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2017 May

Contents

Looking for Louisiana Hot Spots
Oyster Reefs Form a Line of Defense Against Marsh Loss
When Fresh Water and Rising Seas Collide
Eyes on the Storm

Looking for Louisiana Hot Spots

Hot spots are not just for celebrities: Juvenile fish and crabs use them too

It is widely known that many commercially and recreationally important fish and shellfish use coastal habitats as nursery grounds during their juvenile life stage.  Managers tasked with conserving, protecting, and restoring these coastal habitats need further definition of species-specific habitat preferences in order to pinpoint and prioritize “hot spots” where species prevalence may be highest. Knowing where these hot spots occur today can help managers consider how their actions, as well as future environmental conditions, may impact the locations of these hot spots tomorrow.  A recent study used a coupled modeling approach to develop species distribution models for early juvenile stages of white and brown shrimp, blue crab, and spotted seatrout in coastal Louisiana in order to find such hot spots.

Investigators used fisheries-independent monitoring data to describe habitat preferences of the four species with respect to temperature and salinity conditions. These results were integrated with a more comprehensive, spatially explicit model of hydrodynamics, vegetation, and morphology to produce an aggregated species distribution model for all four species. The model results revealed numerous areas of suitable habitat, but only 1% of the habitat was identified as a true hot spot with high suitability for all four species. These areas were mostly high fringing marsh, some of the most rapidly changing habitats in the region, while “cool spots” tended to be interior fresh marshes, fresh forested wetlands, and areas of solid marsh. The species distribution model developed here will be used in the Louisiana Coastal Master Plan to evaluate how future management projects, in the context of a changing landscape, will impact the habitats of these important species. The approach used here could be used in other locations as well.

Source: Commagere Hijuelos, A., S. E. Sable, A. M. O’Connell, J. P. Geaghan, D. C. Lindquist, and E. D. White. 2016. Application of species distribution models to identify estuarine hot spots for juvenile nekton. Estuaries and Coasts (December 2016). DOI: 10.1007/s12237-016-0199-5.

Oyster Reefs Form a Line of Defense Against Marsh Loss

North Carolina fringing oyster reefs may also help keep “blue carbon” in place

Fringing marshes can protect coastal habitats from erosion and inundation from storms and sea level rise, but what protects the marshes? One recent study reveals that fringing oyster reefs can have a protective function, not only slowing the landward retreat of coastal marshes, but possibly protecting against loss of carbon sequestered in marsh sediments as well. The investigators used a variety of imaging and coring techniques to examine historic extents of marsh and natural and restored oyster reefs in Back Sound, North Carolina. They combined methods that allowed them to look at marsh elevation and marsh and oyster reef histories.

Results indicate that natural fringing reefs in the sound are in a state of landward retreat, as suggested by the fact that modern oyster reef locations were occupied by salt marsh within the past two centuries. At the local scale, however, marsh retreat appears to be slowed by the presence of mature reefs. In addition, the formation of natural oyster reefs in areas previously occupied by marshes has prevented erosion of marsh sediments, thereby preventing stored organic carbon, so-called “blue carbon,” from being washed into the estuary. In retreating marshes lacking fringing oyster reefs, all of the stored sediment carbon was lost over time. The amount of marsh sediment preservation under the reef scales with the reef’s relief (an indication of reef maturity): reefs with the greatest relief were level with the marsh platform, providing maximal buffering of the marsh from erosional processes. The authors conclude that coupled restoration and preservation of reef and marsh habitats will help ensure prolonged ecosystem services, retain blue carbon, and ultimately protect uplands against rising seas by protecting marshes from the effects of inundation.

Source: Ridge, J. T., A. B. Rodriguez, and F. J. Fodrie. 2017. Salt marsh and fringing oyster reef transgression in a shallow temperate estuary: Implications for restoration, conservation and blue carbon. Estuaries and Coasts (November 2016). DOI: 10.1007/s12237-016-0196-8.

When Fresh Water and Rising Seas Collide

What happens to water quality in the face of both river diversions and sea level rise?

Mississippi River Delta marshes need sediment to help restore degraded marshes and keep up with sea level. Planning is underway for several large river diversions in the Mississippi system, which have been shown to be effective at sediment delivery. However, not only might these diversions have other impacts on water quality and the ecosystem, but sea level rise itself may also bring water quality changes. What happens to water quality in the face of both increased freshwater delivery from upstream river diversions and saltwater intrusion from a rising sea?

A recent modeling effort examined the effects on water quality (salinity, suspended solids, and chlorophyll a) in the Breton Sound Estuary, LA of large and small river releases from the Caernarvon Freshwater Diversion under scenarios of low and high sea level rise. Model results demonstrate that a large diversion plays a larger role in affecting water quality parameters than sea level, and that high sea level rise plays a larger role than small diversions alone. Coupled sea level rise/river diversion scenarios resulted in higher ranges and increased spatial and temporal variability of all three parameters.

The authors point out that it is critical to understand the combination of river diversions and sea level rise on water quality in order to predict impacts and manage estuarine resources, from vegetation to fish and shellfish. For example, this type of approach could be used to predict changes to oyster populations, as oysters have very specific salinity tolerances.

Source: Wang, H., Q. Chen, K. Hu, and M. K. La Peyre. 2017. A modeling study of the impacts of Mississippi River diversion and sea-level rise on water quality of a deltaic estuary. Estuaries and Coasts (December 2016). DOI: 10.1007/s12237-016-0197-7.

Eyes on the Storm

Researchers brave two coastal storms to quantify storm attenuation by a Chesapeake wetland 

Coastal wetlands are considered a natural barrier to storm surge, attenuating wave action and protecting the upland areas behind them from the wrath of coastal storms. But how do we know this? Because it’s quite difficult and inconvenient to make measurements during an actual storm, most studies of wave and storm surge attenuation involve modeling simulations. One group of intrepid researchers decided to brave the punishing conditions of two storms in the Chesapeake Bay to take some real-world field data on marsh attenuation of storm surge.

During two tropical cyclones in the fall of 2015, each five days long, the investigators placed sensors along transects in a marsh at the mouth of the Chesapeake (Eastern Shore of Virginia National Wildlife Refuge) to measure wave heights, currents, and water levels. During calmer weather, they had already collected data on vegetation and topography parameters. Results suggested significant wave attenuation and reduction in current velocity and water level during storms, all of which varied over a tidal cycle and differed depending on location along the marsh transect. Wave height attenuation was greatest at the lower part of the marsh during rising tides. Current velocity was strongly attenuated between the seaward edge of the marsh and the upland marsh reaches. Tidal range was reduced by the marsh during storms but responses differed by marsh section, in part because of inputs from mid-marsh tributaries. The higher the water level above the marsh surface, the lower the amount of wave attenuation at that site, because proportionally less water was in contact with marsh surface.

The authors suggest that future work should focus on modeling and laboratory studies that examine the effect of marsh and vegetation morphology on water level attenuation, work that could provide key information for managers interested in the use of natural coastlines as defense against storm impacts on coastal areas.

Source: Paquier, A., J. Haddad, S. Lawler, and C. M. Ferreira. 2017. Quantification of the attenuation of storm surge components by a coastal wetland of the US Mid Atlantic. Estuaries and Coasts (November 2016). DOI: 10.1007/s12237-016-0190-1.