Spartina alterniflora (Spartina) and J. romerianus (Juncus) dominated the plant community at our sampling sites. However, Spartina cynosuroides, Spartina bakeri, Salicornia depressa, Borrichia frutescens, Iva frutescens, and Schoenoplectus sp. were all observed in at least one plot. The dominant invertebrates in marsh plots were L. irrorata (Littoraria) and crabs. Crab species associated with crab burrows included Armases cinereum, Uca pugnax, Uca minax, Uca pugilator, Sesarma reticulatum, and Eurytium limosum. There was no evidence of sessile marine invertebrates fouling any of the bulkheads. Armases were found on bulkheads at 11 locations, Littoraria were found on five bulkheads, and Uca pugnax were found on two bulkheads. In addition, we found Anolis carolinensis lizards on one bulkhead, Eumeces faciatus or Eumeces inexpectatus (five-lined skink or southeastern five-lined skink) on three bulkheads, and an unidentified snake on one bulkhead.
Elevations at all study sites were above 0 m, and the extent of the marsh from the upland-creek distance ranged from 13 to 487 m (Table 1). Armored sites had the minimum elevation, upland-creek distance, soil water content (a proportion of 0.22), phosphate concentration (0 μM) and crab burrow density (0 m−2), the maximum salinity (52), and percent sand (92%), and the minimum and maximum proportion of soil organic matter (0.02 and 0.80) and crab burrow density (0 and 560 m−2; Table 1). Unarmored sites had the minimum salinity (2.0), proportion of soil organic matter (0.02), ammonium (0.5 μM) and phosphate concentrations (0 μM), and the maximum elevation (1.5 m) and soil water content (0.81; Table 1). Forested sites had the minimum percent sand (4.0%) and the maximum upland-creek distance (487 m), Littoraria density (720 m- 2), and ammonium (157 μM), nitrate (378 μM), and phosphate (65 μM) concentrations (Table 1). Other parameters (Spartina and Juncus cover, nitrate concentration, fraction of bare surface and wrack cover) maximum and minimum overlapped among site types (Table 1). All data from this survey are available at (Gehman 2016).
Question 1: How does site type affect the physical and environmental characteristics of the upper marsh?
Site types varied in terms of elevation and upland-creek distance, both of which were correlated with other environmental variables. Elevation was significantly lower at armored than unarmored or forested sites (Table 2, Fig. 2). Higher elevations were associated with lower salinity, soil organic matter, ammonium and phosphate concentration, and higher percent sand, soil water content, and nitrate concentration (Table 2; Figs. 3 and 4). Upland-creek distance was longer at forested than at armored or unarmored sites (Table 2; Fig. 2). Longer upland-creek distance was associated with higher ammonium and phosphate concentrations, more bare (unvegetated) space, and lower salinity (Table 2; Figs. 3 and 4).
After accounting for the effects of elevation and upland-creek distance, several significant effects of site type remained. First, salinity was lower at the unarmored sites than at the forested sites (Table 2, Fig. 3A). Second, soil water content was higher at unarmored sites than at armored and forested sites (Table 2, Fig. 3C, d). Third, the fraction bare of vegetation was higher at unarmored sites than at armored sites (Table 2, Fig. 4I, J). In one case, the interaction of Spartina wrack cover and elevation was significant (Table 2); wrack cover at armored sites was greater at lower elevations, whereas at unarmored sites it was greater at high elevations (Table 2, Fig. 4K).
Question 2: How does site type affect the biological community of the upper marsh?
Four members of the biological community accounted for ~ 70% of the dissimilarity between marsh communities adjacent to the different site types: Spartina, Juncus, Littoraria, and crabs (as indexed by their burrows; Table 3). Although the biological communities were different by site type and sampling point along the transect, the multivariate ANOVA model explained little of the variability in the data (Table 4).
Question 3: How does site type affect the relationship between the physical and biological characteristics in the upper marsh?
Seven environmental variables were correlated with the structure of the biological community: elevation, upland-creek distance, porewater salinity, soil water content, porewater concentrations of nitrate and phosphate, and wrack cover (multivariate ANOVA, Table 5).
Spartina had greater coverage at armored versus unarmored sites (generalized mixed modeling, Table 6, Fig. 5). It also had greater coverage at sites with lower elevations, higher salinity and soil water content, and lower wrack cover and porewater phosphate concentrations (Table 6, Fig. 5). Spartina coverage was lower at stations with longer upland-creek distances. Juncus coverage was not affected by site type, but there was greater coverage at sites with lower soil water content and wrack cover (Table 6, Fig. 5). There were more crab burrows at forested sites, followed by armored sites and then unarmored sites. More crab burrows were also found at stations with shorter upland-creek distance and sites with lower elevation, porewater phosphate, and wrack coverage, and higher salinity, soil water content, and porewater nitrate concentrations (Table 6, Fig. 5). Littoraria densities were not affected by site type, but there were more Littoraria at sites with longer upland-creek distances and with lower nitrate concentration and wrack coverage (Table 6, Fig. 5).
Question 4: How does site type affect the use of terrestrial habitats by organisms that routinely move between the upland and marsh?
The number of Armases found in the upland varied by site type, with the highest densities at forested sites, then unarmored sites, and the lowest counts at armored sites (Table 7, Fig. 6). Additionally, the number of Armases found in the marsh was positively correlated to the number found in the upland (Table 7, Fig. 6).