Carolyn A. Currin

Determination of Food Web Support and Trophic Position of the Mummichog, Fundulus heteroclitus, in New Jersey Smooth Cordgrass (Spartina alterniflora ), Common Reed (Phragmites australis), and Restored Salt Marshes

The invasion ofPhragmites australis into tidal marshes formerly dominated bySpartina alterniflora has resulted in considerable interest in the consequences of this invasion for the ecological functions of marsh habitat. We examined the provision of trophic support for a resident marsh fish,Fundulus heteroclitus, in marshes dominated byP. australis, byS. alterniflora, and in restored marshes, using multiple stable isotope analysis. We first evaluated our ability to distinguish among potential primary producers using the multiple stable isotope approach. Within a tidal creek system we found significant marsh and elevation effects on microalgal isotope values, and sufficient variability and overlap in primary producer isotope values to create some difficulty in identifying unique end members. The food webs supportingF. heteroclitus production were examined using dual isotope plots. At both sites, the δ13C values ofF. heteroclitus were clustered over values for benthic microalgae (BMI) and approximately midway between δ13C values ofSpartina andPhragmites. Based on comparisons of fish and primary producer δ13C, δ15N, and δ34S values, and consideration ofF. heteroclitus feeding habits, we conclude that BMI were a significant component of the food web supportingF. heteroclitus in these brackish marshes, especially recently-hatched fish occupying pools on the marsh surface. A 2‰ difference in δ13C betweenFundulus occupying nearly adjacentSpartina andPhragmites marshes may be indicative of relatively less reliance on BMI and greater reliance onPhragmites production inPhragmites-dominated marshes, a conclusion consistent with the reduced BMI biomass found inPhragmites marshes. The mean δ13C value ofF. heteroclitus from restored marshes was intermediate between values of fish from naturally occurringSpartina marshes and areas invaded byPhragmites. We also examined the isotopic evidence for ontogenetic changes in the trophic position of larval and juvenileF. heteroclitus. We found significant positive relationships betweenF. heteroclitus δ15N values and total length, reflective of an increase in trophic position as fish grow.F. heteroclitus δ15N values indicate that these fish are feeding approximately two trophic levels above primary producers.

Engineering away our natural defenses: an analysis of shoreline hardening in the US

Rapid population growth and coastal development are primary drivers of marine habitat degradation. Although shoreline hardening or armoring (the addition of concrete structures such as seawalls, jetties, and groins), a byproduct of development, can accelerate erosion and loss of beaches and tidal wetlands, it is a common practice globally. Here, we provide the first estimate of shoreline hardening along US Pacific, Atlantic, and Gulf of Mexico coasts and predict where future armoring may result in tidal wetland loss if coastal management practices remain unchanged. Our analysis indicates that 22 842 km of continental US shoreline – approximately 14% of the total US coastline – has been armored. We also consider how socioeconomic and physical factors relate to the pervasiveness of shoreline armoring and show that housing density, gross domestic product, storms, and wave height are positively correlated with hardening. Over 50% of South Atlantic and Gulf of Mexico coasts are fringed with tidal wetlands that...

Epiphytic nitrogen fixation associated with standing dead shoots of smooth cordgrass, Spartina alterniflora

N2 fixation associated with the epiphytic community on standing dead Spartina alterniflora shoots was examined in both a natural and transplanted salt marsh in North Carolina. Acetylene reduction (AR) assays were conducted over a 24-mo period to estimate N2 fixation rates on standing dead stems and leaves. In the natural salt marsh, mean AR rates ranged from 0.5 nmol C2H4 cm−2 h−1 to 14 nmol C2H4 cm−2 h−1, while in the transplanted marsh mean AR rates ranged from 1 nmol C2H4 cm−2 h−1 to 33 nmol C2H4 cm−2 h−1. Diel AR activity of epiphytic communities in both marshes varied seasonally. Midday incubations yielded higher AR rates than nighttime incubations in the spring, while midday incubations in late summer and fall generally yielded AR rates equal to or lower than nighttime incubations. Desiccation during low tides occasionally repressed AR activity, although AR rates quickly rebounded with wetting. AR activity was localized in the epiphytic community, rather than in the underlying Spartina stem material. Based on the measured AR rates and the density of standing dead stems, the annual input of new N to the natural salt marsh via epiphytic N2 fixation is estimated to be 2.6 g N m−2 yr−1. The estimate of annual input of new N to the transplanted marsh is 3.8 g N m−2 yr−1. These estimates should be added to previous estimates of N2 fixation in marsh sediments to estimate the total contribution of new nitrogen to salt marsh nitrogen budgets.

