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Estuaries and Coasts

, Volume 41, Issue 1, pp 25–35 | Cite as

Understanding the Impacts of Climate Change: an Analysis of Inundation, Marsh Elevation, and Plant Communities in a Tidal Freshwater Marsh

  • P. Delgado
  • P. Hensel
  • A. Baldwin
Article

Abstract

Tidal freshwater marshes around the world face an uncertain future with increasing water levels, salinity intrusion, and temperature and precipitation shifts associated with climate change. Due to the characteristic abundance of both annual and perennial species in these habitats, even small increases in early growing season water levels may reduce seed germination, seedling establishment, and late-season plant cover, decreasing overall species abundance and productivity. This study looks at the distribution of tidal freshwater marsh plant species at Jug Bay, Patuxent River (Chesapeake Bay, USA), with respect to intertidal elevation, and the relationship between inundation early in the growing season and peak plant cover to better understand the potential impacts and marsh responses to increased inundation. Results show that 62% of marsh plant species are distributed at elevations around mean high water and are characterized by narrow elevation ranges in contrast with species growing at lower elevations. In addition, the frequency and duration of inundation and water depth to which the marsh was exposed to, prior to the growing season (March 15–May 15), negatively affected peak plant cover (measured in end-June to mid-July) after a threshold value was reached. For example, 36 and 55% decreases in peak plant cover were observed after duration of inundation threshold values of 25 and 36% was reached for annual and perennial species, respectively. Overall, this study suggests that plant communities of tidal freshwater marshes are sensitive to even small systematic changes in inundation, which may affect species abundance and richness as well as overall wetland resiliency to climate change.

Keywords

Tidal freshwater marsh Patuxent River Jug Bay Climate change Marsh elevation Inundation Marsh species and elevation 

Introduction

Tidal freshwater wetlands are a common feature in the upper reaches of estuaries around the world (Baldwin et al. 2009). A distinctive characteristic of many of these systems is their high species diversity, particularly in the mid-high marsh zone, comprised mostly of salt-intolerant species (Barendregt and Swarth 2013; Perry et al. 2009; Odum 1988). This diversity, coupled with high rates of biological productivity, renders these marshes critical for the survival of both resident and migratory fauna, in addition to a host of other ecosystem functions. Recent studies have investigated the vulnerability of tidal freshwater marshes to climate change such as rising temperatures (Bullock et al. 2013; Baldwin et al. 2014), salinity increase (Gao et al. 2014; Neubauer 2013), and sea level change (Delgado et al. 2013; Craft et al. 2009; Morris et al. 2002), but there is still much uncertainty in how these systems will respond to the predicted changes (Swarth et al. 2013).

Sea level rise in the Chesapeake Bay is faster than other regions within the country due to factors including isostatic rebound and subsidence (Williams 2013). Rates range between 2.9 and 5.8 mm year−1 (Boon et al. 2010). The nearest active long-term National Oceanic and Atmospheric Administration (NOAA) tide gage to Jug Bay, at Annapolis, has a published sea level rise rate of 3.5 ± 0.21 mm year−1 (http://tidesandcurrents.noaa.gov/sltrends/sltrends.html). Future sea level rise along the mid-Atlantic seaboard is also predicted to be higher due to expected changes in ocean currents (Ezer et al. 2013) as well as geological and other considerations (Miller et al. 2013).

Increased water levels translate into increased inundation in tidal wetlands along the Patuxent, including Jug Bay, a large complex of tidal freshwater marshes in Maryland and along the eastern coast of the USA. How vulnerable a particular marsh would be to increased inundation would depend on factors including the rate of water level change itself, the local tidal regime, the marsh’s ability to compensate through positive elevation change, and species tolerance levels to inundation (Kozlowski 1984; McKee and Mendelssohn 1989; Ward et al. 1998; Casanova and Brock 2000; Baldwin et al. 2001; Morris et al. 2002; Stevenson and Kearney 2009).

