Engineers, CERC, Fort Belvoir, Va. Kraft, J. C. 1971. Sedimentary facies patterns and geologic histo- ry of a Holocene transgression. Geol. Soc. Amer. Bull. 82:.
Vegetatio 71:139-144 (1987) © Dr W. Junk Publishers, Dordrecht - Printed in the Netherlands
139
Shoreline processes and establishment of Phragrnites australis in a coastal plain estuary Jonathan D. Phillips
Department of Geography, Arizona State University, Tempe, A Z 85287 USA Accepted9.3.1987
Key words: Coastal submergence, Delaware Bay, Estuarine shoreline, Salt marsh Abstract
Phragmites australis occurs extensively along undisturbed salt-marsh shorelines of Delaware Bay. The species has been considered indicative of human disturbance when found in estuarine marshes in the USA. It is suggested that geomorphic processes associated with coastal submergence provide an analog of human disturbances which can enable Phragmites australis to become established naturally. Deposition of sand bodies (or rafted debris) can suppress existing vegetation and allow Phragmites to become established. Subsequently, even if the sand or debris is moved, erosional truncation of the intertidal profile can inhibit recolonization by the original dominant shoreline species, Spartina alterniflora. Nomenclature follows G. M. Silberhorn 1982. Common plants of the Mid-Atlantic coast. John Hopkins University Press, Baltimore, Md.
Introduction
Phragmites australis dominates the shore zone vegetation of the New Jersey shoreline of Delaware Bay. In light of a previous vegetation study in the area, the physical characteristics of the bayshore, and the ecology of the species, this domination is unexpected. The purpose of this paper is to examine the relationship between Phragmites australis colonization at the water's edge and geomorphic processes now occurring along the shore. Phragmites is a common, native species in New Jersey and is globally ubiquitous in wetlands, littor•als, and moist uplands. However, the prevalence of the reed in the shore zone of the Delaware Bay estuary is surprising for four reasons. First, Phragmites may be inhibited by the high soil salinity characteristic of the Delaware Bay shore zone (Jaworski &
Tedrow, 1985). Second, along the USA Atlantic coast the reed is typically found in sites disturbed by human activity, while much of the Delaware Bay shore is relatively undisturbed. Third, the shore zones of high-energy, saline estuaries such as Delaware Bay on the Atlantic coast are typically dominated by species of Spartina (Howard et aL, 1977; Sipple, 1971; Silberhorn, 1982). Finally, previous surveys and studies of vegetation along the shore of Delaware Bay did not find Phragmites australis to be a significant component of shoreline vegetation communities (see below).
Regional biogeography of Phragmites australis The biology and ecology of Phragmites has been reviewed by Haslam (1972), Howard et al. (1977),
140 Dykyjova & Kvet (1978), Mook & Van der Toorn (1982) and Van der Toorn & Mook (1982). Phragmites australis is a rapid colonizer of wet, disturbed sites, but is thought to rarely invade established plant communities. In New Jersey it is typically found on wet, disturbed sites, and is common in spoilbanks, roadside ditches, and dredged or filled marshes (see Sipple, 1971). Studies in southern New Jersey have shown that Phragmites can radiate into surrounding vegetation by rhizome extension, but that initial colonization appears to require disturbance of existing plant communities (Ferren et aL, 1981). The reed can become established in disturbed salt marshes, by some combination of surface elevation, suppression of existing vegetation, lowering of the water table, or decreasing soil salinity (Dexter, 1981; Roman et al., 1984). Phragmites australis is common in marshes disturbed by dredging and filling throughout New Jersey and the mid-Atlantic coast (Siple, 1971; Silberhorn, 1982). In general, it has been found that the mean high water mark in estuarine marshes is critical for Phragmites. Seedlings cannot become established under periodically flooded conditions (Haslam, 1972). Any factor elevating the intertidal substrate as much as 1 - 2 cm above mean high water can enable the reed to become established (Roman et al., 1984, Howard et al., 1977). While Phragmites is considered to be common in freshwater marshes in the region, salt marshes in New Jersey are usually dominated by Spartina alterniflora (in the intertidal zone) and