Extent of Sedge/Grass Meadow in a Lake Michigan Drowned-River-Mouth Wetland Dictated by Topography/Bathymetry and Lake Level

Water-level uctuations are critical in maintaining the diversity of plant communities in Great Lakes wetlands. Sedge/grass meadows are especially sensitive to such uctuations. We conducted vegetation sampling in a sedge/grass-dominated Lake Michigan drowned-river-mouth wetland in 1995, 2002, and 2010 that followed high lake levels in 1986 and 1997. We also conducted photointerpretation studies in 16 years dating back to 1965 to include responses to high lake level in 1952 and 1974. Topographic/bathymetric data were collected to assess their inuence on areal extent of sedge/grass meadow. Dominant species in short emergent and submersed/oating plant communities changed with water availability from 1995 to extreme low lake levels in 2002 and 2010. Sedge/grass meadow was dominated by Calamagrostis canadensis and Carex stricta in all years sampled, but Importance Values differed among years partly due to sampling in newly exposed areas. Photointerpretation studies showed a signicant relation between percent of wetland in sedge/grass meadow and summer lake level, as well as the number of years since an extreme high lake level. From the topographic/bathymetric map created, we calculated the cumulative area above each 0.2-m contour to determine the percent of wetland dewatered in select years following extreme high lake levels. When compared with percent sedge/grass meadow in those years, relative changes in both predicted land surface and sedge/grass meadow demonstrated that accuracy of lake level as a predictor of area of sedge/grass meadow is dependent on topography/bathymetry. Our results regarding relations of plant-community response to hydrology are applicable to other Great Lakes wetlands. survey extent may have led to a poor depiction of actual ground surface and/or bathymetric elevation values. LIDAR DEMs from ights conducted in 2008. Portions of these data were clipped and included to supplement the products produced as described above. The interpolated DEM was used to create a topographic/bathymetric map with 20-cm contour intervals. Using the contour tool within ArcPro, the contour interval was set to 0.2m, and the base contour was set to an elevation of 175m, with a Z factor of 1. The contour type was “contour polygon” to enable summarizing the area between the contours. The contour polygon layer was then clipped to the extent of the study area, and using the summarize-within tool, the area between each contour was calculated in square meters.


