Signals of Shoreline Disturbance in a Southeastern Salt Marsh Terrestrial Fringe

1. Signals of Shoreline Disturbance in a SoutheasternSalt Marsh Terrestrial Fringe Mauldin, Ashley1, Walters, K.1, Koepfler, E.1, Luken, J.2 and Hutchens, J.2, Dept. of Marine Science1 and Dept. of Biology2, Coastal Carolina University, Conway, SC 29528 Abstract Locations, sites and plots were selected haphazardly and sampled in October, 2003. Sediment characteristics were determined from samples collected using a 2.1 cm diameter core to a depth of 5 cm. Salinity was measured using a refractometer after samples were hydrated with a known volume of deionized water, agitated multiple times and allowed to settle. After salinity measurements, sediments were wet sieved and dried to constant weight to determine grain size distributions. Organic content was determined from an additional core collected within each plot from which a subsample was removed after sediments had been homogenized. The subsample was dried to constant weight and ashed at 500 ºC for 5 hrs. to calculate sediment ash-free dry mass (AFDM). Littoraria irrorata and M. bidentatus densities were recorded from each plot and L. irrorata lengths, shell dry mass and tissue AFDM measured. Juncus roemerianus live and dead stems were counted and stems collected from haphazardly selected 625 cm2 subplots within each 0.25 m2 plot. The height of 25+ haphazardly selected stems from within each 0.25 m2 plot also was measured. Collected stems were dried to constant weight and a subsample ashed at 500 ºC for 2-3 hrs to calculate stem biomass. All data were analyzed using a nested ANOVA. Development of terrestrial shorelines likely will have pronounced effects on coastal marine environments directly at the upland-marsh boundary. For example, the success of high-marsh plants in US East Coast salt marshes is dependent on freshwater inflow, and any alteration in either the volume or timing of runoff resulting from development may affect plant success and the nature of the high marsh environment. To examine possible shoreline development effects on high-marsh environments we collected data on sediment grain size, organics, and salinity,snaildensity and biomass, microbial composition and respiration, and plant density, biomass and stem height within marshes bordering developed and undeveloped shorelines. Samples were collected from mainland and barrier island locations within Murrells Inlet, SC, a densely populated ocean-dominated estuary. At barrier island locations, Juncus roemerianus stems were significantly taller and Littoraria irrorata densities were significantly greater within developed sites, but Melampus bidentatus densities were significantly greater within undeveloped sites. The balance between heterotrophic and autotrophic processes in barrier island soils, as indicated by O2 and CO2 fluxes in respirometry experiments on ambient and nutrient addition sediments, also suggest significant differences between developed and undeveloped sites. Site differences among other variables measured, although not significant, suggest possible directions and mechanisms of change in the high-marsh when the shoreline boundary is altered. Accounting for both the short- (e.g., rain events) and long-term history (e.g., age of development) may reduce measurement variability and increase the transparency of shoreline development effects on salt marshes. Figure 8.Juncus roemerianus mean (+1 SE) stem heights (n = 5) within developed and undeveloped high-marsh sites at Murrells Inlet mainland and barrier island locations. Figure 7.Juncus roemerianus mean (+ 1 SE) live and dead stem densities (n = 5) within developed and undeveloped sites at Murrells Inlet mainland and barrier island locations. Results • Sediment salinity was not significantly different between developed and undeveloped sites (F2,15=0.71, p>0.05), but the trend was for salinity at undeveloped sites to be less than developed sites (Fig. 4). Salinity was significantly greater in the island sites (F1,2=25.87, p<0.04). Figure 9.Juncus roemerianus mean (+1 SE) stem AFDM (n = 5) within developed and undeveloped sites at Murrells Inlet mainland and barrier island locations. • Respirometry of island location sediments indicated that ambient and nutrient addition induced fluxes of O2 and CO2 differed markedly between developed and undeveloped locations.  Results suggest that the balance between heterotrophic and autotrophic processes in high marsh soils are sensitive to shoreline development. Introduction Coastal environments continue to attract an increasing number of tourists and residents, which has led to a transformation of natural habitats to ones dominated by humans (Beach, 2002). NOAA’s National Estuarine Eutrophication Assessment (Bricker et al. 1999) indicates that southeastern estuaries currently are the least impacted of all US coastal regions, however future population growth, especially in South Carolina and Georgia, is expected to exceed that of all other regions. Development such as construction of residences, hotels, tourist attractions, roads, and parking lots likely will lead to deterioration of the structure and function of adjacent estuarine salt marshes. These developments next to salt marshes remove native vegetation and soils that