CESN Main Page

Coastal & Estuarine Science News (CESN)

Coastal and Estuarine Science News (CESN) is an electronic publication providing brief summaries of select articles from the journal Estuaries and Coasts that emphasize management applications of scientific findings. It is a free electronic newsletter delivered to subscribers on a bi-monthly basis.

You can have future issues delivered to your email inbox on a quarterly basis. Sign up today!

2023 Issue 1

Table of Contents

Spartina Outperforms Phragmites in Wave Attenuation Tests

But the invasive plant still offers more protection than bare sediment

The spread of invasive Phragmites australis can significantly alter the structure and function of a marsh, potentially reducing the ecosystem services the marsh provides. But on the other hand, Phragmites has been known to improve water filtration, accrete sediment, and reduce the bioavailability of toxic metals more rapidly than native wetlands species. One ecosystem service that has been under-studied with respect to Phragmites invasion is coastal protection. To better understand the ability of Phragmites to attenuate waves, a team of researchers measured waves and water levels in a Phragmites-dominated wetland in the Chesapeake Bay, as well as a nearby reference site that was dominated by native Spartina alterniflora. They then used a hydrodynamic model to simulate wave propagation and compare wave attenuation in the two systems under different conditions.

Both species were capable of attenuating incoming waves better than bare ground. However, their performance varied with both water level and season: Spartina significantly outperformed Phragmites at lower water levels and during summer and especially the fall, which coincides with hurricane season along the U.S. Atlantic coast. These seasonal differences are likely due to the fact that Spartina stem density peaks later in the growing season, as the study found that stem density was the vegetation characteristic with the strongest correlation to wave attenuation. Both plants offered a similar and reduced level of protection at higher water levels and during the winter and spring.

This study provides an additional argument for the benefits of Spartina as compared to Phragmites. However, since both plant species were a lot better at attenuating waves than bare sediment, it would be inadvisable to remove Phragmites without the restoration of native species—especially since wave hazards are likely to occur more frequently with climate change.

Source: Coleman, D.J. et al. 2022. The Role of Invasive Phragmites australis in Wave Attenuation in the Eastern United States. Estuaries and Coasts. DOI: 10.1007/s12237-022-01138-x

Should Wood be Part of Tidal Marsh Restoration?

A survey of large wood around Puget Sound

Restoration planners often recommend the addition of large wood to tidal channels in marsh restoration sites—applying fluvial paradigms to estuarine systems. However, there’s no guidance on how much or where large wood should be added. To help inform restoration designs, GIS analysis of high-resolution aerial photos was used to map the distribution of large wood longer than two meters on the marsh surface and in tidal channels within five of Puget Sound’s largest river deltas and two much smaller deltas in Hood Canal on the west side of the sound.

The presence of wood was influenced by distributary networks, channel size, marsh size, storm fetch, topography, and vegetation. Storms are important for trapping large wood in deltas, but where storm fetch is negligible, flood tides are the primary process that pushes wood into channels and onto banks. Small tidal channels generally accumulated wood near the outlet, whereas larger channels accumulated wood near the head. Large marsh islands have disproportionately more and longer logs than small islands. Finally, watersheds with lower anthropogenic alteration had more intact riparian zones that lead to greater recruitment of large wood.

According to the study author, uncritically applying the fluvial large wood paradigm to estuarine habitat restoration is probably unwise given their significant differences in hydrodynamics and geomorphology. Additionally, large wood supply or retention is probably lower in estuarine than riverine systems: The density of large wood in reference lowland streams was as much as 50 times more than in the tidal channels of river deltas. The study provides guidance on when adding large wood is appropriate (e.g. when sources are limited by dams or deforestation) and where it might be placed (e.g. narrower reaches of tidal channels), and recommends that habitat restoration designers consider the details of their particular site in order to add appropriate quantities of large wood at the right locations.

Source: Hood, W.G. 2022. Distribution of Large Wood in River Delta Tidal Marshes: Implications for Habitat Restoration. Estuaries and Coasts. DOI: 10.1007/s12237-022-01122-5

Return to Top

Floating Oyster Aquaculture Can Remove Nitrogen

Enhancing denitrification in a Cape Cod tidal salt pond

Floating oyster aquaculture can help increase the removal of anthropogenic nitrogen through assimilation into oyster biomass, long-term sediment burial, and enhanced sediment denitrification. To determine the effectiveness of this approach, a team of researchers quantified N cycling associated with floating oyster aquaculture in a small tidal salt pond on Cape Cod. The team deployed millions of oysters in mesh bags attached to floats and compared sediment oxygen demand and denitrification from sediment cores collected under oyster bags in high biodeposition areas with cores collected elsewhere in the pond.

Of the three pathways of N removal by oyster aquaculture, assimilation into tissue and shell and the subsequent harvest contributed the most (64%), followed by enhanced denitrification (22%) and burial (14%). Enhanced sediment denitrification was observed in all three years, ranging from 265% to 388% of the (non-oyster) background. Sediment biodeposition rates, as well as sediment oxygen demand, were also significantly higher under the floating bags than in non-aquaculture areas, especially during warmer months. Biodeposition actually enhanced denitrification, as long as surface sediments remained oxidized; denitrification rates were inhibited during times of bottom water hypoxia.

Floating oyster aquaculture can be a viable strategy for reaching N reduction goals, but it won’t work everywhere: Stratified systems that experience substantial bottom water hypoxia will not show an N removal effect from enhanced denitrification. Similarly, too much aquaculture could enhance sediment oxygen demand to the point of creating hypoxic bottom water conditions.

Source: Labrie, M.S. et al. 2022. Quantifying the Effects of Floating Oyster Aquaculture on Nitrogen Cycling in a Temperate Coastal Embayment. Estuaries and Coasts. DOI: 10.1007/s12237-022-01133-2

Return to Top

Balls or Sills: Which Living Shoreline Design Dissipates Waves Best?

Comparing two different designs in the same Virginia bay

Living shorelines combine structural and natural elements to stabilize the coastline by dissipating waves while also providing habitat. To investigate the efficiency of different types of living shorelines in wave attenuation, researchers turned to a small, sheltered bay in Norfolk, Virginia, where a marsh sill (marsh planted over an elevated fill behind a low- crested stone breakwater) and an array of oyster reef balls (low-crested, concrete, hollow breakwaters designed to attract and grow oysters) were installed. The team measured waves around the two features and performed a spectral analysis to quantify the relative loss in wave energy associated with each living shoreline structure.

The study found that both types of living shorelines can reduce wind-driven wave energy. However, the marsh sill was much taller and was submerged only a quarter of the time as compared to the reef balls, which were almost continuously submerged. When exposed or near exposure, the marsh sill attenuated waves by 40% compared to 13% attenuation by reef balls. Once submerged at high tide, the two designs were equally inefficient in wave damping, causing only minor wave reduction. This finding suggests that structure crest elevation is a key factor, perhaps more important than structure type.

This low-energy environment is typical of good candidates for nature-based erosion reduction, and understanding how different designs function can help coastal managers select the most suitable alternative. Although sills were better for wave dissipation, oyster reef balls have a smaller footprint, and oysters also enhance water quality. Comparing the relative benefits of different living shorelines exposed to the same conditions can help managers make more informed assessments about the types of designs that best suit their location and goals.

Source: Leone, A. & N. Tahvildari. 2022. Comparison of Spectral Wave Dissipation by Two Living Shoreline Features in a Sheltered Tidal Bay. Estuaries and Coasts. DOI: 10.1007/s12237-022-01140-3

Return to Top