Introduction
The relations between water-level uctuations and plant communities of Great Lakes wetlands have been extolled for decades (e.g., Keddy and Reznicek 1986;Hudon 1997;Wilcox 2004; Wilcox and Nichols 2008; Keddy and Campbell 2020; Smith et al. 2021). High lake levels cause die-back of upland and canopydominating wetland species, and succeeding low lake-level periods expose sediments and allow regrowth of smaller-statured wetland plants from seeds or propagules. Over the past nearly 5000 years, water levels on Lake Michigan-Huron (one lake hydrologically) have uctuated on a quasi-periodic cycle (Baedke and Thompson 2000; Argyilan et al. 2018). Large, longer-term uctuations from high to low lake levels recur about every 160 years, with uctuations of about 30-33 years riding on them. The latter uctuation pattern is responsible for much of the vegetation change (Wilcox 2004).
Models have been created that demonstrate the relation of such hydrology on vegetation. Wilcox and Nichols (2008) modeled the response of a bulrush marsh in Saginaw Bay of Lake Huron to the reduction of water levels following an extreme high in 1986. The model made use of plant community data collected along transects that followed elevation contours with speci c water-level histories to make predictions of plant-community change as a function of meters above water vs. number of years out of water. Keddy and Campbell (2020) made use of literature knowledge to model relative marsh area vs. duration of dewatering in relation to invasion by and ooding-out of woody species.
The plant community most affected by water-level uctuations is sedge/grass meadow (SGM), which occurs along the higher elevation fringe of the wetlands and, in relation to ooding and dewatering, is in competition with emergent vegetation at the lower extent and woody vegetation at the upper extent (Keddy and Reznicek 1986;; Keddy and Campbell 2020). Gathman et al. (2005) sampled wetland plant communities in northern Lake Huron in 1996 when water levels were above average, 1997 with high water levels, and 1998 when levels were back near those of 1996. Stem counts of dominant sedge and grass species in the wet meadow decreased by >70% from 1996 to 1997 and increased in 1998, but only by 16%. Their study did not capture changes that would have occurred by 1999, when lake levels decreased an additional half meter, but we expect that increases were much greater. They also did not collect data on areal extent of SGM nor on elevations. Werner and Zedler (2002) and Peach and Zedler (2006) addressed microtopographic surface area in SGM provided by Carex stricta tussocks but not wetland topography or surface area.
On Lake Ontario,  and Wilcox and Bateman (2018) assessed changes in SGM in relation to water levels using photointerpretation of aerial photographs. Wilcox and Xie (2007) brought in topographic relief data to model response of SGM to water-level uctuations. However, Lake Ontario water levels have been regulated since about 1960 when the St. Lawrence Seaway began operation (Hudon et al. 2006; Wilcox and Xie 2007), so the results do not necessarily re ect natural Great Lakes processes. Although the Keddy and Campbell (2020) model imposed the concept of areal extent, it did not contain numerical area data, which they acknowledged would require site-speci c topography/bathymetry.
As part of three studies at Arcadia Marsh on the eastern shore of Lake Michigan, we had the opportunity to sample changes in SGM and other plant communities in 1995, 2002, and 2010 related to water-level decreases from extreme lake-level highs in 1986 and 1997 (Fig. 1). Observations made prior to the last study in 2010 led us to recognize that area of SGM in various years was likely determined by the relation between topography/bathymetry and lake levels, which was not yet quanti ed. Thus, we added collection of elevation data to the study, as well as photointerpretation analyses, which allowed us to include decreases from extreme lake-level highs in 1952 and 1974 to the study also (Fig. 1). Our overall study objectives were to assess changes in composition of plant communities across the three study years that had different water-level histories and to draw correlations between topography/bathymetry and lake level in determining areal extent of SGM, which had previously not been made.

Study Area
Arcadia Marsh is a 170-ha drowned-river-mouth wetland ) near the village of Arcadia in Manistee County, Michigan, USA (Fig. 2). The wetland follows the corridor of a stream formed by the con uence of Bowens, Tondu, and Lucker Creeks crossing a wide basin upstream from Arcadia Lake, which is connected to the eastern shore of Lake Michigan by a channel. The wetland is separated from Arcadia Lake by a 1.0-km-long road-crossing (M- 22) with two large culverts that restrict ow during high ow periods. Surface sediments in the aquatic zone are largely decomposed peat, with some sand and silt. Numerous ditches were constructed through much of the wetland in an attempt to drain it for agriculture, which was unsuccessful because water levels are controlled by Lake Michigan levels. The adjacent land is used for agriculture. The most prominent vegetation type is sedge/grass meadow dominated by Calamagrostis canadensis and Carex stricta, although some areas have been invaded by species such as Phalaris arundinacea, Typha angustifolia, Typha × glauca, and more recently Phragmites australis.

