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2018 January

Contents

Phragmites May Hatch More Male Terrapins
Chesapeake Marsh Area Remains Constant
Effects of Shoreline Stabilization
Horseshoe Crab Haven?

 Phragmites May Hatch More Male Terrapins

Phragmites does not damage nests, but does make them cooler

The invasive reed Phragmites australis can spread quickly into disturbed areas, replace native vegetation, and fill tidal creeks so that they're nearly impassable. But does it affect diamondback terrapin nests? For terrapins, risks include direct damage from the growth of Phragmites roots as well as the potential for a hatchling sex ratio skewed toward males if shading by Phragmites causes nest temperatures to drop. A new study examines what this means for terrapins living in the Virginia’s Fisherman Island National Wildlife Refuge, a barrier island at the mouth of the Chesapeake Bay.

Researchers searched for nests and measured plant cover and stem density at each nest site they located. To evaluate the threats Phragmites might pose depending on terrapins’ choice of nest location, they also buried temperature loggers and root in-growth bags at hypothetical nest sites with low (0-20%), medium (21-50%), and high (51-75%) Phragmites cover. Root ingrowth was modest and suggested that Phragmites roots are a relatively low risk to nests. However, sites with high Phragmites cover had significantly lower temperatures than those in areas with low cover. These differences were enough to cause a potential shift to predominantly male offspring.

Most actual terrapin nests located by the researchers were at locations with little to no Phragmites. However, anthropogenic activity and the spread of other invasive species could eventually force terrapins to nest in lower-quality sites where Phragmites shading will pose a problem. Although complete Phragmites eradication would be a daunting task, a lesser degree of control will likely be enough to maintain nesting habitat—this research suggests that terrapins can be successful in areas with limited Phragmites presence, as long as density is kept low.

Source: Cook, C.E., A.M. McCluskey, and R.M. Chambers. 2017. Impacts of Invasive Phragmites australis on Diamondback Terrapin Nesting in Chesapeake Bay. Estuaries and Coasts. DOI: 10.1007/s12237-017-0325-z

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Chesapeake Marsh Area Remains Constant

Upland migration of marshes kept pace with erosion in the twentieth century

When it comes to assessing how sea level rise might affect marshes, a lot of attention has been focused on inundation causing marshes to drown at their seaward edges. However, increasing sea levels also have the potential to help marshes expand into what are currently upland areas. Changes in the total extent of marsh habitat depend on the balance between these competing processes, and new research looks into how this is playing out in the Chesapeake Bay.

The researchers used historical maps called “T-sheets,” dating from between 1846 and 1912, to locate the upland and seaward edges of the area's marshes in the late nineteenth century. Comparing this to aerial photographs from 2013, they found that marshes tended to expand in the southern portion of the Bay and contract in the mid and northern portions. However, erosion at the seaward edge and migration at the upland edge were essentially balanced over the twentieth century, and the total amount of marshland remained remarkably constant across the study area—311 square kilometers of tidal marshes in the late nineteenth century and 318 in 2013. The results also showed a weak relationship between a marsh's inland migration rate and the slope of the land, which could potentially be affected by land use and development.

Overall, this study demonstrates the importance of considering both erosion and migration when evaluating the potential effects of sea level rise. Despite the threats they face, marshes, at least in some regions, have the potential to maintain their spatial extent by migrating inland.

Source: Schieder, N.W., D.C. Walters, and M.L. Kirwan. 2017. Massive Upland to Wetland Conversion Compensated for Historical Marsh Loss in Chesapeake Bay, USA. Estuaries and Coasts. DOI: 10.1007/s12237-017-0336-9

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Effects of Shoreline Stabilization

Different techniques have different effects on sedimentation and SAV

Stabilizing shorelines to protect property from erosion and sea level rise is already common, and climate change and expanding coastal human populations will only accelerate this trend. However, detailed information on how different shoreline structures affect nearshore sediment and submerged aquatic vegetation is lacking.

In a recent study, researchers collected sediment cores and recorded the presence or absence of submerged aquatic vegetation at sites around Chesapeake Bay with differing shoreline structures (offshore breakwaters, riprap, and “living” shoreline) as well as at naturally eroding shorelines, which served as controls. Linear regression models were able to predict nearshore sedimentary changes based on a combination of factors including structure type, shoreline erosion rates, dominant sediment source, and land use, and a few generalizations emerged. Overall, changes in sedimentation rates and content at naturally eroding sites were minimal and reflected broader environmental trends, whereas changes at hardened sites reflected the effects of structures. Breakwaters trap sediment, and therefore changes after their installation are related to the characteristics of the dominant sediment source. Riprap, on the other hand, severs the sediment flow link between land and sea, which can cause the loss of marshes and aquatic vegetation but also lead to local water quality improvements. SAV results were inconsistent, with coverage increasing at some breakwater and riprap sites but remaining stable or decreasing at others.

Bottom line? There’s no “one size fits all” strategy for determining the likely effects of shoreline stabilization on sedimentation and aquatic vegetation. Managers need to consider historical shoreline erosion rates, land use, and dominant sediment type as well as the type of shoreline stabilization being contemplated in order to make the best possible decisions for sites under their care.

Source: Palinkas, C.M., L.P. Sanford, and E.W. Koch. 2017. Influence of Shoreline Stabilization Structures on the Nearshore Sedimentary Environment in Mesohaline Chesapeake Bay. Estuaries and Coasts. DOI: 10.1007/s12237-017-0339-6

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Horseshoe Crab Haven?

Beach nourishment leads to small but detectable gains for horseshoe crabs

Horseshoe crabs depend on sandy beaches for spawning. New York’s Plumb Beach, part of the urbanized estuary of Jamaica Bay, is a critical spawning location for the state’s horseshoe crab population. A beach nourishment project in 2012, done to protect a major highway, provided an opportunity for researchers to monitor the activity of horseshoe crabs and evaluate how beach nourishment affects a site's suitability for spawning.

The study compared the number of mating horseshoe crabs, the density of eggs deposited, and the sediment characteristics of the freshly nourished western end of the beach with conditions at the more natural eastern end. Before the nourishment project, very few horseshoe crabs spawned at the western site and this remained true throughout the study. There was little to no change in the first year after nourishment, with small increases during the second and third years. Egg deposition did not change at the western site, remaining at low levels.

Although the two beach sections looked superficially similar after the nourishment project, sediment at the nourished beach was finer and more uniform, creating a harder surface with less oxygen diffusion. As a result, female horseshoe crabs may have approached the beach but laid their eggs elsewhere after discovering the sediment conditions. However, the differences between the two sites became smaller over time, and the sediments at the nourished beach may eventually become more suitable for the crabs.

Despite the limited benefits to horseshoe crabs after beach nourishment, the study’s authors see it as preferable to the use of bulkheads or revetments. The likelihood of successfully restoring horseshoe crab spawning grounds, they suggest, can be enhanced with a better understanding of a specific site’s hydrodynamics and geomorphology.

Source: Botton, M.L, C.P. Colón, J. Rowden, S. Elbin, D. Kriensky, K. McKown, M. Sclafani, and R. Madden. 2017. Effects of a Beach Nourishment Project in Jamaica Bay, New York on Horseshoe Crab (Limulus polyphemus) Spawning Activity and Egg Deposition. Estuaries and Coasts. DOI: 10.1007/s12237-017-0337-8

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