Vertical accretion through sediment deposition may help counterbalance sea level rise: long-term sedimentation records from soil cores at Jug Bay showed a positive gain in elevation post-European settlement with additional peaks of sediment deposition during the late-1960s and early-1980s following rapid urbanization in the upstream watershed (Khan and Brush 1994). A more recent study, however, has shown spatial variability of marsh elevation change within the Jug Bay estuarine system with some areas gaining and others losing elevation (Delgado et al. 2013; Delgado, unpublished data).

Species’ tolerance levels to inundation differ among different species, and their location along the intertidal elevation gradient may affect their vulnerability to increased inundation (Bockelmann et al. 2002; Silvestri et al. 2005; Suchrow and Jensen 2010). Overall, wetland vegetation communities tend to be found in relatively narrow elevation ranges (e.g., McKee and Mendelssohn 1989), so increased inundation due to sea level rise can cause spatial shifts in wetland communities as well as wetland loss. In addition, the low tidal amplitude at Jug Bay (about 0.75 m) renders the wetlands even more susceptible to sea level rise (SLR) (Stevenson and Kearney 2009). Species zonation in tidal freshwater marshes, especially within the highly diverse mid-high marsh zone, is not always clear; on the contrary, species appear to be distributed in a random manner forming a mosaic of species assemblages (Barendregt and Swarth 2013; Odum et al. 1984). The low marsh zone, however, because of its lower elevation and proximity to intertidal mudflats or open water, may be expected to be affected first. This zone is often colonized by a low diversity of perennial (e.g., Nuphar lutea) and annual (Zizania aquatica) species adapted to more frequent and prolonged inundation (Odum et al. 1984; Field and Philipp 2000). The mid-high marsh zone, because of its higher elevation, is often colonized by a more diverse plant community (over 80 species, including Typha spp., Persicaria arifolia, Peltandra virginica, Leerzia orizoides, Hibiscus moscheutos, Sagittaria latifolia, Impatiens capensis, and Acorus calamus), but less tolerant to inundation (Barendregt and Swarth 2013; Odum et al. 1984).

Inundation and hydrology are widely referred as main drivers of marsh dynamics. A tidal freshwater marsh in South Carolina exposed to increasing freshwater water inputs showed a decrease in gross ecosystem productivity (Neubauer 2013). Baldwin et al. (2001), Leck et al. (2009), Parker and Leck (1985), and Gosselink and Turner (1978) point to the importance of hydrology and seed banks for annual species as main factors governing local vegetation patterns of tidal freshwater wetlands. Baldwin et al. (2001) found that inundation patterns can control plant species composition from year to year and that species diversity may be reduced by significant hydrological change. Gosselink and Turner (1978) indicate the importance of the local hydrologic regime (velocity, duration of flooding, water depth) determining species composition and richness in tidal freshwater wetlands.

In 2008, the NOAA National Estuarine Research Reserve System (NERRS) initiated a network of climate change “sentinel sites,” establishing permanent monitoring infrastructure in estuarine systems to obtain long-term data that will enable monitoring the response of coastal habitats to changing water levels and other climate-related impacts (NERRS 2012). In that same year, Jug Bay was one of the early adopters of this strategy and became a NERRS sentinel site. As such, Jug Bay established infrastructure to monitor groundwater levels, marsh vegetation, wetland surface elevation change, water quality, and weather conditions, all connected together through high-precision surveying to a local vertical control network. Data collected at Jug Bay through this sentinel site effort were analyzed in this study to address two main hypotheses regarding the potential impacts and response of Jug Bay marshes to increased inundation along the Patuxent river: (1) vegetation species distribution within the marsh is related to differences in surface elevation, and (2) there exists a negative relationship between peak plant percent cover and the degree of inundation the marsh is exposed to early during the growing season. Understanding species distribution in relation to elevation is key as we try to better understand species vulnerability to local inundation changes. The impact of increased inundation in tidal freshwater marsh species, particularly annual species, during a key period (early growing season) of the growing season was indicated through a greenhouse experiment conducted by Baldwin et al. (2001). Data collected through the Jug Bay sentinel site program provides an opportunity to test this finding in the field and to bring to light yet another potential vulnerability of these systems to increased inundation.

