Efforts to restore the Eastern oyster (Crassostrea virginica) reef habitats in Chesapeake Bay typically begin with the placement of hard substrata to form three-dimensional mounds on the seabed to serve as a base for oyster recruitment and growth. A shortage of oyster shell for creating large-scale reefs has led to widespread use of other materials, such as Surf clamshell (Spisula solidissima), as a substitute for oyster shell. Oyster recruitment, survival and growth were monitored on intertidal reefs constructed from oyster and Surf clamshell near Fisherman’s Island, Virginia, U.S.A. and on a subtidal Surf clamshell reef in York River, Virginia, U.S.A. At the intertidal reefs, oyster larvae settlement occurred at similar levels on both substrate types throughout the monitoring period but higher levels of post-settlement mortality occurred on clamshell reefs. The oyster shell reef supported greater oyster growth and survival and offered the highest degree of structural complexity. On the subtidal clamshell reef, the quality of the substrate varied with reef elevation. Large shell fragments and intact valves were scattered around the reef base, whereas small, tightly packed shell fragments paved the crest and flank of the reef mound. Oysters were more abundant and larger at the base of this reef and less abundant and smaller on the reef crest. The availability of interstitial space and appropriate settlement surfaces is thought to account for the observed differences in oyster abundance across the reef systems.
Fishermans Island, 1, Virginia, USA, 37.0929212, -75.96354109999999
Country or Territory:
United States of America
Coral Reef, Seagrass & Shellfish Beds
Area being restored:
U.S. Environmental Protection Agency
Planning / Design
Primary Causes of DegradationMining & Resource Extraction, Urbanization, Transportation & Industry
In Chesapeake Bay, years of poor resource management of both live oysters and shell, mortality from diseases caused by the protistan parasites, Dermo (Perkinsus marinus) and MSX (Haplosporidium nelsoni), and increased sedimentation and environmental degradation have contributed to a dramatic decline in oyster populations and reduced reefs to mere footprints (Hargis & Haven 1988; Hargis 1999).
To examine and contrast the oyster recruitment, growth and survival at different reef elevations on a subtidal S. solidissima shell reef at Goodwin Island in lower York River, and intertidal reefs constructed of C. virginica shell and S. solidissima shell near the mouth of Chesapeake Bay at Fisherman’s Island.
The project does not have a monitoring plan.
Description of Project Activities:
---Project Sites--- The study area included two created oyster reef sites protected from any commercial harvesting in lower Chesapeake Bay. One site was situated at the mouth of the York River approximately 1 km north of Goodwin Island. An oyster reef base was constructed in spring 1995 of 30,000 bushels (1,057 m3) fractured Surf clam (S. solidissima) shell on a subtidal sandflat (Meisner 1995). The reef measured approximately 1,350 m2 and extended approximately 1.5 m above the seabed. At low tide, the water over the subtidal reef crest was 1.5 m deep. The other site was located at the Fisherman's Island National Wildlife Refuge near the mouth of Chesapeake Bay at Virginia's eastern shore. During summer 1996, 11 subtidal and intertidal oyster reef habitats were constructed of three substrate materials: Crassostrea virginica shell (market size valves, â‰¥ 7.6 cm shell height), crushed S. solidissima shell, and, although not evaluated in this study, pelletized coal ash. The reefs ranged in size from 162 to 364 m2 (O'Beirn et al. 2000). ---Sampling--- Sampling of the reef to determine oyster abundance and size at Goodwin Island took place from fall 1999 through summer 2001. Using reference stakes permanently positioned at the reef margins, the surface of the reef was divided into a grid and coordinates were assigned to each cell of the grid. The reef was further subdivided into three depth strata: crest (1.5 m above the seabed), flank (0.8 m above the seabed), and base (0.2 m above the seabed). Within each depth stratum, the coordinates on the reef surface were selected randomly without replacement for each sample. Within the cell, divers placed a square plastic frame (0.25 x 0.25 m) on the reef surface and all substrate material within the frame was removed by hand to a depth of 10 cm (below this depth, shell and associated sediments were black and indicative of anoxic conditions) and placed in a cloth bag. Six replicate quadrat samples per sampling period were collected from each elevation stratum. All live adult (oysters > 30 mm shell height, measured from the hinge to the ventral shell margin), juvenile (â‰¤30 mm shell height), and recently dead (with empty, paired, articulated valves with no evidence of interior fouling) oysters were counted and measured. Similar methods were used at Fisherman's Island for assessing oyster stocks on the created intertidal reefs (described in detail in O'Beirn et al. 2000). Briefly, in May 1997, 1998, and 1999, three 0.25 x 0.25-m quadrats were collected from each of the three tidal elevations on two replicate reefs of each substrate type. The elevations were subtidal (0.25 m below mean low water), low intertidal (at mean low water), and high intertidal (0.25 m above mean low water). The crests and flanks of different reefs were exposed to different tidal inundation regimes because of settling and erosion of the reefs over time (in particular, one of the oyster shell reefs). Because the tidal elevation of the reef crests ranged from high to low intertidal, the higher tidal elevations from all reefs were not sampled during the entire study. Therefore, to compare oyster density by reef substrate type, analysis was restricted to the samples