S. patens (Silberhorn, 1982; Robichaud & Buell, 1973). Studies in marshes of the Delaware estuary have not found R australis in undisturbed saline marshes (Walton & Patrick, 1973; Good, 1965; McCormick & Ashbaugh, 1972). The mapping of vegetation in the area by Walton & Patrick (1973), which did not indicate major shoreline populations of R australis, is believed to be accurate. It is therefore inferred that the invasion of the shoreline by the reed in the lower estuary occurred between the early 1970s and early 1980s. The reasons for this timing are unclear, and may merely reflect regional dispersion trends. However, at the time of the Walton and Patrick survey, construction was underway for a nuclear power plant along the
lower Delaware River, just upstream of the study area as described below. A major portion of the river was filled to create '~,rtificial Island." This fill area was completely vegetated with Phragmites. While Phragmites existed along the Delaware River shore at several upstream locations, and at scattered points in interior marshes and marsh-upland contacts, it is believed that this construction activity provided a massive seed source which led to downstream shoreline colonization in subsequent years.
Study area
The study area is a 52 km reach of the northeast shore of Delaware Bay. Nearly 20 000 ha of tidal salt and brackish marsh lie between the shoreline and upland in the study area. Development is minimal. There are I0 small, scattered clusters of shoreline cottages, where in most cases erosion protection structures have obliterated shoreline vegetation. Dikes built to facilitate the harvest of Spartina patens for hay, and drainage ditches for mosquito control in interior marshes constitute the only other significant human disturbance. Most of the shoreline is relatively undisturbed by human activity. The Delaware Bay and surrounding wetlands are underlain by unconsolidated Pleistocene sediments. Holocene estuarine and marine sediments have been deposited over this substrate (Kraft, 1971; Weil, 1977). As the locus of sediment deposition migrated upstream during Holocene sea level rise, the bay-wetland complex began a transformation from a constructive to a destructive estuarine delta characterized by low sediment input and extensive reworking (Weil, 1977). The general response of the wetlands to continued coastal submergence has been erosion of the bayside fringe, and landward extension on the inland edge of the marshes (Kayan & Kraft, 1979; Washburn, 1982). Shoreline erosion data from Delaware Bay suggest that bay-edge erosion and drowning is occurring more rapidly than landward/upward extension, resulting in a net loss of wetlands (Phillips, 1986b). The alongshore pattern of erosion is complex, but in general the erosion rate is severe, averaging more than 3 m/yr of linear shoreline retreat (Phillips,
141 1986a). Geomorphic conditions along the bay shore suggest that rapid erosion and drowning is likely to persist (Phillips, 1986a, 1986b).
Data collection The study area shoreline was surveyed for a general description of shoreline types based on morphology and vegetation. Erosion rates were determined photogrammetrically over the period 1948- 78 (see Phillips, 1986a). Detailed data were collected at 48 sites in a fourstage nested sampling design (described in Phillips, 1986a). At each site a transect of the shore zone was surveyed, normal to the general trend of the shoreline. The shore zone was defined as the nearshore wave-influenced bottom up to and including a dune crest or high marsh inundated only by storm tides. The general substrate type was surveyed along the entire transect. Other variables not used in this study were collected and are described elsewhere (Phillips, 1986a). Methods of data collection were based on standard techniques of estuarine shoreline site evaluation (see Knutson & Woodhouse, 1983; Sharp et al., 1979). Vegetation was sampled in one-meter quadrats along the entire transect at each site, with presence and percent ground cover of each species noted in each quadrat. The Braun-Blanquet scale of cover estimation was used (methods described by Kiichler, 1967). Shallow auger cores were also taken at selected locations in an attempt to determine whether existing vegetation was immediately preceded by different vegetation types. These data were inconclusive, however, due to inability to take intact core samples of the runny surficial sediments.