Study Location

This field study was conducted in the tidal freshwater marshes of Jug Bay (38.7846° N, 76.7008° W), located about 65 km upriver from the mouth of the Patuxent River. Jug Bay tidal marshes are very diverse, featuring approximately 95 species of wetland plants, of which 11 (12%) are annual and 84 (88%) perennial (Jug Bay Wetlands Sanctuary’s wetland plant list: http://www.jugbay.org/research/species_lists). These species are distributed along the intertidal gradient through a range of elevations from the low marsh to scrub-shrub habitat. The low marsh has been generally characterized as the portion of the marsh that gets flooded more frequently (3–12 h per day), where depth of flooding is the deepest (0–200 cm), and which is often colonized by species tolerant to prolonged inundation such as N. lutea, Z. aquatica, P. cordata, Sagitaria lattifolia, and P. virginica. The mid-high marsh gets flooded for a period of 2–8 h per day and contains the highest species plant diversity (Simpson et al. 1983; Delgado et al. 2013). Characterized as a shallow tidal freshwater marsh system, Jug Bay has an average water depth of about 1 m and its salinity is generally lower than 0.5 ppt, but spikes of up to 2.4 ppt have been documented, particularly during drought years (Delgado, unpublished data).

Monitoring infrastructure installed within the Jug Bay marsh complex is distributed among three main study sites: Iron Pot Landing (IPL; along Western Branch), Railroad Bed (RR; along the main stem of the Patuxent River), and Mataponi Creek (MC; along Mataponi Creek) (Fig. 1). These sites are located along a 6-km reach within the tidal fresh portion of the Patuxent River and include direct interaction with either the main stem of the river or tributaries and tidal creeks. The monitoring components found at each study site include one continuous water quality monitoring station (CONMON), four surface elevation tables (SETs; Lynch et al. 2015), and five marsh vegetation transects (exception: railroad bed has only four transects). One weather station located at the railroad bed site collects information for the entire marsh complex (Fig. 1). Most of the data collected at these sites are available at http://cdmo.baruch.sc.edu/.
Fig. 1

a Approximate location of Jug Bay estuarine system within the Patuxent river watershed; b map showing the three study sites: iron pot landing, railroad bed, and mataponi creek; the circle represents the location of the weather station, the diamonds the location of the CONMON stations, and the stars the location of the emergent vegetation transects; c close-up of the iron pot landing study site, showing the transects and the location of the surface elevation tables (SETs) represented as circles along transects 1 and 2

Methods

Local Vertical Control Networks and Datums

To allow comparisons of geospatial datasets among all study sites (IPL, RR, MC), a survey was conducted in 2011 to connect all SET stations, CONMON stations, and marsh vegetation transect plots together through the establishment of local vertical control networks at each site. Multiple simultaneous, ≥ 5-h static GPS observations were taken on deep rod SET marks (one at each site) and nearby published NAVD 88 bench marks over the course of four separate days in April 2011. All GPS observations were post-processed using the NGS Online Position User Service (OPUS) Projects software. Mean sea level (MSL) was computed from the monthly means of the hourly water levels at the RR CONMON station for the period 2010–2015. All other tidal datums were derived from MSL using the tidal datum separation from the nearest NOAA tide station at Lower Marlboro (station 8579542—not active), located at approximately 15 km from the RR station.

Species Distribution and Marsh Surface Elevation

Surface elevation measurements at single-species-dominated stands and species assemblages were collected during the summer of 2015 from the three study sites (IPL, RR, and MC; Fig. 1) using a Leica Geosystems Sprinter 250M digital barcode level. The level was used to measure the height difference between the soil surface where a particular plant was found and a local vertical control mark (nearby surface elevation table, or SET mark) with a known NAVD 88 elevation. Which species were sampled at each study site depended on their presence and dominance there.

The locations for these elevation measurements were selected following three rubrics: (1) random samples were taken at species homogeneous patches. A homogeneous patch was defined as an area of at least 1 m2 dominated by a single species; (2) in areas with mixed species, measurements were randomly taken at the right, center, and left of temporary transects drawn perpendicular from the water; and (3) to capture species growth ranges, random measurements were taken along the lowest vegetation line in the lower marsh zone and in the most upper marsh ranges. If two or more plant species were present at the same point, all of the species were assigned the same elevation. A minimum of 20 measurements were taken under each rubric.