collected from the subtidal (reef base) and low intertidal (reef crest or flank, depending on the reef) reef elevations. All live and recently dead (with paired, articulated valves with no interior fouling) oysters were enumerated and measured to the nearest 0.1 mm. Visual comparisons of the size of Surf clamshell (S. solidissima) fragments that made up the Goodwin Islands reef at different elevations were striking and led to a characterization of the reef substrate in July 2000. A subsample of at least 50 shell fragments was randomly selected from each 0.25 m2 reef quadrat sample and the largest dimension of each fragment was measured to the nearest millimeter to characterize the size of the substrate material at each reef elevation. This analysis was not repeated on the Fisherman's Island reefs. Dead oysters collected from Goodwin Islands were examined for evidence of predation by crabs in 2000 and 2001. Predation by crabs on oysters was distinguished from other sources of mortality by the presence of chipped or cracked valve margins, puncture holes within the umbo region, crushing of the umbo region, and complete crushing of the valves. This analysis was not repeated on the Fisherman's Island reefs.
Ecological Outcomes Achieved
Eliminate existing threats to the ecosystem:
---Goodwin Island--- Oyster densities on the Goodwin Island reef showed a clear pattern relative to reef elevation at all sampling times with the base of the reef having greater oyster densities than reef crest. Elevation on the reef, but not date, influenced densities of both live and dead mature oysters (>30 mm). These oysters were significantly more abundant at the reef base compared with the flank and crest. The size of clamshell fragments making up the reef substrate reflected the distribution of larger oysters and varied among elevation strata. Statistical analysis revealed that fragments of clamshell were significantly larger at reef base compared with flank and crest. Shell fragments from the flank and crest did not differ significantly. Densities of live juvenile oysters (â‰¤30 mm) were significantly affected by reef elevation, date and their interaction. Statistical tests revealed the following: (1) during the 1999 and 2000 sampling events, densities of small live oysters were greater at the flank and base than densities at the crest; (2) during the 2001 sampling period, no statistically significant difference in juvenile oyster density according to reef elevation was detected, although as in 1999 and 2000, densities tended to be lowest at the crest; and (3) recruitment of juvenile oysters was lowest in 2000 across all elevations. Densities were greatest in 1999 and intermediate in 2001 at both the reef flank and the base, but no significant differences were detected in densities in these years at the reef crest. Densities of dead juvenile oysters (â‰¤30 mm) were significantly affected by reef elevation and date. These oysters were significantly more abundant at the reef base than at the flank and crest. Size frequency distributions reveal that for each of the 3 years, juvenile oysters numerically dominated all samples. Reef crest oysters had a unimodal population distribution each year. Flank and base reef strata exhibited bimodal size distributions in 2001. Juvenile oysters dominated all three strata throughout the sampling period, with mature oysters (>30 mm) rare. A greater proportion of mature oysters were collected from the reef base than from the reef flank, although the difference was small. Dead oysters (with articulated shells) were present at each elevation each year and, although fewer in number, tended to reflect the distribution of live oysters at each elevation. Dead oysters exhibiting evidence of predation by crabs were collected from each reef elevation but were proportionally more abundant at reef crest and flank compared with the reef base. ---Fisherman's Island--- The mean density of oysters at Fisherman's Island varied significantly according to tidal reef elevation, substrate type, and date. Subtidal oyster densities on the clamshell sustrate reef were greater than those densities at low intertidal reef elevations in 1997. During all other sampling events on both reef types, oyster densities exhibited the reverse pattern with greater oyster densities at the low intertidal reef elevation compared with those located subtidally. Densities of oysters increased over time at the subtidal elevation of the oyster shell reefs and at the low intertidal reef elevation of the clamshell reefs. This pattern was not evident at the low intertidal reef elevation on the oyster shell reef where the density of oysters was lowest in 1998. Oyster densities remained low throughout the study at the clamshell reefs' subtidal elevation. Oysters were consistently more abundant on the oyster shell than on the clamshell reef habitat. Overall abundance patterns on clamshell were similar to that found on the clamshell reef at Goodwin Island with a population dominated by small oysters and few oysters surviving to larger sizes (>30 mm). By May 1997, nearly 1 year after reef construction, oysters were notably more abundant on the oyster shell reef compared with the clamshell reef. By May 1998 and through 1999, the size distribution of oysters on the oyster shell reef was bimodal with relatively large numbers of large live oysters, whereas a weaker bimodal size distribution of small live oysters, likely masked by low recruitment, was found on clamshell. Recently, dead oysters with articulated shells were present on both reef types all years and tended to reflect the distribution of live oysters. There appeared to be increased survival on the oyster shell habitats because the ratio of live oyster to recently dead oyster abundance was greater on the oyster shell reefs than on the clamshell reefs each year.