Conceptual model Sand deposition on marsh surfaces and barrier beach transgression appear to be the most common mechanisms enabling Phragmites australis establishment on the marsh fringe. Delaware Bay is a transgressive system. The sand barrier beaches common along the bay shore respond in a well-known
manner, migrating landward by storm overwash even as the bayward fringe retreats (Maurmeyer, 1978; Washburn, 1982; Kayan & Kraft, 1979). The migration of these barriers represents a natural disturbance, smothering the existing vegetation (Spartina) and creating a higher surface for Phragmites to colonize. Similar effects could occur without sand bodies. Large volumes of rafted organic debris were often observed on the marsh fringe surface after storms. These organic deposits can also smother existing vegetation and could provide an elevated surface for Phragmites colonization. Sand barriers may be temporary, being deposited (and later removed) by storms. It is not uncommon for a sand barrier to be transient if a constant supply of coarse-grained material is not available. Delaware Bay sand barriers have been shown to be highly mobile in both shore-normal and littoral directions (Maurmeyer, 1978). If the disturbance lasts long enough for the reeds to become established, the species would likely remain in place as the dominant species after barrier removal. Though Phragmites is not as tolerant of salinity and tidal flooding as Spartina, rapid peat formation may enable the reed to extend itself above normal tides (Bird, 1963: Howard et al., 1977; Roman et al., 1984). In any case, evidence from Haslam (1972) suggests that once established reeds could persist despite frequent salt or brackish water flooding. The failure of Spartina alterniflora to recolonize the intertidal zone is linked to the nature of the rapid erosion now occurring. Though sand barriers sometimes overlie the marsh sediments, the shoreline is chiefly composed of fine-grained, saturated, organic-rich salt marsh sediments. This cohesive material tends to form sharp slopes or scarps as it is eroded, truncating the shore profile (Phillips, 1986a). The formation of these erosional scarps in the intertidal zone and the continued focis of wave attack on these slopes (see Phillips, 1986b) can effectively destroy existing vegetation and inhibit recolonization. Knutson & Woodhouse (1983) present experimental results showing that S. alterniflora cannot become established in zones of strong wave attack and high sediment mobility. Similar findings under field conditions were reported by Van Eerdt (1985) for S. anglica, which occupies a similar niche
142 in marshes in the Netherlands. The conceptual model has fou/~stages: 1. Sand or rafted organic debris is deposited on, or transported over, Spartina marsh. 2. Spartina i s s u p p r e s s e d or destroyed. 3. Phragmites australis colonizes the disturbed area, above the mean high water mark. 4. Bayside erosion o f salt marsh sediments forms a scarp or steep slope which truncates the shore profile. This inhibits Spartina alterniflora in the intertidal zone. I f the model is correct, all Phragmites-dominated shoreline sites should be characterized by either the presence o f sandy deposits or severe erosional truncation o f the intertidal profile. The presence o f rafted organic debris is not included due to difficulty in separating residual rafted debris from in situ detritus.
Results Field evidence supports the model. At the time of survey in spring, 1984, 39 o f the 48 study sites were vegetated. Phragmites was present at 25, and dominant at 17. All 17 sites where Phragmites australis is dominant show evidence of either sand deposition or profile truncation. All but one o f eight other sites where the reed was present exhibited either sand deposition or profile truncation. Further, of 18 sites with sand deposition, all but one exhibited Phragmites populations at the time o f data collection. This site, ironically, ultimately provided an observation o f the Phragmites establishment process. During data collection in early spring it was obvious the site had recently been overwashed. In situ Spartina alterniflora and S. patens were found buried under the sand. When this site was revisited in mid-May, scattered Phragmites australis plants were observed in the sand body, with no other vegetation. Severe erosional profile truncation was defined as areas where at least half of the profile between mean high and mean low water featured a near-vertical scarp. Evidence of sand deposition included presence of sand barriers, overwash deposits, or a veneer on marsh sediments above mean low water. O f the 17 reed-dominated sites, eight had evidence of sand deposition, six of profile truncation, and three of
Table 1. Relationship between shoreline conditions (S = sand deposition present; T = profile truncation; ST = both sand and truncation; Z = neither sand rlor truncation) and presence or absence of Phragmitesaustralisat 39 vegetated study sites. Numbers indicate number of sites where a particular shoreline condition and vegetation state were found.