All elevation data collected for a particular species across the different study sites were combined for statistical analyses using the statistical programming language R. Average elevations of plant species were compared within a one-way analysis of variance model. Post hoc multiple comparisons were run using a Tukey adjustment.

Inundation During Early Growing Season and Peak Plant Cover

Early growing season was defined as the period between March 15 and May15, and the peak of the growing season occurred between end-June and mid-July. Three main datasets were collected from two of the study sites (IPL and RR): (1) water depth (2010–2015), (2) plant plot surface elevation (2015), and (3) plant species composition and percent cover (2010–2015).

Water Depth

Water depth was recorded every 15 min at each IPL and RR CONMON stations (Fig. 1) using a non-vented YSI Series 6600 multi-sonde. All data were corrected for barometric pressure from the weather station at the railroad bed. The sensor orifice (sensor “zero” point) was surveyed to each site’s local vertical control network using either leveling or real-time kinematic (RTK) GPS.

Plant Plot Surface Elevation

Surface elevation measurements were taken at each corner of every plant plot in April 2014 using a Trimble R8® single-base RTK system. Rover occupations lasted a full 3 min per point. The base station was set up on a deep rod SET mark that had been previously surveyed (see above). Checks were made to other control points (other local deep rod SET marks) throughout the survey to ensure accuracy.

Water depth data from IPL and RR stations from March 15 to May 15 (early growing season) for each year of the study (2010–2015) and plot elevation data (2014) were used to estimate three inundation parameters: (a) average water depth per plot in each year (March 15–May 15); (b) duration of inundation (total fraction of time each plot was inundated in each year, March 15–May 15, represented as percent of time inundated); and (c) frequency of inundation (total number of inundation events per plot in each year, March 15–May 15, represented as number of inundation events.

Duration and frequency of inundation were estimated through a simple algorithm using the CONMON water level data (expressed with respect to NAVD 88) and marsh surface elevations (also on NAVD 88). The algorithm tracked water levels over the entire time period (March 15–May 15) and computed the cumulative time that a particular plot was expected to have been inundated based on its elevation with respect to the local water depth (to derive the total duration of inundation). Each occurrence of expected inundation was also tallied to provide an estimate of the number of inundation events. These inundation calculations assume water surfaces are level between the nearby CONMON station and the wetland sites. We recognize that this assumption introduces uncertainties related to bathymetry differences, hydrodynamics, wind fields, etc. The expected accumulated error contribution would lie in the range of 0.01–0.02 m (NOAA CO-OPS, personal communication). Similarly, the inundation calculations also assume that the elevation at the plots remained relatively unchanged during the 5-year study period. We recognize that sedimentation, compaction, and decomposition processes affect changes in elevation over time; however, there was no way to correct for this unknown source of error.

Marsh Plant Species Composition and Percent Cover

Five marsh emergent vegetation transects were established in 2008 at each of the study sites following an orientation perpendicular from the water. Each transect contains between four and six 1-m2 plots (Fig. 1), which are sampled yearly during the peak of the growing season (end-June to mid-July). Measurements of species composition, percent cover, density, and maximum height taken at each plot follow established National Estuarine Research Reserve’s (NERRS) protocols for sampling marsh emergent vegetation (Moore 2009). Following this protocol, plant percent cover can reach values higher than 100% as it estimates cover for species growing not only at the canopy level but also under the canopy. In 2008, one transect at the railroad bed site was lost, and none of the transects were sampled in 2012.

All the plant percent cover data from IPL and RR stations (2010–2015) were combined and statistical analyses were conducted by annual and perennial species as well as individual species using Excel. It is important to clarify that these are not cause and effect analyses.