Living Shorelines: Coastal Resilience with a Blue Carbon Benefit

Living shorelines are a type of estuarine shoreline erosion control that incorporates native vegetation and preserves native habitats. Because they provide the ecosystem services associated with natural coastal wetlands while also increasing shoreline resilience, living shorelines are part of the natural and hybrid infrastructure approach to coastal resiliency. Marshes created as living shorelines are typically narrow (< 30 m) fringing marshes with sandy substrates that are well flushed by tides. These characteristics distinguish living shorelines from the larger meadow marshes in which most of the current knowledge about created marshes was developed. The value of living shorelines for providing both erosion control and habitat for estuarine organisms has been documented but their capacity for carbon sequestration has not. We measured carbon sequestration rates in living shorelines and sandy transplanted Spartina alterniflora marshes in the Newport River Estuary, North Carolina. The marshes sampled here range in age from 12 to 38 years and represent a continuum of soil development. Carbon sequestration rates ranged from 58 to 283 g C m-2 yr-1 and decreased with marsh age. The pattern of lower sequestration rates in older marshes is hypothesized to be the result of a relative enrichment of labile organic matter in younger sites and illustrates the importance of choosing mature marshes for determination of long-term carbon sequestration potential. The data presented here are within the range of published carbon sequestration rates for S. alterniflora marshes and suggest that wide-scale use of the living shoreline approach to shoreline management may come with a substantial carbon benefit.

Impacts of Climate-Related Drivers on the Benthic Nutrient Filter in a Shallow Photic Estuary

In shallow photic systems, the benthic filter, including microphytobenthos and denitrifiers, is important in preventing or reducing release of remineralized NH4+ to the water column. Its effectiveness can be impacted by climate-related drivers, including temperature and storminess, which by increasing wind and freshwater delivery can resuspend sediment, reduce salinity and deliver nutrients, total suspended solids, and chromophoric dissolved organic matter (CDOM) to coastal systems. Increases in temperature and freshwater delivery may initiate a cascade of responses affecting benthic metabolism with impacts on sediment properties, which in turn regulate nitrogen cycling processes that either sequester (via microphytobenthos), remove (via denitrification), or increase sediment nitrogen (via remineralization, nitrogen fixation, and dissimilatory nitrate reduction to ammonium). We conducted a seasonal study at shallow stations to assess the effects of freshwater inflow, temperature, wind, light, and CDOM on sediment properties, benthic metabolism, nitrogen cycling processes, and the effectiveness of the benthic filter. We also conducted a depth study to constrain seasonally varying parameters such as temperature to better assess the effects of light availability and water depth on benthic processes. Based on relationships observed between climatic drivers and response variables, we predict a reduction in the effectiveness of the benthic filter over the long term with feedbacks that will increase effluxes of N to the water column with the potential to contribute to system eutrophication. This may push shallow systems past a tipping point where trophic status moves from net autotrophy toward net heterotrophy, with new baselines characterized by degraded water quality.

Succession of Microphytobenthos in a Restored Coastal Wetland

Sediment microphytobenthos, such as diatoms and photosynthetic bacteria, are functionally important components of food webs and are key mediators of nutrient dynamics in marine wetlands. The medium to long-term recovery of benthic microproducers in restored habitats designed to improve degraded coastal wetland sites is largely unknown. Using taxon-specific photopigments, we describe the composition of microphytobenthic communities in a large restoration site in southern California and differences in the temporal recovery of biomass (chlorophylla), composition, and taxonomic diversity between vegetatedSpartina foliosa salt marsh and unvegetated mudflat. Visually distinct, spatially discreet, microphytobenthic patches appeared after no more than 7 mo within the restoration site and were distinguished by significant differences in biomass, taxonomic diversity, and the relative abundance of cyanobacteria versus diatoms. Sediment chlorophylla concentrations within the restored site were similar to concentrations in nearby natural habitat 0.2–2.2 yr following marsh creation, suggesting rapid colonization by microproducers. RestoredSpartina marsh very rapidly (between 0.2 and 1.2 yr) acquired microphytobenthic communities of similar composition and diversity to those in naturalSpartina habitat, but restored mudflats took at least 1.6 to 2.2 yr to resemble natural mudflats. These results suggest relatively rapid recovery of microphytobenthic communities at the level of major taxonomic groups. Sediment features, such as pore water salinity andSpartina density, explained little variation in microphytobenthic taxonomic composition. The data imply that provision of structural heterogeneity in wetland construction (such as pools and vascular plants) might speed development of microproducer communities, but no direct seeding of sediment microfloras may be necessary.

