aquatic macrophyte, Nelumbo lutea.

626

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tions which have declined and/or disappeared with rising levels should reappear with the lower levels presently being experienced. The aquatic vascular flora of OWC estuary is not representative of floras reported from other Great Lakes coastal wetlands. Nelumbo lutea, the dominant macrophyte at OWC, is considered a southern taxon (Marshall 1977) with northern limits of distribution being the southern Great Lakes. Within the Great Lakes, N. lutea is largely confined to the Lake Erie and Lake St. Clair regions

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(Argus et al. 1982-1987), although it has been reported once from the southern Georgian Bay area and inland from counties bordering western Lake Ontario and the southern tip of Lake Michigan (The Nature Conservancy per.comm.). This taxon is not considered common in either Lake Erie wetlands or Lake St. Clair wetlands (Herdendorf 1987, Herdendorf et al. 1986). Nelumbo lutea is frequently observed with the other floating-leaved plants Nuphar advena and Nymphaea tuberosa, although it may form dense monospecific stands. Such dense stands tend to inhibit colonization by potential competitors (Sculthorpe 1967). Although all three taxa are considered relatively intolerant of wind and wave activity (Sculthorpe 1967), turbulence may playa role in determining which taxon assumes dominance in a particular wetland area. Heslop-Harrison (1955) reported that Nuphar sp. (N. lutea(L.) Sibth. & Sm.) appeared to be more tolerant to water movement than Nymphaea sp. (N. alba L.). Leaves with a deep notch such as those of Nuphar sp. and Nymphaea sp. are more prone to tearing in turbulent water than are entire leaves such as those of Nelumbo sp. (Sculthorpe 1967). Therefore, Nymphaea sp. would be replaced by Nuphar sp., which in turn would be supplanted by Nelumbo sp. in waters of increasing wind and wave turbulence. The emergent vegetation characteristic of many of the coastal marshes in the Great Lakes is noticeably sparse in OWC estuary. The emergent vegetation in the estuary is currently confined to the marginal fringes, primarily due to higher lake levels. Since the 1973 study by Marshall (1977, Marshall and Stuckey 1974), the distribution of emergent vegetation has become even more restricted than they reported. The common emergent plants reported in 1973, Peltandra virginica west and northwest of the island, and Polygonum coccineum southeast of the island, have disappeared. An extensive bed of Typha spp. formerly covered the area west of the island but had disappeared by 1973 due to higher water levels in the late 1960s and early 1970s (Marshall and Stuckey 1974). Marshall (1977) considered that aquatic vascular plants were very poorly represented in OWC estuary. He attributed this to two factors: turbidity, and morphometry of the land surrounding the estuary. The steep banks surrounding OWC estuary greatly limit shallow water habitats during high water periods. Therefore, the shallow water vegetation is largely extirpated during high water conditions. Additionally, the high turbidity levels in

BOTANICAL RESEARCH IN GREAT LAKES WETLANDS

the water column inhibit the presence of many submerged macrophytes including Elodea sp., Myriophyllum sp., and Potamogeton spp. Robel (1961) also reported that decreased macrophyte biomass resulted from increased turbidity. Carp (Cyprinus carpio L.) may be important in determining the aquatic macrophyte composition in OWC and in other Great Lakes coastal wetland areas. King and Hunt (1967) considered that the uprooting of macrophytes during carp feeding and spawning may be a major factor in regulating wetland vegetation in carp infested wetlands. Marshall (1977) believed that the paucity of both cattails and sedges in OWC may be due to carp. In addition, the scarcity of submerged macrophytes in OWC estuary and in other impacted coastal wetlands could be the result of carp activity which increases turbidity levels in the water column. The diversity in Great Lakes coastal wetlands is largely dependent upon fluctuating lake levels and existing seed banks (Keddy and Reznicek 1985). High water levels prevent the invasion of terrestrial and woody species into the wetland area. Fluctuating water periods also help prevent the creation of monoculture marshes. During low water periods, the seed banks in exposed mudflat areas provide a major source for revegetation of these areas. Siegley et al. (1988) reported that the seed bank in a recently developed mitigation marsh in Sandusky Bay, Lake Erie closely mirrored the aquatic flora that previously had been reported from the area. They also noted that the seed bank was rich in species important for wildlife and waterfowl. The major pest species present in the marsh were apparently introduced from outside the marsh, since they were not well represented in the seed bank. However, preliminary work conducted in Old Woman Creek has indicated a different pattern (R. Laushman, Oberlin College, per. comm., May 1992). Here the seed bank in the surface sediments is dominated by terrestrial weed species with only a smattering of aquatic macrophytes, primarily Peltandra virginica. The dominant aquatic macrophyte, Nelumbo lutea, is very poorly represented in the seed bank. However, this scarcity of aquatic macrophytes in the seed bank may be a result of the scouring and disturbance history of the sediments in the estuary rather than the seed bank not reflecting the vegetation of the estuary (R. Laushman, per. comm., May 19~2). Other researchers (e.g., van der Valk and Davis 1979) have questioned whether a seed bank actually reflects the macrophyte flora. They have observed that