Excel exploratory data analysis was used to detect patterns in the data. A plot of plant percent cover vs. inundation parameters (frequency or duration) or water depth suggested the presence of an inundation or water depth threshold above which a marked decline in percent cover occurred (lower inundation mapped to higher percent vegetation cover). The concept of “threshold” was tested by iteratively dividing the data into two parts: above-threshold and below-threshold inundation—for values of “threshold” ranging from the lowest to the highest metric of inundation in the data (0% inundation vs. 1–94% inundation, 0–1% inundation vs. 2–94% inundation, etc.). At each step along the way, a T test was used to compare the average percent cover among the two groups thusly divided. The P values for the tests were then plotted according to the corresponding metric of inundation. The value of inundation corresponding to the minimum P value determined to be the threshold (typically where the P value plummeted to close to zero). Once threshold values for all inundation parameters were identified, calculations were conducted to estimate decrease of percent plant cover.

Results

Species Distribution and Marsh Surface Elevation (Hypothesis 1)

Using computed tidal datums from Jug Bay (MLLW, MLW, MSL, MHW) as a reference and each species’ minimum elevation as the sorting parameter, results indicate that Pontederia cordata and Nuphar lutea were the only two species out of 26 species sampled found below the MLLW elevation mark. Zizania aquatica was the only species below MLW. Six species (23%) were found at elevations between MLW and MSL, while the majority of the species (62%) were found between MSL and MHW. Only Decodon verticillatus was found above the MHW mark (Table 1). The species that showed the largest ranges of elevation (maximum minus minimum elevation) included N. lutea, P. cordata, Peltandra virginica, and Z. aquatica (Fig. 2). All of these are most often found within the low marsh zone, although P. virginica is also common at mid-high elevations.
Table 1

Distribution of Jug Bay tidal freshwater marsh plant species, sorted by their minimum surface elevation value and with respect to the local Jug Bay mean lower low water (MLLW), mean low water (MLW), mean sea level (MSL), and mean high water (MHW) tidal datums. All elevations are given with respect to NAVD 88

Species (code)

Minimum elevation (m)

Maximum elevation (m)

Average elevation (m)

Standard error (m)

Pontederia cordata (POCO)

− 0.42

0.35

− 0.04

0.03

Nuphar lutea (NULU)

− 0.34

0.44

0.14

0.01

Mean lower low water

  

− 0.22

 

Zizania aquatica (ZIAQ)

− 0.21

0.30

0.12

0.01

Mean low water

  

− 0.16

 

Bidens laevis (BILA)

− 0.08

0.34

0.20

0.01

Persicaria hydropiper (PEHY)

− 0.08

0.39

0.20

0.02

Murdannia keisak (MUKE)

− 0.03

0.36

0.18

0.03

Peltandra virginica (PEVI)

0.00

0.57

0.32

0.01

Persicaria arifolia (PEAR)

0.09

0.52

0.35

0.01

Typha spp. (TY)

0.09

0.52

0.30

0.01

Mean sea level

  

0.12

 

Sparganium eurycarpum (SPEU)

0.14

0.52

0.42

0.01

Impatiens capensis (IMCA)

0.18

0.55

0.41

0.00

Schoenoplectus tabernaemontani (SCTA)

0.19

0.42

0.32

0.01

Schoenoplectus pungens (SCPU)

0.20

0.44

0.35

0.01

Hibiscus moscheutos (HIMO)

0.20

0.55

0.40

0.01

Leersia oryzoides (LEOR)

0.20

0.48

0.36

0.01

Bolboschoenus fluviatilis (BOFL)

0.27

0.52

0.38

0.01

Sagittaria latifolia (SALA)

0.28

0.52

0.44

0.01

Spartina cynosuroides (SPCY)

0.30

0.37

0.34

0.00

Galium spp. (GA)

0.35

0.48

0.39

0.01

Eleocharis spp. (EL)

0.35

0.52

0.44

0.01

Onoclea sensibilis (ONSE)

0.36

0.57

0.47

0.01

Cephalantus occidentalis (CEOC)

0.36

0.52

0.45

0.02

Juncus effusus (JUEF)

0.36

0.52

0.45

0.02

Symphyotrichum puniceum (SYPU)

0.36

0.52

0.45

0.01

Carex spp. (CA)

0.36

0.55

0.46

0.01

Mean high water

  

0.38

 

Decodon verticillatus (DEVE)

