John M. Rybczyk

Numerical models of salt marsh evolution: ecological, geomorphic, and climatic factors

Salt marshes are delicate landforms at the boundary between the sea and land. These ecosystems support a diverse biota that modifies the erosive characteristics of the substrate and mediates sediment transport processes. Here we present a broad overview of recent numerical models that quantify the formation and evolution of salt marshes under different physical and ecological drivers. In particular, we focus on the coupling between geomorphological and ecological processes and on how these feedbacks are included in predictive models of landform evolution. We describe in detail models that simulate fluxes of water, organic matter, and sediments in salt marshes. The interplay between biological and morphological processes often produces a distinct scarp between salt marshes and tidal flats. Numerical models can capture the dynamics of this boundary and the progradation or regression of the marsh in time. Tidal channels are also key features of the marsh landscape, flooding and draining the marsh platform and providing a source of sediments and nutrients to the marsh ecosystem. In recent years, several numerical models have been developed to describe the morphogenesis and long-term dynamics of salt marsh channels. Finally, salt marshes are highly sensitive to the effects of long-term climatic change. We therefore discuss in detail how numerical models have been used to determine salt marsh survival under different scenarios of sea level rise.

Estimating the potential for submergence for two wetlands in the Mississippi River Delta

We used a combined field and modeling approach to estimate the potential for submergence for one rapidly deteriorating (Bayou Chitigue Marsh) and one apparently stable (Old Oyster Bayou Marsh) saltmarsh wetland in coastal Louisiana, given two eustatic sea level rise scenarios: the current rate (0.15 cm year−1); and the central value predicted by the Intergovernmental Panel on Climate Change (48 cm by the year 2100). We also used the model to determine what processes were most critical for maintaining and influencing salt marsh elevation including, mineral matter deposition, organic matter production, shallow subsidence (organic matter decomposition + primary sediment compaction), deep subsidence, and sediment pulsing events (e.g., hurricanes). Eight years of field measurements from feldspar marker horizons and surface elevation tables revealed that the rates of vertical accretion at the Bayou Chitigue Marsh were high (2.26 (0.09) cm yr−1 (mean ± SE)) because the marsh exists at the lower end of the tidal range. The rate of shallow subsidence was also high (2.04 (0.1) cm yr−1), resulting in little net elevation gain (0.22 (0.06) cm yr−1). In contrast, vertical accretion at the Old Oyster Bayou Marsh, which is 10 cm higher in elevation, was 0.48 (0.09) cm yr−1. However, there was a net elevation gain of 0.36 (0.08) cm yr−1 because there was no significant shallow subsidence. When these rates of elevation gain were compared to rates of relative sea level rise (deep subsidence plus eustatic sea level rise), both sites showed a net elevation deficit although the Bayou Chitigue site was subsiding at approximately twice the rate of the Old Oyster Bayou site (1.1 cm yr−1 versus 0.49 cm yr−1 respectively). These field data were used to modify, initialize, and calibrate a previously published wetland soil development model that simulates primary production and mineral matter deposition as, feedback functions of elevation. Sensitivity analyses revealed that wetland elevation was most sensitive to changes in the rates of deep subsidence, a model forcing function that is difficult to measure in the field and for which estimates in the literature vary widely. The model also revealed that, given both the current rate of sea level rise and the central value estimate, surface elevation at both sites would fall below mean sea level over the next 100 years. Although these results were in agreement with the field study, they contradicted long term observations that the Old Oyster Bayou site has been in equilibrium with sea level for at least the past 50 years. Further simulations showed that the elevation at the Old Oyster Bayou site could keep pace with current rates of sea level rise if either a lower rate for deep subsidence was used as a forcing function, or if a periodic sediment pulsing function (e.g., from hurricanes) was programmed into the model.

THE IMPACT OF WASTEWATER EFFLUENT ON ACCRETION AND DECOMPOSITION IN A SUBSIDING FORESTED WETLAND

Insufficient sedimentation, coupled with high rates of relative sea-level rise (subsidence plus eustatic sea-level rise), are two important factors contributing to wetland loss in coastal Louisiana, USA. We hypothesized that adding nutrient-rich, secondarily treated wastewater effluent to subsiding wetlands in Louisiana could promote vertical accretion in these systems through increased organic matter production and subsequent deposition and allow accretion to keep pace with estimated rates of relative sea-level rise (RSLR). However, we also hypothesized that nutrient enrichment could stimulate the decomposition of organic matter, thus negating any increase in accretion due to increased organic matter accumulation. To test these hypotheses, we measured leaf-litter decomposition, litter nutrient dynamics, and sediment accretion in a permanently flooded and subsiding forested wetland receiving wastewater effluent and in an adjacent control site, both before and after effluent applications began. We also measured organic and mineral matter accumulation in the treatment site before and after effluent applications began. A Before-After-Control-Impact (BACI) statistical analysis revealed that neither leaf-litter decomposition rates nor initial leaf-litter N and P concentration were affected by wastewater effluent. A similar analysis revealed that final N and P leaf-litter concentrations did significantly increase in the treatment site relative to the control after effluent was applied. Total pre-effluent accretion, measured 34 months after feldspar horizon markers were laid down, averaged (±SE) 22.3±3.2 mm and 14.9±4.6 mm in the treatment and control sites, respecitvely, and were not significantly different. However, total accretion measured 68 months after the markers were installed and 29 months after effluent additions began in the treatment site averaged 54.6±1.5 mm in the treatment site and 19.0±3.2 mm in the control site and were significantly different. Additionally, after wastewater applications began, the estimated rate of accretion in the treatment site (11.4 mm yr−1) approached the estimated rate of RSLR (12.3 mm yr−1). Most of this increase in accretion was attributed to organic matter inputs, as organic matter accumulation increased significantly in the treatment site after effluent application began, while mineral accumulation rates remained constant. These findings indicate that there is a potential for using wastewater to balance accretion deficits in subsiding wetland systems.

