Preliminary assessment of bioengineered fringing shoreline reefs in Grand Isle and Breton Sound, Louisiana

Abstract

Restoration of three-dimensional shell habitats in coastal Louisiana presents a valuable and potentially self-sustaining approach to providing shoreline protection and critical nekton habitat and may contribute to water quality maintenance. The use of what has been called “living shorelines” is particularly promising because in addition to the hypothesized shoreline protection services, it is predicted that, if built and located in viable sites, these living shorelines may ultimately contribute to water quality maintenance through filtration of bivalves and may enhance nekton habitat. This approach, however, has not been tested extensively in different shallow water estuarine settings; understanding under what conditions a living shoreline must have to support a sustainable oyster population, and where these reefs may provide valuable shoreline protection, is key to ensuring that this approach provides an effective tool for coastal restoration. This project gathered preliminary data on the sustainability and shoreline stabilization of three large bioengineered fringing reefs located in Grand Isle, Lake Eloi, and Lake Fortuna, Louisiana. We collected preconstruction and postconstruction physiochemical and biological data by using a before-after-control-impact approach to evaluate the effectiveness of these living shoreline structures on reducing marsh erosion, enabling reef sustainability, and providing other ecosystem benefits. Although this project was originally designed to compare reef performance and impacts across three different locations over 2 years, delays in construction because of the Deepwater Horizon oil spill resulted in reefs being built from 12 to 18 months later than anticipated. As a result, monitoring postconstruction was severely limited. One reef, Grand Isle, was completed in March 2011 and monitored up to 18 months postcreation, whereas Lake Eloi and Lake Fortuna reefs were not completed until January 2012, and only 8 months of postconstruction data are available. Data for the latter two sites thus reflect only the 2012 spring/summer seasons, which were further impacted by a direct hit by Hurricane Isaac in August 2012, which resulted in shoreward movement of approximately 14 percent of the bioengineered structures at Lake Fortuna. Given the shortened monitoring timeframe and significant differences in construction schedules, we were not able to provide a full postconstruction assessment of the sites or a full comparison of site success based on local site characteristics. Because many of the impacts that were identified for monitoring reflect long-term processes, results and data presented should be interpreted cautiously. Sustainable oyster reefs require recruitment, growth, and survival at a rate that keeps pace with mortality and shell disarticulation. Although one site failed to recruit (establishment plus survival > 50 millimeters [mm]) over two spawning seasons, two sites only had 6 months postconstruction data available for assessment. Although there are good data on the requirements for oyster growth, there is limited explicit information on the site-specific water quality, hydrodynamic, and biological interaction effects that may determine successful reef establishment. Furthermore, interannual variability can significantly affect reef establishment, and our shortened timeframe of sampling (less than one spawning season for two of the reefs; two spawning seasons for one reef), combined with a lack of prerestoration monitoring data, limit our ability to draw any conclusions about long-term reef sustainability. Bioengineered reefs are thought to provide some benefits to shoreline stabilization through their structure by immediately attenuating wave energies and directly reducing erosion rates at shorelines sheltered by the reefs but also by increasing sediment deposition behind the reefs. Preliminary data indicate differences in reef impact by site; given the short timeframe of postconstruction data at two of the sites, and differences in reef placement between sites, however, it is difficult to draw any conclusions. Longer-term data collection and further analyses comparing reef placement; local wind, wave energy, sediment transport processes; and local bathymetry may help in parameterizing sites where fringing reefs may be most beneficial for shoreline protection. In addition to basic reef sustainability and shoreline stabilization, we measured both water quality parameters and nekton abundances around the newly created reefs and adjacent reference sites. Within the timeframe of monitoring, no effect of reefs on water quality was detected at any site. Given that water quality effects are hypothesized to result from the filtration activities of bivalves, and reefs either failed to recruit (settlement plus survival to > 50 mm) or successfully recruited but only had a couple months of growth prior to this report, it was not expected that an effect would be detectable in this timeframe. Nekton such as blue crab, gulf menhaden, and anchovies were found to be more abundant on the reefs; larger, more transient species were not found to be affected by reef presence. Future work examining smaller organisms and juveniles, including more explicit studies examining why and how these organisms preferentially use oyster reefs, would be useful in the design of other bioengineered reefs and help in understanding the role of the reefs in supporting the nekton community. It is clear from the initial work that ensuring correct site selection by better understanding what local site factors influence oyster populations is key to establishing successful living shoreline reefs. Ultimately, the success of the reefs in providing any ecosystem service relies on their ability to build a viable oyster population that is self-sustaining over the long term. As many of the ecosystem processes hypothesized to result from reefs develop over the long term (4–6 years), some level of monitoring over the next few years is highly recommended in order to accurately assess the long term viability of the reefs, their provision of ecosystem services, and to provide better guidance for future projects.