627

many shallow-marsh species such as Carex spp., produced a very limited number of seeds so that these plants were greatly under-represented in the seed bank. In addition, the seeds of other plants were difficult to germinate. These plants could be well represented in the seed bank, but very sparse in the extant vegetation. Leck et al. (1989) reported that seed bank diversity is considerably greater than seedling diversity or the extant marsh vegetation diversity. ALGAE

Lake Erie, particularly its archipelago, has been the subject of extensive phycological research. The lake's importance as a natural resource and the subsequent, well-publicized deterioration of its water quality by increased nutrient loading have resulted in many algal studies (e.g., Tiffany 1934; Chandler 1940, 1942, 1944; Taft and Taft 1971; Taft and Kishler 1973; Hohn 1969; and Munawar and Munawar 1976). Davis (1964, 1969) attributed three distinct changes in the phytoplankton since the 1920s to the increased nutrient loading: 1) a three-fold increase in total phytoplankton; 2) a shift from brief vernal and autumnal population pulses to pulses of greater intensity and duration leading to a reduction in the length of the summer minima; and 3) a shift in algal dominance with Aulacoseira (=Melosira) spp. replacing Asterionella formosa Hass. as the dominant diatom during the vernal pulse and from Fragilaria, subgenus Alterasynedra (= Synedra) spp. to Aulacoseira spp. to Fragilaria, subgenus Fragilaria spp. along with increases in green and blue-green algae in the autumn pulse. By the 1970s the autumn pulse had become dominated by blue-green algae (Kline 1981). Despite great interest in algae of Lake Erie proper, the algae of wetlands bordering this and other Great Lakes have largely been ignored. This is readily illustrated by the small number of algal studies conducted in Lake Erie wetlands and the large number of newly-reported taxa for the lake arising from these studies (Table 2). The number of epiphytic taxa is most notable. Millie and Lowe (1981) attributed this high number to a lack of previous littoral algal studies (particularly the diatom component), problems associated with sampling the three-dimensional epiphytic matrix, and the diversity of habitats in the various types of Lake Erie coastal wetlands. Despite the extensive investigations of Lake Erie phytoplankton, 83 of

628

KLARER and MILLIE

TABLE 2. Studies documenting newly-reported algal taxa from wetlands in central and western Lake Erie. Habitat Planktonic Benthic Epiphytic Epiphytic Phanktonic Epiphytic Epilithic

Number of New Taxa Reported 49

11

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FIG. 5. Dynamics of dominant phytoplankton groups during 1981.

the 263 newly-reported taxa are planktonic or tychoplanktonic. The algal populations in Lake Erie estuaries were first theorized to be extensions of lake populations (Sullivan 1953). However, the large number of newly-reported planktonic taxa appears to contradict this hypothesis. The near-shore phytoplankton of Lake Erie typically exhibits a bimodal pattern with maxima in the late winter/early spring and in late summer/early autumn. The spring pulse is dominated by diatoms, particularly Fragilaria subgenus Fragilaria spp., Diatoma sp., Stephanodiscus spp., and Asterionella formosa. The autumn pulse is dominated by the blue-green algae, Aphanizomenon flos-aquae (L.) Ralfs and Microcystis aeruginosa Kuetz., emend Elenkin (Kline 1981). Bimodal pulses of the dominant phytoplankton were not observed in the owe estuary during 1981. Rather, four distinct pulses were observed (Fig. 5). An initial, minor pulse followed ice-out in the estuary and was most pronounced at the creek mouth. This pulse was composed primarily of the diatoms Cye/otella spp., Navicula spp., Fragilaria subgenus Fragilaria spp., and Asterionella formosa. The pulses in May and July to mid-August were dominated by small diatoms, particularly Cye/otella spp. The pulse in October was typified by the diatom, Aulacoseira alpigena (Grun.) Krammer (=Melosira distans var. alpigena Grun.), the chrysophyte, Cryptomonas erosa Ehren., the green algae, Lagerheimia spp. and Tetrastrum spp., and the blue-green algae Gomphosphaeria sp. Periodicity and spacial distribution of estuarine phytoplankton are influenced by factors such as fluctuating nutrient concentrations, flow patterns, and light availability (Day et al. 1989). During 1984 and 1985, the role of storm-water inputs in regulating phytoplankton dynamics in owe estu-

