The Oconto Marsh #2 shoreline experienced considerable change over the monitoring period. From satellite imagery (years 2013 and 2018), we estimate the total vegetated wetland area decreased from 1.2 km2 in 2013 to 0.6 km2 in 2018 (Figure 1). Using the lake access walkway located between transects 2 and 3 as a point of reference, the walkway and shoreline vegetation (where the majority of 2011 wet meadow and emergent zone sampling points were placed) are largely absent from the 2018 satellite imagery (Figure 1b & d). While we did observe wet meadow zone expansion farther inland in 2016 (discussed below), the total wetland area gained did not offset the area lost, because the wet meadows that developed are fragmented by uplands and housing settlements. The loss of vegetated wetland area farthest lakeward was almost certainly amplified by P. australis herbicide treatment in 2014. Overall, we suggest that the combination of herbicide treatment and factors associated with higher water levels (e.g., high energy wave action and ice scour penetrating farther inland) are likely both responsible for causing shoreline alterations at the site. Tracking the return of vegetation back to its previous extent is a future monitoring objective.
The lowest growing season (e.g., May to October) Lake Michigan water levels occurred in 2012 and 2013 (mean = 176.0 MASL) and the highest were in 2019 and 2020 (mean = 177.4 MASL) (Figure 1). Because several sampling points moved farther inland towards higher elevation areas in later years, the increase in water depths recorded at each sampling point do not entirely reflect the > 1 meter water level increases that occurred. The shifting of coastal wetland plant guilds farther inland to maintain water depth preference in response to rising water levels was documented by Smith et al. (2020) for Lake Ontario coastal wetlands. In response to rising Lake Superior water levels, Hartsock et al. (2022) documented that a 100 cm increase in submergent zone water depths caused a reduction in submergent zone extent and a decline in aquatic macrophyte species richness at a coastal wetland in Superior, Wisconsin. At Oconto Marsh #2, submergent and emergent zone water depths (measured within each quadrat) increased by about 35 cm from 2011 to 2021 (Figure 2). Because of these extreme water depth changes that occurred over a relatively short period, turnover in species composition should be expected (discussed below). Annual water depth changes were less extreme across the wet meadow zones. Water depths were highest in the wet meadow zones in 2017 (about 8 cm), but generally were stable at near 0 cm over the entire study period, which was largely due to expansion of the wet meadow zone farther inland towards higher elevation areas.
Transects and sampling points
Table 1 summarizes the number of sampling points surveyed in each vegetation zone in each year. In 2011, all three transects had a definable submergent, emergent, and wet meadow vegetation zone, however, only 37 of the quadrats contained vegetation. Eight of 15 quadrats in the submergent zone were unvegetated in 2011. Sampling points in 2011 were located farther lakeward compared to later years and the emergent zone was less than 11 meters wide along transects 1 and 2, thereby warranting use of the narrow transect procedure (Figure 1). In 2016, a total of 35 sampling points were present along the transects and all sampling point quadrats contained vegetation. Submerged aquatic vegetation was present along transect 3 only. In accordance with the CWMP protocol, the transect 3 bearing shifted slightly to avoid sampling a shrub-dominated area. While the submergent zone extent decreased in 2016, emergent and wet meadow zones expanded and shifted farther inland following trends similar to those reported by Hartsock et al. (2022) and Smith et al. (2021). In contrast to 2011, all emergent zones along the transects exceeded 11 meters in 2016. Additionally, the farthest inland wet meadow sampling points in 2016 were over 50 meters farther inland compared to the 2011 farthest inland points. In 2017, due to a GPS malfunction, we were unable to display sampling point locations. Transect 2 was unable to be resampled in 2017 due to high water preventing safe access to the farthest lakeward areas. In total, 20 sampling points along transects 1 and 3 were sampled in 2017 and a submergent zone was not present along either of the two transects. In 2021, a total of 30 sampling points were sampled that were in similar locations as those from 2016. However, in contrast to 2017 observations, an emergent vegetation zone could not be defined along any of the three transects. Thus, between 2017 and 2021 there was turnover in plant composition at the farthest lakeward sampling points from an emergent vegetation-dominated community to a submerged aquatic-dominated community.