Vegetation Sampling
Initial vegetation mapping and plant sampling were conducted during 27-28 July 1995 (Wilcox et al. 2002). Major vegetation types clearly de nable on aerial images, which were photographed in 1994 in anticipation of their need, were identi ed and ground-truthed in the eld with photographs in hand. Vegetation types were categorized as interior sedge/grass meadow, invaded sedge/grass meadow, short emergent, cattail, and submersed/ oating. We then sampled ten 1-m x 1-m quadrats in each vegetation type according to a randomly dispersed, haphazard design (blind toss over shoulder). All taxa in each quadrat were identi ed to species, if possible, and estimates of percent cover were assigned to each taxon in the quadrats at one-percent increments to 10, then at vepercent increments. Data on T. angustifolia and T. × glauca were combined due to the tedious task of identifying each plant individually. Similar sampling was conducted during 13-16 August 2002 and 20-22 July 2010, with 20 quadrats sampled in interior sedge/grass meadow and submersed/ oating. Photointerpretation New 1:5000 CIR photos in 1994 and 1:6000 CIR photos in 2002 and 2010 were contracted through private vendors. Following an extensive search, we selected the best available imagery from 1965 through 2010 for analysis (Table 1). Available orthophotos were loaded into ArcMap GIS (ESRI) such that each site had a layer le projected to UTM zone 16N. Heads-up digitizing was used to identify the boundaries of plant community types and delineate them with polygon features. Due to variation of photo resolution and quality, the resolution at which vegetation was delineated differed across years, although a scale of 1:1500 or better was used when drawing polygons. The border extent followed the description provided below for topographic data collection. We used auto-complete polygons within the editor toolbar in ArcMap to produce polygon features around vegetation stands identi ed in each year (e.g., 2010: Fig. 3). To ensure that the extent of wetland delineated was the same in each year, the clip feature in ArcMap was used so that only the delineated polygons within the wetland border were used in the analyses. Total area was determined and compared to ensure that they matched.

Topographic Data Collection
Topographic/bathymetric survey data were collected during July 2010 sampling using two methods. A Trimble RTK GPS was used for real-time, differentiallycorrected elevations in areas above water, totaling 647 points with estimated elevation accuracy of 3 cm. A Garmin hand-held GPS in combination with waterdepth measurements from a boat and a benchmark on the bridge culvert that was surveyed by RTK GPS was used for below water areas, totaling 381 points (Appendix Fig. 1).
All Digital Elevation Models (DEMs) used the International Great Lakes Datum 1985 (IGLD85) for reference. All spatial data sets were created based on the Universal Transverse Mercator Zone 16 North coordinate system, the North American Datum of 1983, and used internal units of the SI (metric) system.

Processing Methods
Extent. Prior to processing survey data to generate a DEM, we needed to establish an extent polygon for subsequent processing steps. We began by constructing a polygon within a Geographic Information System (GIS) based on the outermost points from the July 2010 topographic surveys. The polygon boundaries were then adjusted to limit the number of seemingly extraneous surveyed data points.
Merging hand-held GPS boat-based and RTK GPS data sets. GPS data were merged by selecting point features from each set, copying those features with the select tool in Arcmap's editor tool bar, and pasting them into a new shape le with the same tool. Horizontal positional accuracy, vertical elevation values, and naming conventions were used to cross-validate the independent attribute elds with the input data that were used to produce the merged shape le.
Adjusting elevations to the IGLD85 datum. Vdatum (version 2.3.0) produced by the National Oceanic and Atmospheric Administration was used to convert North American Vertical Datum of 1988 (NAVD88) elevation values to the IGLD85 system.
Interpolation. An Inverse Distance Weighted (IDW) interpolation method was applied to the survey point data within Arcmap's Spatial Analyst extension using the default settings. These settings included a second power weighting function combined with a variable search radius that required twelve points to estimate grid cell values.
Clipping. The resulting DEM was clipped based on the extent polygon we created. This eliminated unrealistic data from beyond the July 2010 survey extent that may have led to a poor depiction of actual ground surface and/or bathymetric elevation values.

LIDAR DEMs were provided by the United States Army Corps of Engineers Joint Airborne LIDAR Bathymetry Technical Center of Expertise (USACE JLBTCX)
from ights conducted in 2008. Portions of these data were clipped and included to supplement the products produced as described above.
The interpolated DEM was used to create a topographic/bathymetric map with 20-cm contour intervals. Using the contour tool within ArcPro, the contour interval was set to 0.2m, and the base contour was set to an elevation of 175m, with a Z factor of 1. The contour type was "contour polygon" to enable summarizing the area between the contours. The contour polygon layer was then clipped to the extent of the study area, and using the summarize-within tool, the area between each contour was calculated in square meters.