Phragmitesdominant Phragmitespresent Phragmitesabsent
s
T
ST
Z
8 4 1
6 1 9
3 2 0
0 1 4
both sand deposits and profile truncation. O f the eight sites where Phragmites australis was present but not dominant, only one showed no evidence of sand or truncation. Four sites had sand, one exhibited truncation, and two had both. A contingency table was constructed relating Phragmites and shoreline conditions (Table 1). Each vegetated site was classified according to whether the reed was dominant, present but not dominant, or absent. Sites were also categorized as exhibiting sand deposits, profile truncation, both sand and truncation, or neither. A chi-square test showed the relationship to be statistically significant at the 98°7o confidence level (X2 = 16; 6 d.f.). A second test of a 2 × 2 table (Phragmites present or absent; sand and/or truncation present or absent) was significant at the 9907o confidence level.
Discussion and conclusions The data strongly support the suggestion that Phragmites australis establishment along Delaware Bay marsh fringe shorelines is associated with deposition o f sand or debris at or near the mean high water mark' and erosional truncation of the intertidal profile. Natural processes thus provide an analog to h u m a n disturbances which are known to facilitate establishment ofP. australis. At only one site was the reed present with neither sand nor truncation. At this location reed was mixed with Spartina alterniflora and other species. While deposition on the marsh fringe facilitates Phragmites invasion, profile truncation need not be
143 associated with reed invasion. Erosional t r u n c a t i o n seems necessary if Phragmites australis stands are to be preserved as the fringe vegetation after removal o f sand deposits or debris rafts, b u t apparently does not itself p r o m o t e reed invasion. Nine study sites exhibited severe t r u n c a t i o n but h a d n o Phragmites plants. There is n o reason to believe t r u n c a t i o n would p r o m o t e invasion by t h e species unless it was a c c o m p a n i e d by some disturbance o f existing shoreline vegetation. Estuarine shorelines, especially those in zones o f coastal submergence, are extremely dynamic. A l o n g the U S A Atlantic and G u l f coasts, m a n y are erosional. T h e process o f coastal submergence is often associated with deposition and subsequent migration o f sand deposits on the m a r s h surface. Further, in any relatively high-energy estuary with a coarse sediment supply transient sand deposits m a y be c o m m o n . Rafted organic debris can play a similar role in suppressing m a r s h surface vegetation. The processes n o w active in Delaware Bay fringe marshes are also present, to greater or lesser extents, in other m a j o r coastal plain estuaries, including Chesapeake Bay, Pamlico Sound, and the Missisippi Delta (Salinas et al., 1986; Stevenson et al., 1985; Phillips, 1986b). T h e intensity o f the reed invasion in Delaware Bay is p r o b a b l y due to the c o m b i n a t i o n o f especiaUy rapid erosion and proximity to m a j o r Phragmites seed sources. Because similar conditions probably exist in m a n y estuaries, the future spread o f Phragmites into estuarine marshes should c o m e as n o surprise. The species should be recognized as a natural c o m p o n e n t o f estuarine marshes, and not necessarily an indicator o f h u m a n disturbance. Invasion o f estuarine marshes by Phragmites australis can occur as a result o f natural processes, and is not necessarily associated with h u m a n disturbance. Deposition o f sand (or rafted organic debris) at or near m e a n high water o n m a r s h shores can suppress existing vegetation and provide e n o u g h elevation for Phragmites to b e c o m e established. Subsequently, even if the sand or debris is moved, erosional t r u n c a t i o n o f the intertidal zone can inhibit recolonization by the original d o m i n a n t species.
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