Plant community dynamics
A total of 101 vascular plant species were encountered during our four sampling campaigns at Oconto Marsh #2. The complete species list is shown in Table S1. Table 2 shows the ten most abundant plant species within sampling point quadrats for all years Oconto Marsh #2 was surveyed and mean relative cover. Total species richness was lowest in 2011 (32 species) and ranged from 52 to 56 species in years 2016, 2017 and 2021. Our observations of increasing species richness follow patterns from Lishawa et al. (2019) who documented plant richness increases after cattail removal at two Lake Huron marshes. However, some of our added species are likely due to sampling farther inland along the transects. Dominant species present in 2011 were notably dissimilar from later years (Table 2). Invasive P. australis was most abundant in 2011 with relative cover ranging from 5 to 70 percent when present in quadrats. Other dominant species in 2011 within the submergent, emergent, and wet meadow zones included Utricularia intermedia, Schoenoplectus pungens, and Carex stricta, respectively. Except for C. stricta, these species were less abundant in later years of monitoring. Following treatment of P. australis with herbicide in 2014, exposure to rising water levels, and subsequent loss of vegetated wetland area resulted in considerable species turnover at the site. Sampling point quadrats containing P. australis declined from 18 quadrats in 2011, 6 quadrats in 2016, and 3 quadrats in 2021. While P. australis was dominant across the site in 2011, dominant species in years 2016, 2017, and 2021 were mostly comprised of Calamagrostis canadensis, Phalaris arundinacea, Solidago canadensis, Potamogeton spp., and Stuckenia pectinata. P. australis was not encountered along the research transects in 2021, highlighting the success of management. However, despite reduction of P. australis cover, mean C and adjusted FQI progressively declined across the site from 2011 to 2021 (Table 3). This occurred because more non-native species were present in later years. In 2011, four non-native species were encountered: Lythrum salicaria, P. arundinacea, P. australis (invasive), and Typha angustifolia. In later years eight, additional non-native species were found that included: Cirsium palustre, Convolvulus arvensis, Equisetum arvense, Potentilla norvegica, Rumex crispus, Stellaria media, Typha x glauca, and Xanthium strumarium. Overall, higher proportions of low quality herbaceous and non-native species in years 2016, 2017, and 2021 are responsible for site-wide declines in floristic quality. Many plant species encountered in later years are typical colonizers of disturbed soils. Thus, the opening of niche space following P. australis treatment, repeat disturbance related to rising water levels, and a changing shoreline have likely facilitated establishment of several opportunistic and non-native species at Oconto Marsh #2. Similar to our observations, Lishawa et al. (2019) reported an increase of early successional species in the wet meadow zone and a shift in the emergent zone community from a cattail-dominated community towards an aquatic macrophyte-dominated community following cattail removal at two Lake Huron marshes.
Ordinating the quadrat-based percent cover data using NMDS (3D stress = 2.0) highlights the dissimilarity between several 2011 sampling points from those in later years (Figure 3). In general, 2011 sampling points are concentrated in the positive region of axis 1 and negative region of axis 3. Overlaying dominant species on the ordination emphasizes the close association between several 2011 sampling points with U. intermedia and P. australis. Elevated cover of Juncus canadensis, Schoenopectus tabernaemontani and Leersia oryzoides in 2021 contributed to the dissimilarity between 2021 sampling points and the other years. Using year as a grouping factor, PERMANOVA analysis revealed years 2016 and 2017 were not different from each other, whereas years 2011 and 2021 were different from all other groups (Pseudo F = 3.4; p<0.001). Overall, PERMANOVA and NMDS demonstrate considerable turnover in species composition has occurred. Species richness increased in the wet meadow areas after 2011 monitoring, and both emergent and submergent zones were in a state of flux as water levels progressively increased. While coastal wetlands are documented to be resilient ecosystems, prolonged exposure to recent record high Lake Michigan water levels over multiple growing seasons has potentially affected coastal wetland vegetation structure to a greater magnitude than other studies have shown. Treatment of P. australis with herbicide and its contribution to shoreline destabilization is also a factor not to be ignored. More studies aimed at assessing whether our observations of declining floristic quality and vegetated wetland area losses are the norm throughout Green Bay, or a unique circumstance should be encouraged.