Data Analyses
Plant community data from eld sampling were sorted by vegetation type and year. For evaluation, Importance Values (IV) for all taxa were then calculated by vegetation type in each year as relative mean percent cover + relative frequency x 100.
We determined the area of wetland mapped in each vegetation type in each photo year and converted data to percent of wetland. To assess the relations of the prominent sedge/grass meadow with variable Lake Michigan water levels, we ran regressions of %SGM in each photo year from the photointerpretation data against the mean, three-month, growing season lake levels (June, July, August) in photo years and against the number of years since each photo year had an extreme high lake level (> 177m IGLD85).
To assess the effects of lake-level reductions following extreme highs (> 177m), we made comparisons between %SGM from photointerpretation results and topographic/bathymetric data. We calculated the cumulative area above each 0.2-m contour and interpolated to 0.01m levels to determine the percent of wetland that would be predicted to dewater when Lake Michigan water levels receded from extreme highs to those for speci c photo years.

Vegetation Sampling
The interior sedge/grass meadow was dominated by C. canadensis and C. stricta in all years sampled, with other prominent taxa including Impatiens capensis and Persicaria amphibia in 1995, Campanula aparinoides in 2002, and Carex aquatilis and Carex lacustris in 2010 ( Table 2). The decrease in summertime peak water levels in 2002 was accompanied by an increase in C. canadensis, but it was not maintained in 2010 sampling. Carex stricta decreased in 2002 and even more in 2010.  Species  1995  2002  2010  1995  2002  2010  1995  2002  2010  1995  2002  2010  1995  2002 Bidens sp.

Salix exigua
Nutt. - Scutellaria galericulata L. Engelm. - Schleiden. - The invaded sedge/grass meadow was dominated by C. stricta, C. canadensis, and Typha sp. in 1995 (Table 2). However, sampling of this vegetation type was largely in a different area in 2002 and 2010 because lower water levels exposed areas of previously ooded sedge/grass meadow. Carex stricta and Typha then decreased in sampling, with P. arundinacea, C. lacustris, and C. canadensis becoming the dominant species.
Areas sampled for short emergents also changed with changes in water level. During higher water levels in 1995, dominant species were Sparganium eurycarpum and Lemna minor, with Ceratophyllum demersum and Sagittaria latifolia prominent (

Topography/Bathymetry
The topographic/bathymetric map showed little variation in elevation across much of the study area (Fig. 4). Elevations from 175.8m to 176.8m (IGLD85) contained 89.8% of the total study area, which ranged in elevation from 175.0m to 178.0m (Table 4). Elevations from 176.0m to 176.4m accounted for 47.3% of the total study area. For later analyses, the cumulative area of wetland above each contour interval was also calculated, which con rmed that much of the wetland area had little relief.

Short Emergents
Many changes in IV in the short emergent community from 1995 to 2002 to 2010 (Table 2) were likely related to decreases in growing season water levels across years, resulting in changes in locations of areas that were sampled -the short emergents were in different places. However, species typical of more shallow waters (Swink and Wilhelm 1979; Chadde 2012; Voss and Reznicek 2012), such as E. palustris, L. oryzoides, and S. tabernaemontani, increased substantially with lower lake levels in 2002.

