Individual study: Creation of artificial Eastern oyster Crassostrea virginica reefs protects brackish marsh in low-energy wave environments at Sister Lake, Louisiana, USA
Piazza B.P., Banks P.D., & (2005) The potential for created oyster shell reefs as a sustainable shoreline protection strategy in Louisiana. Restoration Ecology, 13, 499-506
Protection and maintenance of shorelines is often a goal of coastal protection programs. This study investigated the potential shoreline protection role of created, Eastern oyster Crassostrea virginica shell reefs fringing eroding marsh shorelines in Louisiana, southeastern USA.
Study site: The study was conducted in Sister (Caillou) Lake, Terrebonne Parish, Louisiana. This area is a typical Louisiana coastal brackish marsh system and experiences high marsh loss (>2,500 ha/yr). Sister Lake is primarily an open water system, fringed by brackish marsh. Water depths ranges from 1 to 3 m. Winds are typically southeastly, except in the winter when northerly winds occur. Fetch distance is quite large, and wind-induced erosion is the dominant mechanism of shoreline loss. Tides range from −0.8 to 1.1 m.
Experimental reefs: Experimental reefs (25 × 1.0 × 0.7 m; intertidal) were created in June 2002 at both high and low wave energy shorelines. Six 25-m study sites (three oyster-reef and three controls i.e. no reef) were established at each shoreline in June 2002, for a total of 12 sites.
Reef deployment: Experimental oyster shell reefs were created in June 2002. A total of 17.5 m³ of shucked Eastern oyster Crassostrea virginica shell was off-loaded at each reef site, and an experimental reef (25 × 1.0 × 0.7 m) was constructed. Reefs were built as close to the shoreline as possible (within 5 m of the shoreline) and were intertidal. Monitoring occurred monthly from June 2002 to June 2003.
Marsh characterization: Water quality (salinity, temperature, dissolved oxygen), vegetation, and soils data were collected monthly to characterize the study sites and detect any changes. A survey of marsh elevation was conducted in January 2003 at each shoreline with a survey transit and staff. Water quality data were obtained from a U.S. Geological Survey data collection platform located between study shorelines. Hourly data (June 2002–June 2003) were downloaded to calculate salinity, water temperature, stage and flooding frequency and duration.
Shoreline change: Shoreline advance or retreat was measured at each site. To ensure consistent measurements throughout the study, monthly shoreline position was measured by the same investigator. Shoreline edge was defined as the farthest waterward extent of the wetland macrophytes. Average retreat rates were calculated for each site.
Reef sustainability: Triplicate, randomly selected, 0.06-m² shell samples were removed from each reef monthly. Oyster spat (≤30 mm) on each shell were counted, measured, and categorized as live/dead.
Marsh characteristics: Environmental characteristics during our study were typical of long-term (18 years) averages. On average, marshes were flooded 8.8 ± 1.1 hour/day. No significant differences were found between sites in temperature, salinity, or dissolved oxygen. Vegetation cover and biomass were similar at all study sites. Marsh areas in Sister Lake were dominated by smooth cordgrass Spartina alterniflora (38%), saltgrass Distichlis spicata (27%) and black needlerush Juncus romerianus (27%). Aboveground vegetation averaged 76 stems/m² and 75.4 g/m². Belowground biomass averaged 6.6 g/cm³. Average soil bulk density was 0.44 ± 0.01 g/cm³ and organic content 21%.
Shoreline change: Over the 1-year study, average monthly retreat ranged from 0.03 to 0.15 m. Shoreline retreat differed significantly by treatment and energy. Shoreline retreat from June 2002 to June 2003 was significantly lower at reef sites and at low-energy shorelines as compared to non-reef controls (0.12 m/month) and high-energy shorelines (0.14 m/month). Significant differences were found between reef treatments only in low-energy sites. Highest shoreline erosion rates occurred in October and November following two significant storm events.
Reef sustainability: A total of 30,527 oyster spat were counted on 6,044 sampled shells (4.9 spat/shell). Recruitment of oysters began immediately upon reef creation reef in June 2002. Oysters began setting within 1 month of shell placement and peaked in October 2002, with an average of 9.5 spat/shell. No significant difference in oyster spat numbers was detected between low- and high-energy sites. Spat growth averaged 3.4 mm after 1 month (July 2002; N= 579) and 23.0 mm (N= 2,252) after 1 year (June 2003). No significant difference in oyster spat size was detected between low- and high-energy reefs.
Conclusions: Shoreline retreat was reduced along low-energy shorelines with oyster-reefs as compared to the control low-energy shorelines but was not significantly different between reef and control sites in higher-energy environments. These results demonstrate that in low-wave energy environments, small created fringing oyster reefs may be useful in slowing shoreline erosion. The reefs were found to have high spat recruitment and growth, suggesting potential sustainability over time. In coastal Louisiana where coastal oyster reefs are extensive, the use of fringing oyster shell reefs has the potential to provide a useful means of shoreline stabilization along low-energy coastal stretches. Their usefulness may be limited in high-energy environments.
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