Paper: La Peyre MK, Serra K, Joyner TA, Humphries A. (2015) Assessing shoreline exposure and oyster habitat suitability maximizes potential success for sustainable shoreline protection using restored oyster reefs. PeerJ 3:e1317 https://doi.org/10.7717/peerj.1317
Like any other project, oyster reef restoration and shoreline protection projects require planning! But how do you get the most bang for your buck? You need to know where the need for protection is greatest and where the oysters will be able to thrive. That way, you can maximize the benefit of your restoration project, best utilizing whatever funds you have.
A little background first: Why restore oyster reefs at all? Because
Oysters = One Outstanding Organism!
Oysters provide a number of ecosystem services. As filter feeders, they improve water quality by removing pollutants, microalgae, and excess nutrients and contributing to sediment deposition. They build bars or reefs that buffer the shore from wave energy (Fig. 1). These reefs also provide habitat for many other organisms by acting as a hard substrate for settlement or a refuge. They attract fish that eat the oysters themselves or the critters hiding between their shells.
Unfortunately, in the last 100 years, 85% of oyster reefs have been gone the way of so many habitats: disappearance and degradation. Loss of oysters means loss of all those services they provide! Many studies have looked at oyster restoration projects and seen improvements in fisheries, water quality, etc. But the effect of oyster loss or restoration on coastal erosion is unknown, even though restoration projects are already being used for
Coastal lands are eroding and being lost for a number of reasons, including sea level rise, coastal development, and removal of plants and other organisms that had previously stabilized shorelines (such as oysters?). Therefore, resources and effort are devoted to stabilizing shorelines to prevent further loss of these coastal lands, including marshes. While some methods of coastal protection utilize barriers such as seawalls, there is a recent focus on creating “living shorelines,” natural and sustainable systems such as oyster reefs. Louisiana, the site of this study, has rapidly eroding coastlines. They have already begun restoring oyster reefs to combat erosion, create habitat and improve water quality.
Oyster reefs buffer wave action, redirect water flow, and trap sediment while also bonding together and creating a calcium carbonate cement (essentially a kind of wall). Despite the many reasons that oyster reefs should prevent coastal erosion, there is not much evidence to support that. La Peyre et al. sought to determine whether they do and where they would likely be most successful based on two factors: 1) suitability for oysters and 2) shoreline orientation.
The “How” of the Study:
The researchers used a number of technologies, including GIS (Geographic Information Systems) and wave models to assess shoreline exposure and shoreline movement (is the shore advancing or retreating?) at five sites that have had oysters restored as well as control sites where no restoration had occurred (Fig. 2). At these sites and other randomly selected ones in Breton Sound estuary, LA, they evaluated shoreline exposure and habitat suitability.
They applied a habitat suitability index (HSI) to determine which areas are most likely to support oyster populations. That index (used in the Louisiana Coastal Master Plan 2012) incorporates bottom habitat and a few salinity metrics. The HSI maps show multiple years that vary by river discharge (which affects salinity). As salinity decreases, suitability for oysters also decreases. They then overlay those to find where oyster restoration efforts should be focused.
Erosion was Reduced!
Marsh erosion was indeed lower where reefs had been restored than at other sites without oyster reefs (Fig. 3). Where exposure was intermediate or high, the shoreline benefited most from restoration, with erosion reduced by 1.07 m/yr where reefs were restored compared to shoreline without reefs. Erosion was also reduced where exposure was low, but it had less of an effect since erosion was lower there to begin with.
Where Should Oyster Reefs Go?
I told you at the beginning: where the benefit is greatest and the oysters will thrive. Specifically, that means where? Given that oyster reefs do reduce marsh retreat on shorelines with intermediate and high exposure, it seems a good first step to focus on those levels of exposure. But it is important to consider the suitability of the habitat for oysters and choose sites where oysters are most likely to succeed. Maps (ex. Fig. 4) which overlay exposure and suitability are great tools for site evaluation and selection. And the variation in suitability based on river discharge seen in the three different maps shows how important it is to consider various scenarios and trade-offs when making decisions. It will be important to predict what future river discharge is likely or to prioritize selecting sites that will do well under any of the salinity scenarios.
Note: Marsh retreat was not reversed with restoration, so utilizing this shoreline stabilization strategy may need to be combined with strategies to manage other impacts beyond wave energy, such as sea level rise.
As always, there are many questions that need to be answered to improve our ability to site successful projects. To name a few, how does reef length, height, or shape affect success? How fast do oysters build the reef and is it more in expansion outward or upward? Stay tuned over the years as more restoration sites are created and monitored to answer these questions.
Because I found it cool, here’s a few links to publicly available databases the authors used to find high-resolution shoreline maps: Gulf of Mexico Coastal Ocean Observing System (GCOOS) and NOAA