0.40

0.57

0.48

0.01

Fig. 2

Distribution of Jug Bay tidal freshwater plant species arranged based on the size of their elevation range in which they were found: plants with the greater ranges are to the left; those with smaller ranges are to the right. Mean lower low water (MLLW), mean low water (MLW), mean sea level (MSL), and mean high water (MHW) were added as a reference. The meaning of species codes are indicated in Table 1. Elevations are expressed with respect to NAVD 88

The distributions of most species are concentrated at higher elevations, specifically around the MHW mark. P. cordata, N. lutea, and Z. aquatica, which are characteristically found in the low marsh zone, are distributed along almost the entire elevation range and showed a peak distribution between the MSL and MHW marks (Fig. 3). Multiple comparisons by average elevation grouped species in four main groups (Fig. 4; P ≤ 0.001). The last two groups blend into each other, so the distinction between them was based on the result that Bolboschoenus fluviatilis was significantly different from both Typha and P. virginica.
Fig. 3

Histogram of Jug Bay tidal freshwater marsh plant species along marsh surface elevation values. Upper panel shows the distribution of plants typically found in the low marsh zone; lower panel shows the distribution of plants typical of the mid-high marsh zone. Elevations are expressed with respect to NAVD 88

Fig. 4

Grouping of species by average marsh elevation resulting from multiple comparisons (P ≤ 0.001). Open symbols within a same group indicate a significant difference between those two species. Division between LEOR and BOFL based on significant differences between BOFL and both TY and PEVI, the lowest members of the previous group. Elevations are expressed with respect to NAVD 88

Inundation During Early Growing Season and Peak Plant Cover (Hypothesis 2)

Figure 5 shows a representation of the relationship between peak plant percent cover (end-June to mid-July) and duration of inundation (percent inundation) for annual and perennial species for a time period during early growing season (March 15–May 15). Instead of seeing a smooth decreasing trend of peak plant cover with increased inundation, a sharp and significant (P ≤ 0.01) drop in peak plant cover was observed at what appears to be a “threshold” value of duration of inundation. The threshold values were 25 and 36% inundation for annual and perennial species, respectively. These threshold values correspond to 36 and 55% decrease in peak percent plant cover for annual and perennial species, respectively.
Fig. 5

Relationship between peak plant cover (end-June to mid-July) of annual and perennial species and duration of inundation estimated for a time period during early growing season: March 15 to May 15. The vertical dash line indicates the threshold value where a significant decline on peak plant cover was observed

Similar comparisons were conducted between peak plant percent cover and average water depth and frequency of inundation (number of inundation events), and a similar “threshold” phenomenon was suggested by the data. The threshold value for water depth was 8 cm for both annual and perennial species, which corresponded to a 45 and 77% decrease on peak percent plant cover for annual and perennial species, respectively (P ≤ 0.01). For frequency of inundation, a significant (P ≤ 0.001) decline in plant cover for both annual (35%) and perennial (63%) species was observed when the marsh was exposed to 76 inundation events between March 15 and May 15. The data were also disaggregated into separate species and analyzed separately. These analyses showed the same general pattern observed for all the data combined: a sharp decline of peak plant cover when a specific threshold value of duration of inundation, number of inundation events, or water depth was reached.

Discussion

Climate change tends to be very gradual and almost imperceptible, but the accumulation of its effects may eventually lead to dramatic changes if critical thresholds are reached. This study provides a first attempt to discern early signs of sea level rise impacts in the vegetation communities of Jug Bay by looking at the distribution of marsh plant species with respect to intertidal elevation and the relationship between inundation early in the growing season (March 15–May 15) and plant cover measured later during the peak growing season (end-June to mid-July).

Our results support the hypothesis that elevation plays an important role in plant species distribution within the marsh. Only 3 out of 26 species studied (P. cordata, N. lutea, Z. aquatica) are found below mean low water (MLW; Table 1). These species, in addition to P. virginica and P. hydropiper, which are also found in the lower marsh zone, showed the broadest elevation distribution ranges (0.48–0.78 m; Fig. 2). Z. aquatica for example has been found growing along the entire intertidal gradient from low to high marsh areas in a tidal freshwater marsh in the Delaware River (Field and Philipp 2000).