An important added benefit of the CWMP is the total amount of wetland area observed by on-the-ground professional botanists while accessing and surveying research sites. In 2021, we discovered the invasive species European frogbit (EFB) (Hydrocharis morus-ranae L) at Oconto Marsh #2 near the research transects, but not within the 1 m2 quadrats (Figure 4). Our observation of EFB is the first documented occurrence in the state of Wisconsin. EFB is a perennial free-floating aquatic plant native to parts of Europe, Asia, and Africa (Catling et al. 2003), but extremely invasive in Great Lakes wetlands. It is typically found in still or slow-moving aquatic habitats that include, but are not limited to ponds, ditches, marshes, canals, backwaters, and coastal wetlands. EFB can become dominant or co-dominant within five years after introduction and drastically alter aquatic ecosystems. Rapid growth in summer leads to formation of dense intertwined mats at the water’s surface (Monks et al. 2019). Subsequent attenuation of available sunlight can negatively affect aquatic macrophytes below (Catling et al. 2003; Catling et al. 1988). While seed production is supposedly rare, EFB’s rate of invasion is enhanced by asexual reproduction strategies that include stoloniferous growth and turion production. A single plant can produce up to 150 turions that are viable for up to two years (Catling et al. 2003). At Oconto Marsh #2, several relatively small colonies of EFB were observed in open water areas within the south ditch adjacent to County Road Y (Figure 1), and additional EFB colonies were found in proximity to the bridge crossing. An unnamed stream that bisects the site and flows directly into Lake Michigan is likely facilitating the spread of EFB propagules to new areas. A voucher specimen from Oconto Marsh #2 was collected and sent to the University of Michigan Herbarium (R.D. Rutherford 151 MICH) for documentation. EFB population coordinates were immediately reported to the Wisconsin Department of Natural Resources (WDNR). Action was taken by WDNR personnel to mitigate the EFB infestation and further investigation revealed the infestation range spanned from Oconto, WI (10 km south of the original discovery) to Marinette, WI (21 km northeast). Many infestation sites are state properties and state natural areas. Over 2,000 pounds of EFB was hand-pulled as of October 1, 2021. The larger and more concentrated infestations are slated for treatment in early 2022 (Amanda Smith WDNR personal communication).
Across the Great Lakes basin, Lakes Ontario, Erie, and Huron have the most documented EFB observations, while Lake Michigan detections are more recent and less widespread. While no documented EFB observations are known for Lake Superior, its arrival is highly probable because EFB is present in the St. Marys River, the connecting channel between Lakes Huron and Superior (Dr. Dennis Albert personal observation, 2010). The presence of EFB in Lake Huron’s Munuscong Bay in the eastern Upper Peninsula of Michigan has reduced the cover of several aquatic macrophytes, including common bladderwort (Utriculata vulgaris) (Monks et al. 2019, Wellons 2018). Robichaud and Rooney (2021) showed that following invasive P. australis removal from some Lake Erie coastal wetlands, EFB subsequently colonized the open water areas within two years. This is especially relevant to our study, as many Green Bay coastal wetlands have been treated with glyphosate-based herbicide to remove invasive P. australis with great success. However, some of these herbicide-treated sites have transitioned into open water wetlands. The opportunity for invasion by non-native colonizers such as EFB has already been documented at this and other sites. We recommend follow-up monitoring in areas treated with herbicide to mitigate recolonization by secondary invaders.