Shoreline Change in the New River Estuary, North Carolina: Rates and Consequences

ABSTRACT Currin, C.; Davis, J.; Cowart Baron, L.; Malhotra, A., and Fonseca, M., 2015. Shoreline change in the New River Estuary, North Carolina: Rates and consequences. Aerial photography was used to determine rates of shoreline change in the New River Estuary (NRE), North Carolina, from 1956 to 2004. The NRE shoreline was digitized from aerial photographs taken in 1956, 1989, and 2004, and shoreline type was determined by ground-truthing the entire shoreline by small boat in 2009. Major shoreline type categories included swamp forest (6% of total), salt marsh (21%), sediment bank (53%), and modified/hardened (19%). Ground-truthing provided additional details on relief, marsh species composition, and structure type. A point-based, end-point rate approach was used to measure shoreline change rate (SCR) at 50 m intervals for the periods 1956–89, 1989–2004, and 1956–2004. Representative wave energy (RWE) was modeled for each interval using local bathymetry and wind data. Average SCR across all shoreline types for the entire time period ranged from −2.3 to +1.0 m y−1, with a mean SCR of −0.3 m y−1. This translates to an average loss of ~13 m for any given point over the 48-year period covered by this study. The most negative average SCR (greatest erosion) occurred along unvegetated sediment bank shorelines (−0.39 m y−1). Change along marsh shorelines (−0.18 m y−1) was lower than along sediment banks, and narrow fringing marsh associated with sediment bank shorelines significantly reduced bank erosion. Modeled RWE values were positively correlated with erosion only in the highest wave-energy settings. Erosion of sediment bank shorelines provides a conservative estimate of 17,660 m3 of sediment each year to the estuary, with marsh erosion contributing up to an additional 1900 m3 y−1. Based on analysis of the sediment volume required to maintain marsh surface elevation with respect to sea level, we hypothesize that shoreline erosion plays a vital role in supporting growth and maintenance of downstream marshes.

Geomorphic Implications of Particle Movement by Water Surface Tension in a Salt Marsh

Measurements and modeling of salt marsh morphodynamics typically focus on the sediment fraction suspended in water, but another fraction moves at the air-water interface on water surface tension. We compared the geomorphic significance of this surface microlayer sediment fraction to the suspended fraction in a mesohaline, microtidal salt marsh in North Carolina. Conventional methods of suspended sediment and surface microlayer sampling were used and a method was developed to measure the surface microlayer material generated per area of benthic surface. The mass of material held in the surface microlayer was 4% to 16% of the suspended sediment mass in the water column overlying the marsh. This surface microlayer material mass was lower than the mass generated from benthic surfaces and it declined over time, indicating that the surface microlayer released sediment to the water column. Loss on ignition, carbon:nitrogen, and particle size were similar in the surface microlayer and suspended sediment, although the surface microlayer material was higher in chlorophyll a. We estimate that 19% to 100% of the suspended sediment that settled onto the marsh was lifted by surface tension during the next rising tide. This process helps explain the topography of tidal marshes and patterns of sediment accretion.

Living on the Edge: Increasing Patch Size Enhances the Resilience and Community Development of a Restored Salt Marsh

Foundation species regulate communities by reducing environmental stress and providing habitat for other species. Successful restoration of biogenic habitats often depends on restoring foundation species at appropriate spatial scales within a suitable range of environmental conditions. An improved understanding of the relationship between restoration scale and environmental conditions has the potential to improve restoration outcomes for many biogenic habitats. Here, we identified and tested whether inundation/exposure stress and spatial scale (patch size) can interactively determine (1) survival and growth of a foundation species, Spartina alterniflora and (2) recruitment of supported fauna. We planted S. alterniflora and artificial mimics in large and small patches at elevations above and below local mean sea level (LMSL) and monitored plant survivorship and production, as well as faunal recruitment. In the first growing season, S. alterniflora plant survivorship and stem densities were greater above LMSL than below LMSL regardless of patch size, while stem height was greatest in small patches below LMSL. By the third growing season, S. alterniflora patch expansion was greater above LMSL than below LMSL, while stem densities were higher in large patches than small patches, regardless of location relative to LMSL. Unlike S. alterniflora, which was more productive above LMSL, sessile marine biota recruitment to mimic plants was higher in patches below LMSL than above LMSL. Our results highlight an ecological tradeoff at ~LMSL between foundation species restoration and faunal recruitment. Increasing patch size as inundation increases may offset this tradeoff and enhance resilience of restored marshes to sea-level rise.