On the other hand, the fact that most plant species within Jug Bay marshes are found at higher elevations and have narrow elevation ranges (Figs. 2 and 3) puts the larger marsh system at a potential risk of dramatically altering its species diversity and composition if increasing inundation trends continue. The 17 species found within the upper elevation distribution range (Table 1, Fig. 2) are the ones expected to be most vulnerable if faced with a rapid and significant inundation change. If these species are to disappear, it would result in a 65% reduction of diversity of the Jug Bay marsh community. This expectation is supported by previous studies which showed how sensitive tidal freshwater marshes can be to changes in inundation. Baldwin et al. (2001) demonstrated in field experiments that exposing a tidal freshwater marsh (mid-high marsh) to 10 cm of additional flooding (which corresponds approximately to one-sixth of the average tidal amplitude at Jug Bay) reduced plant species richness by 26%, while the same marsh submitted to drier condition (10 cm above marsh surface) increased richness by 42%. The study also showed that species such as Eleocharis obtusa, Impatiens capensis, Leersia orizoides, Polygonum arifolium (now Persicaria arifolia), and Pilea pumila, more characteristic of higher elevations, showed a decreased in abundance with additional inundation while a species such as P. virginica also found in the low marsh was not impacted. Similar results showing species diversity decline with inundation were presented by Casanova and Brock (2000). Note that a change of species diversity/dominance caused by increased inundation would not necessarily translate into wetland loss, as we could expect that the more water-tolerant species and those with broad elevation distribution ranges would colonize any denuded areas (Groups 1, 2, and 3 from Fig. 4). Wetland loss, however, could be expected when inundation surpasses the tolerance level of any marsh plant species particularly in areas where marsh elevation change is not keeping pace with SLR.

Past and recent estimates of wetland elevation change within Jug Bay marshes show large temporal and spatial variability, with some areas gaining and others losing elevation (Brush et al. 1982; Childers et al. 1993; Boumans et al. 2002; Delgado et al. 2013). For example, IPL and RR low marsh areas are losing elevation at an average rate of − 8.8 and − 15.7 mm/year, respectively while mid-high marsh areas are gaining elevation at 1.5 and 1.1 mm/year, respectively (Delgado, unpublished data). Because of this local variability, the effects of increased sea level rise may still not be felt in areas gaining elevation at an equal or faster rate. In those areas where the marsh is losing elevation, the impact of increased inundation may be exacerbated and its effects may already be occurring on the vegetation communities.

Field and Philipp (2000) proposed that vegetation can be considered an indicator of hydroperiod, where a change in vegetation can signal a change in the frequency and duration of inundation and water depth. Results of a broad-scale mapping change analysis around the Jug Bay RR marsh area have already shown shifts in the location and extent of some community types associated with changes in elevation (Swarth et al. 2013). At a finer scale, some areas showed a decrease in species diversity and a decrease in the importance values of species such as Acorus calamus and P. virginica; in contrast, other areas, particularly, the mix mid-high marsh showed stability within the plant communities (Swarth et al. 2013).

It is known that seed banks in tidal freshwater marshes such as Jug Bay play an important role in maintaining the plant communities of annual species such as Bidens laevis, Zizania aquatica, and Impatiens capensis (Peterson and Baldwin 2004; Leck and Simpson 1995; Leck et al. 1994). Seed germination in these marshes occurs between March and May, and during this sensitive time, perennial species are also starting to grow new shoots. Interestingly, for both annual and perennial species, a sharp decline in plant cover (which ranged between 35 and 77%) seems to correlate to a threshold value for frequency and duration of inundation and water depth during this early growing season period (Fig. 5).