Submersed/Floating
The most striking change in IV in the submersed/ oating vegetation type was the great reduction in N. variegata from 1995 to 2002 and absence in 2010 sampling (Table 2). By 2002, lower lake levels reduced standing water to a relatively narrow channel that previously contained no Nuphar. The channel was rather turbid in 2002 and 2010, which explains dominance by turbidity-tolerant C. demersum, P. crispus, and S. pectinata (Adamus and Brandt 1990).
Sedge/Grass Meadow and Lake Levels One of our objectives in this publication was to assess the responses of sedge/grass meadow to uctuations in Great Lakes water levels, as SGM is an . We took several approaches for this assessment, including using our dataset to making comparisons of %SGM mapped in each photo year with mean three-month growing season lake level and number of years since the last extreme high summer lake level de ned as >177m (1952:177.26m; 1974:177.27m; 1986;177.37m; 1997:177.16m). Percent SGM vs. mean three-month lake level, which is biologically important because it represents much of the growing season, showed a signi cant relation (R 2 = 0.756, p = 0.000) (Fig. 5A). The relation of %SGM vs. years since high lake level was also signi cant (R 2 = 0.703, p = 0.000) (Fig. 5B) and has importance because sedge/grass meadow may be reduced or even eliminated by extreme ooding, as will be addressed in our next assessment.
The percent of Arcadia Marsh returning to SGM when water levels decreased from high lake-level years differed among years. A 1.24-m reduction in threemonth summer lake level from 1952 to photo year 1965 (Fig. 1)  resulted in an increase from 20.6% (photo year 1998, in which lake level had already decreased by 0.29m) to 61.0% in ve years. In a period of ve to 13 years following a reduction in growing season water levels by 0.83m or more from an extreme high lake level, the increases in %SGM, as could be measured from available photos, ranged widely. Each of those time periods had lag times of ve or more years, suggested as critical by Wilcox and Xie (2007). The disparity in response comes from the starting and ending elevations of lake level in these comparisons combined with the topography/bathymetry of the wetland.
Growing season water levels were 176.02m in 1965, 176.54m in 1992, and 176.31m in 2002. Simply, lower lake levels may expose more of the underlying soils and create more habitat for sedge/grass meadow.
To assess this relationship, we made comparisons between %SGM from photointerpretation results and the topographic/bathymetric data from Arcadia Marsh, which were developed with 0.2-m contour intervals (Table 4). We calculated the cumulative area above each contour and interpolated to 0.01m levels to determine the percent of wetland that would be predicted to dewater when Lake Michigan water levels receded from extreme highs for comparison with %SGM in speci c photo years. Exposure of the land surface would be expected to promote development/reestablishment of sedge/grass meadow (Wilcox 2004; Keddy and Campbell 2020).
The topographic/bathymetric data predicted 76.2% exposure of the wetland land surface in 1965, while %SGM was 84.4% (Fig. 6). The three-month summer lake level was very low (176.02m), and despite an intervening moderate, single-year high in 1960 (Fig. 1), there was a lengthy time lag following the 1952 extreme high lake level that would promote growth of sedge/grass meadow. Following the 1974 extreme high, predicted exposure of land surface was 11.4% in 1978, with lake levels at 176.67m, and %SGM was 24.4%. Although percent land cover was less than %SGM in 1978, both values were much less than for 1965.
Following the 1986 extreme high lake level, predicted land surface exposed was 13 Following all post-extreme years, percent land-surface exposure was always less than corresponding %SGM, likely due to accuracy of topographic/bathymetric mapping based on a nite number of RTK-GPS and water-depth data points and LIDAR DEM data from a relatively at area. However, relative changes in both predicted land surface and %SGM shown in our results demonstrate that accuracy of lake level as a predictor of area of sedge/grass meadow is dependent on topography/bathymetry. Lake Michigan water levels increased again beginning in 2014 and extended to a three-month extreme high of 177.44m in 2020 (Fig. 1). When water levels decrease dramatically again, as expected based on historical data and paleo-lake-level studies (Baedke and Thompson 2000, Argyilan et al. 2018), the response of SGM in Arcadia Marsh and other Lake Michigan/Huron wetlands will depend on the duration and amplitude of the low lake-level period but also on topography/bathymetry.

Relation to Other Great Lakes Wetlands
These relations of plant-community response to hydrology are most applicable to other drowned-river-mouth wetlands along the eastern shore of Lake

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Funding (information that explains whether and by whom the research was supported)  Availability of data and materials: All data produced from this study are provided in this manuscript.

Figure 2
Map showing the Arcadia Marsh study site along the eastern shore of Lake Michigan in Manistee County, MI.    Percent of Arcadia Marsh study area (blue) with elevation greater than three-month summer water levels of Lake Michigan (June-August) and (red) mapped as sedge/grass meadow (SGM) in years with low lake levels following extreme highs in 1952, 1974, 1986, and 1997.

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