Forecasting the Effects of Coastal Protection and Restoration Projects on Wetland Morphology in Coastal Louisiana under Multiple Environmental Uncertainty Scenarios

ABSTRACT Couvillion, B.R.; Steyer, G.D.; Wang, H.; Beck, H.J., and Rybczyk, J.M., 2013. Forecasting the effects of coastal protection and restoration projects on wetland morphology in coastal Louisiana under multiple environmental uncertainty scenarios. Few landscape scale models have assessed the effects of coastal protection and restoration projects on wetland morphology while taking into account important uncertainties in environmental factors such as sea-level rise (SLR) and subsidence. In support of Louisianas 2012 Coastal Master Plan, we developed a spatially explicit wetland morphology model and coupled it with other predictive models. The model is capable of predicting effects of protection and restoration projects on wetland area, landscape configuration, surface elevation, and soil organic carbon (SOC) storage under multiple environmental uncertainty scenarios. These uncertainty scenarios included variability in parameters such as eustatic SLR (ESLR), subsidence rate, and Mississippi River discharge. Models were run for a 2010–2060 simulation period. Model results suggest that under a “future-without-action” condition (FWOA), coastal Louisiana is at risk of losing between 2118 and 4677 km2 of land over the next 50 years, but with protection and restoration projects proposed in the Master Plan, between 40% and 75% of that loss could be mitigated. Moreover, model results indicate that under a FWOA condition, SOC storage (to a depth of 1 m) could decrease by between 108 and 250 million metric tons, a loss of 12% to 30% of the total coastwide SOC, but with the Master Plan implemented, between 35% and 74% of the SOC loss could be offset. Long-term maintenance of project effects was best attained in areas of low SLR and subsidence, with a sediment source to support marsh accretion. Our findings suggest that despite the efficacy of restoration projects in mitigating losses in certain areas, net loss of wetlands in coastal Louisiana is likely to continue. Model results suggest certain areas may eventually be lost regardless of proposed restoration investment, and, as such, other techniques and strategies of adaptation may have to be utilized in these areas.

Using Natural Wetlands for Municipal Effluent Assimilation: A Half-Century of Experience for the Mississippi River Delta and Surrounding Environs

An assimilation wetland is a natural (non-constructed) wetland into which secondarily-treated, disinfected, non-toxic municipal effluent is discharged. In the Mississippi River Delta, the wetland is typically either a freshwater forested wetland (e.g., baldcypress-water tupelo) or a freshwater emergent wetland. These wetlands have been hydrology altered, some extensively, with freshwater input reduced from historical norms. Discharge of freshwater effluent with nutrients and suspended sediments into an assimilation wetland increases vegetation productivity and accretion and combats subsidence. Effluent discharge rate into an assimilation wetland depends on wetland size and effluent nutrient concentrations. Design and construction of an assimilation wetland requires a Louisiana Department of Natural Resources (LDNR) Coastal Use Permit (CUP), a Louisiana Department of Environmental Quality (LDEQ) Louisiana Pollutant Discharge Elimination System (LPDES) permit, a US Army Corps of Engineers (USACE) 404 permit, and an LDEQ Water Quality Certification, along with potential levee board permit applications. Both a feasibility study and an ecological baseline study are conducted before discharge of treated effluent begins. Assimilation wetlands are designed with a minimum of four monitoring sites; three located along a transect from the discharge to the area where surface water leaves the wetland, and the fourth, a reference area, located in an ecologically similar wetland nearby. As part of the LDEQ LPDES permit, study sites within an assimilation wetland are monitored continually for the life of the project, including vegetation productivity and species composition, sediment accretion, hydrology, and surface water nutrient and metals concentrations. There are ten active assimilation wetlands in coastal Louisiana and another four with permit applications pending. Results of annual monitoring show nutrient concentrations of surface waters decrease with distance, reaching background levels before water leaves the wetland. While nutrient concentrations decrease, vegetative productivity is enhanced. In degraded forested wetlands being used as assimilation wetlands, baldcypress and water tupelo seedlings are often planted, which thrive in the nutrient rich environment. However, nutria are attracted to vegetation with increased nutrient concentrations, and herbivory severely damaged one emergent wetland receiving municipal effluent, killing both herbaceous vegetation and unprotected tree seedlings. After culling of nutria, the wetland recovered. This introduced species must be monitored and controlled in any assimilation wetland. Here we review the history of assimilation wetlands in the Mississippi River Delta to show how advances in scientific understanding, growing regulatory sophistication, and controversy have shaped this program.

Introduction to the Skagit Issue—From Glaciers to Estuary: Assessing Climate Change Impacts on the Skagit River Basin

1Author to whom correspondence should be addressed. Email: john.rybczyk@wwu.edu The Skagit River, which flows from its headwaters in British Columbia to Puget Sound in northwestern Washington State, contributes more than 30% of all freshwater entering the Sound (Figure 1). It is the largest river system in the Puget Sound basin and is a tremendous economic and ecological asset to the region, providing water for the largest agricultural center in Western Washington, habitat for many important fish and wildlife populations, regionally significant hydropower resources, water resources for cities and irrigation, and extensive outdoor recreational opportunities. The Skagit basin is home to all six salmon species present in the Sound, including steelhead and the most abundant remaining run of Chinook salmon, and the lower basin is also a haven for migratory waterfowl (Lee and Hamlet 2011).