This concept of an inundation threshold is supported by other studies on seed banks and germination conducted for different tidal freshwater marsh communities and species. A comprehensive greenhouse study on freshwater wetland seed banks showed that longer durations of flooding lowered species richness and biomass especially of the most sensitive species (Casanova and Brock 2000). Similarly, flooding significantly reduced the species richness, diversity, and density of emerging seedlings from restored and natural tidal freshwater marshes within the Anacostia river watershed (Baldwin and Derico 1999). Peterson and Baldwin (2004) showed a significant decrease in seedling emergence of both annual and perennial species due to flooding and sedimentation in marshes from the Nanticoke River watershed. In a study conducted by Baldwin et al. (2001), overall field, greenhouse, and seed bank studies showed that 3–10 cm of flooding can significantly reduce seedling recruitment and growth of many tidal freshwater marsh species resulting in lower plant diversity. Additionally, the field and greenhouse studies showed that annual or annual/perennial species were more inhibited by flooding than perennials. Further, the greenhouse study indicated that shallow flooding early in the growing season can reduce the abundance of certain species, mainly annuals. This last finding was not supported by our study, which showed that both perennial and annual species can experience a significant decrease in plant cover after an inundation threshold is reached early during the growing season.

The suggestion that Jug Bay marsh species, both annuals and perennials, are responding to changes in the depth, duration, and frequency of inundation when exposed early during the growing season also highlights the importance that inundation history over this sensitive time of the year may have on peak biomass, rather than just the inundation at the time of the biomass survey.

In summary, the combination of high local SLR rates, a micro-tidal environment, and evidence of site-specific wetland elevation loss (particularly in low marsh areas; Delgado, unpublished data) is raising the concern that some wetland areas in Jug Bay may be vulnerable to increased inundation. Under these conditions, low marsh species which are somewhat more resistant to inundation and have a broader elevation distribution range than high marsh species may be able to migrate to higher elevations and persist. The same is probably not true for species currently found at higher elevations, which are more sensitive to inundation and have more restricted distribution elevation ranges, and in many instances, are limited for upland migration due to sharp elevation changes within the Jug Bay area. Although current conditions at Jug Bay seem to indicate some stability in the mid-high marsh area (due partly to positive marsh elevation change rates; Delgado, unpublished data), any future sustained increase in inundation may trigger major changes in species composition and dominance in this part of the marsh.

Information gained through this study may provide preliminary evidence for resource managers to consider the benefits of reducing additional stressors such as invasive species and poor water quality. Additionally, restoration efforts (i.e., plantings, distribution of sediments to increase marsh elevation) could be informed by the knowledge gained on species elevation distribution ranges and means, increasing their likelihood of success. Despite what we have learned, there is still much that needs to be studied to better understand the long-term impacts and responses of Jug Bay tidal freshwater marshes to increasing inundation; the continued collection of Jug Bay “sentinel site” monitoring data will be an important piece to accomplish this task. As other NERR sites and coastal partners continue to collect their own wetland “sentinel site” data, we hope to obtain a broader, longer-term regional dataset on tidal freshwater marsh responses to increasing sea levels, which will not only help us detect change and responses to change but also provide important data to validate predictive models to better support decision-making.

Notes

Acknowledgements

The authors would like to thank the Maryland Chesapeake Bay National Estuarine Research Reserve (CBNERR-MD) for providing the resources and facilitating the establishment and continued sampling of all sentinel site monitoring infrastructure through the present day. Special thanks go to CBNERR-MD staff, Jug Bay Wetlands Sanctuary staff, and Friends of Jug Bay volunteers who helped year after year with the collection of data, including Lindsay Carroll, Cathy Ervin, Becky Lang, Julia Pusak, Katrina Keller, Lindsay Hollister, Heather Baden, Chris Snow, Jenny Allen, Amanda Garzio-Hadzick, Erica Loudermilk, and Regina Kreger. The authors wish to thank Jay Howard (NOAA NGS) for his help in processing the water level data. Special recognition goes to Erica Loudermilk for her field and data analytical contribution regarding species and marsh elevation while doing a summer research internship at Jug Bay during the summer of 2015.

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Copyright information

© Coastal and Estuarine Research Federation 2017

Authors and Affiliations

  1. 1.Jug Bay Wetlands SanctuaryLothianUSA
  2. 2.National Oceanic and Atmospheric AdministrationNational Geodetic SurveySilver SpringUSA
  3. 3.Department of Environmental Science and TechnologyUniversity of MarylandCollege ParkUSA

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