ary was examined (Klarer and Millie in preparation). In both the upper and lower estuary, decreases in phytoplankton standing crop corresponded to periods of high turbidity, indicating that storm water flushed plankton out of the estuary (Fig. 6). In the upper estuary, standing crop remained low after passage of the storm water. In the lower estuary, however, standing crop quickly rebounded after passage of the storm water due to rapid growth of Cryptomonas erosa and small diatoms, particularly Cye/otella spp. These differences in population dynamics were attributed

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FIG. 6. Phytoplankton standing crop in response to storm-water inflow.

BOTANICAL RESEARCH IN GREAT LAKES WETLANDS to the presence and absence of refugia in the lower and upper estuary, respectively. The upper estuary is a deep narrow channel with few backwater areas that would escape the flushing action of storm waters. In contrast, the flushing action of storm waters would be diminished in the broad and shallow lower estuary. Additionally, backwater areas in the lower estuary would not be as directly impacted by storm water as those in upper estuary and, therefore, could serve as a source of taxa for repopulation. In OWC estuary, influxes of limiting macronutrients, including nitrate, orthophosphate, and silica, accompany storm-water input (Klarer 1988, Klarer and Millie 1989). This nutrient input, coupled with increased light penetration (as suspended sediment levels subside), provide optimal growth conditions for phytoplankton (see Legendre et al. 1986, Day et al. 1989). In this manner, rapid reestablishment of opportunistic taxa and subsequent flushing of these populations by storm events might contribute to the succession of phytoplankton maxima and minima observed in 1981. Similar regulation of phytoplankton periodicity by riverine flow and turbidity dynamics have also been observed in marine estuaries (Welsh et al. 1972, Cloern et al. 1983, Filardo and Dunstan 1985). Studies on the benthic algal communities in OWC and other Lake Erie coastal wetlands have largely been confined to basic taxonomic surveys and seasonal succession studies. Millie (1979, Millie and Lowe 1983) did examine host specificity. The work completed at OWC has largely been restricted to a taxonomic survey of the epiphytic algae (Millie and Klarer 1980, Klarer 1985). However, a study of impact of various environmental parameters including nutrients, light, and turbulence on the epipelic algal species composition has recently been completed (Jensen 1992). PRIMARY PRODUCTION The recent high-water levels in Lake Erie have resulted in a decrease of macrophyte vegetation in OWC estuary. Presently, the estuary is an algalbased system. Reeder and Mitsch (1989) estimated annual planktonic and macrophyte productivity to be ca. 366 and 75 g carbonem 2e yr respectively. Water column productivity is greatest during summer months when solar influx is greatest, and appears to be limited only by day length (Reeder and Mitsch 1989). Heath (1987) concluded that algal productivity in OWC was light limited rather than

629

nutrient limited. Additionally, Reeder (1990) attributed high water-column productivity values at the edge of N. lutea beds to dense growths of epiphytic algae on the petioles. Reeder and Mitsch (1989) and Robb (1989) estimated annual above ground production of N. lutea at several sites throughout the estuary. Using quadrat and transect methods, Reeder and Mitsch (1989) estimated net production at ca. 160 and 131 g dry wt m 2 respectively. Robb (1989) reported a mean dry weight of 187 with a range of 125-301 g dry wt m2 • These values ate much lower than those previously reported from other coastal wetlands in Lake Erie (Robb 1989) and are, undoubtedly, attributable to the type of dominant vegetation in OWC estuary. The high water levels in OWC estuary in recent years have selected for broad, floating leaf taxa, such as N. lutea, rather than emergent taxa, such as Typha spp. and Scirpus spp. Consequently, the lower biomass values per unit area are not unexpected (Robb 1989, Reeder and Mitsch 1989, Reeder 1990). e