act as important buffers between upland and wetland environments. Despite salt marshes being one of the most productive ecosystems in the world and important nursery habitats for many marine organisms (Mitsch & Gosselink 1993), the impact of development on marshes has been greatly neglected (see however Bertness et al. 2002). Discussion Coastline developments at Murrells Inlet mainland and barrier island locations appear to have generated a number of predictable and some unexpected effects on estuarine high marshes. Greater J. roemerianus stem heights and L. irrorata densities within developed sites are consistent with possible increased nutrient loading at these sites. Residential fertilizer additions, if transported into the high marsh, would increase J. roemerianus and microalgal growth. Observed differences in O2 and CO2 fluxes, although only measured at the island location, and L. irrorata densities may both be attributed to increased nutrient flow from developed sites resulting in increased production of the microphytobenthos. However, the lack of increased stem densities or ash-free dry mass, both expected from increasing nutrient input, is puzzling. Greater M. bidentatus densities within undeveloped sites can be attributed to increased shading and reduced evapo-transpiration stress in a physiologically less tolerant species. One factor that may contribute to the variation in observed results and which our study did not consider directly is the history of the developed sites. The haphazard choice of sites resulted in selection of a relatively newly developed mainland area (<1 yr.), whereas the island developed site was much older (>5 yr.). Additionally, weather patterns (e.g., rainfall) around the sampling dates were not accounted for and possibly contributed to the variation of our observed results. Figure 4. Sediment mean (+1 SE) salinity (n = 5) within Juncus roemerianus plots in developed and undeveloped sites at Murrells Inlet mainland and barrier island locations. Figure 6. Snail mean (+1 SE) densities (n = 5) for developed and undeveloped sites within Murrells Inlet mainland and barrier island locations. Figure 5. Sediment mean (+1 SE) ash-free dry mass (n = 5) from 0-5 cm depth samples collected within developed and undeveloped sites at Murrells Inlet mainland and barrier island locations. • Sediment organic content (ash-free dry mass) was not significantly different between developed and undeveloped sites (F2,16=1.08, p>0.05) or between mainland and island locations (F1,2=15.16, p>0.05). The trend, in part obscured by within-site variation, was for greater sediment organic content at undeveloped sites and within mainland locations (Fig. 5). • Littoraria irrorata (F2,16=61.14, p<0.001) and Melampus bidentatus densities(F2,16=6.58, p<0.01) were significantly different between developed and undeveloped sites. Littoraria irrorata were found almost exclusively within developed sites, but densities of M. bidentatus were greater at undeveloped sites (Fig. 6). • Juncus roemerianus live (F2,16=0.40, p>0.05) and dead stem densities (F2,16=0.65, p>0.05) were not significantly different between developed and undeveloped sites. Stem densities tended to be greater on the mainland and within the developed sites (Fig. 7), but only live stem densities were significantly greater at the mainland locations (F1,2=20.33, p<0.05). • Juncus roemerianus stem heights were significantly taller within mainland and island developed sites (F2,16=14.64, p<0.001), but were not significantly different (F1,2=0.86, p>0.05) between locations. • Stem ash-free dry mass (Fig. 9) was not significantly different between developed and undeveloped sites (F2,16=0.99, p>0.05) or between mainland and island locations (F1,2=0.42, p>0.05). References Figure 1. Aerial photograph of mainland and barrier island sampling locations (stars) in Murrells Inlet, SC. Beach, D. 2002. Coastal sprawl: the effects of urban design on aquatic ecosystems in the United States. Pew Oceans Commission, Arlington VA. (http://www.pewoceans.org/reports/water_pollution_sprawl.pdf) Bertness, M.D., P.J. Ewanchuk, and B.R. Silliman. 2002. Anthropogenic modification of New England salt marsh landscapes. Proceedings of the National Academy of Sciences 99:1395-1398. Bricker, S. B., C. G. Clement, D. E. Pirhalla, S. P. Orlando and D. R. G. Farrow. 1999. National Estuarine Eutrophication Assessment: Effects of Nutrient Enrichment in the Nation’s Estuaries. National Oceanic and Atmospheric Administration, National Ocean Service, Special Projects Office and the National Centers for Coastal Ocean Science. Silver Spring, Maryland. 71p. Mitsch, W.J., and J.G. Gosselink. 1993. Wetlands. 2nd Edition. John Wiley and Sons, Inc., New York. Figure 2. Example of an altered terrestrial shoreline in Murrells Inlet, SC mainland location. Figure 3. Example of unaltered terrestrial shoreline in Murrells Inlet, SC mainland location. Materials and Methods Sediment salinity, grain size and organic content,snail (Littoraria irrorata and Melampus bidentatus) density and L. irrorata biomass, and plant (Juncus roemerianus) density, biomass and stem height were measured within five 0.25 m2 plots at Murrells Inlet mainland and barrier island locations from developed (Fig. 2) and undeveloped sites (Fig. 3).