e

GREAT LAKES VS MARINE COASTAL WETLANDS The most obvious difference between the wetlands bordering the Great Lakes coast and those bordering the sea coasts is the presence of high salinity in the latter. Salinity has been demonstrated to be the dominant factor in regulating species composition in both the aquatic macrophytes (Adam 1963) and the algae (Cholnoky 1968). Odum (1988) reported that macrophyte diversity was much greater in a freshwater tidal wetland than in a salt marsh, due largely to the added stress of high salinity in the latter. The extensive beds of coastal marshes dominated by one species along the Atlantic and Gulf coasts are not found in the Great Lakes. Annual fluctuating lake levels have largely prevented this domination by one or two species in Great Lakes coastal wetlands (Keddy and Reznicek 1985) and may also be important in mitigating the pronounced vascular plant zonation that has been observed in salt marsh systems (e.g., Adam 1963, Nixon 1982). Because water levels are constantly changing in the Great Lakes, there is a diminished opportunity for distinct zonation to become established. Annually fluctuating water levels have also been credited with increasing the diversity in Great Lakes coastal wetlands (see Keddy and Reznicek 1985).

630

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Traditional estuarine and marine coastal wetland are very harsh environments for benthic algae (Admiraal 1984). Apart from the missing salinity gradients, Great Lakes coastal wetlands mirror this harshness. The algae of both systems must endure short periods of exposure and the resulting desiccation caused by tides (marine) or seiches (Great Lakes). Both wind- and storm-induced turbulence may impact the benthic algal communities in both systems. Light penetration is equally variable in both systems, ranging from very high intensity during short periods of exposure to very low intensity during periods of high turbidity. Although it is often assumed that freshwater and marine ecosystems are phosphorus and nitrogen limited, respectively, this may not necessarily be the case in coastal wetlands. In a study of the EmsDollard (marine) estuary, Admiraal (1978) reported that the benthic diatoms were generally not limited by either nitrogen or phosphorus. Heath (1987) reported that the phytoplankton in Old Woman Creek estuary were not nutrient limited. Jensen (1992), however, reported possible evidence of phosphorus limitation and nitrogen limitation in the epipelic algae of OWC estuary during different times of the year. FUTURE RESEARCH The botanical research conducted to date in Great Lakes coastal wetlands has provided only cursory insights into a series of complex and dynamic communities. While the population dynamics of aquatic macrophytes have been well documented, the causative factors are still largely unknown. Fluctuating water levels in the Great Lakes have been identified as an important regulating force in macrophyte composition (Prince and D'Itri 1985). However, knowledge of the long-term effects on c,ommunities by changing water levels is still fragmentary. The role of algae in the wetlands ecosystem is largely unknown and represents a major void in our understanding of wetland ecology. Baseline studies concerning algal temporal and spatial periodicity need to be conducted. Only when this information is known can questions concerning the role of algae in wetland production and trophic interactions be addressed. Additionally, interactions between estuarine and littoral lake algal populations need to be analyzed. The influx of estuarine algae into the littoral zone, particularly during storm events, may have a marked local impact on

structure and production of the lake's littoral algal community. Agriculture is the dominant land-use pattern within the Lake Erie watershed, as it is for most of the Great Lakes. The short- and long-term impacts of agriculturally-derived chemicals, particularly pesticides, on wetlands have only recently been addressed (cf. Krieger et al. 1989). Despite these recent studies, impacts are not fully understood and much work remains to be undertaken. Although pesticide residues remain in OWC estuary throughout the year, storm waters import an additional chemical load (Krieger 1984). Subsequent alterations in water quality may result in short-term changes in community composition and/or physiology of the primary producers. However, the long-term ramifications of these changes on population dynamics and the physiology of the community proper are unknown. Finally, symbiotic and competitive interactions between epiphytic and macrophytic communities have yet to be elucidated. In addition to providing sites of attachment, host substrates may also be a source of nutrients and organic substances for the epiphytic community (Lee et al. 1975). Fitzgerald (1969) and Wetzel (1992) have demonstrated that interactions between algal communities and aquatic macrophytes are more complex than had previously been suspected. Because algal communities form much of the base of the grazer food web, these interactions will have ramifications through the entire wetland system. ACKNOWLEDGMENTS Portions of the work presented here are part of an on-going biological and chemical monitoring program at Old Woman Creek State Nature Preserve & National Estuarine Research Reserve. This program is funded by the Division of Natural Areas and Preserves, Ohio Department of Natural Resources. Additional funding is provided by the National Estuarine Research Reserve System, National Oceanic & Atmospheric Administration, U. S. Department of Commerce. The authors express appreciation to Scott Hoffman and Gary Obermiller for collecting water samples. Roger Laushman provided unpublished data on the seed bank at OWC. We thank Christopher Dionigi, Donna Gibson, Maren Klich, and Kenneth Krieger for critically reviewing the manuscript.

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Submitted: 5 June 1991 Accepted: 25 August 1992