Factors limiting recovery of the ecosystem:
Low recruitment on areas comprising small substrate components (the clamshell reefs at Fisherman's Island and the crest and flank of the Goodwin Islands reef) compared with reef areas of larger reef material fragments (Fisherman's Island's oyster shell reefs and the base of the Goodwin Island reef) may be attributed to differential substrate selection by oysters at the larval stage and post-settlement loss. The former, as evidenced by reduced initial settlement, could be a result of physical processes such as turbulence and flow. If settlement and metamorphosis success were unequal across substrate types, surviving oysters could be expected to be more abundant on reefs with favorable larval habitats, such as demonstrated by the patterns observed on the oyster shell of Fisherman's Island or larger clamshell fragments at Goodwin Island. Conversely, if oyster larval settlement and metamorphosis success were equal across all reefs, the patterns observed could be a result of differing post-settlement mortality pressure on different reef types. If this post-settlement mortality was the result of predation, then different reef substrates may foster predator communities imposing different pressures on newly settled oysters. The matrix of oyster shell reef substrate, having larger interstitial spaces compared with clamshell (O'Beirn et al. 2000), could be more accessible to fish and decapod predators. Although these larger predators may not prey directly upon the small, new recruits, they may feed upon smaller, intermediate predators of oysters, making the reef matrix a predation refuge for young oysters (McDermott & Flower 1952; Grabowski 2004). Smaller interstitial spaces, such as those of the clamshell substrate, may be limiting to larger predators but accessible to small decapods (such as juvenile panopeid and portunid crabs) and flatworms. This reef type may serve as a structural refuge for these individuals, permitting grazing on newly settled oysters. No direct predation comparisons were made between clamshell and oyster shell reefs at Fisherman's Island during this study, hence, further examinations of interactions of newly settled oysters and their predators on different substrate materials are needed to elucidate the potential importance of habitat selection and predation processes in structuring these communities. In this study, the distribution of oysters with evidence of crab predation reflected the size distribution of clamshell fragments on the reef mound of the Goodwin Island reef. Proportionately fewer dead oysters (regardless of size) displaying typical evidence of crab predation (Eggleston 1990) were collected from the base of the reef where the substrate afforded adequate refuge for young oysters from predators. Physical disturbance, through the mechanical grinding of the substrate material by waves and currents, may partially explain distribution of live oysters and the physical structure of the subtidal reef at Goodwin Island. The structure of this reef is analogous to natural mature subtidal oyster reefs of the Gulf Coast exhibiting the "grit principle" (MacKenzie 1977; Gunter 1979). Reefs of this type form barren central ridges consisting of fine dead shell grit on the reef crest and live oysters are only found along the flanks and in deeper water. Constant motion of the crest substrate from effects of wind-generated waves and currents abrades sessile organisms and hinders oyster larvae development (Gunter 1979). Because few live oysters were collected at the reef crest, which was made up of small, unconsolidated broken shell fragments, such physical disturbance may partially explain the distribution of oysters observed on this reef.
Socio-Economic & Community Outcomes Achieved
Key Lessons Learned
Constructed reef design should account for local geophysical and biological conditions and provide shelter for oysters and associated fauna from such stressors as hypoxia, siltation, ice scour, and resident and transient predators. Materials used as reef substrate should provide adequate small-scale structural complexity with ample refugia for newly settled oysters to avoid predation, whether subtidally or intertidally. In fact, considerations of small-scale structural design, such as the availability of proper settlement substrate (i.e. adequate surface heterogeneity), may be more important to the success of reef restoration efforts in some settings than large-scale aspects of reef design, such as mounding vertical relief.
Given the limited supply of oyster shell for restoration, the results of this study should be used to reassess the types of material and reef construction configurations in future oyster reef restoration efforts. Rather than building an entire mound of one substrate material, a mixture of substrates may lead to improved restoration success. Less desirable substrate materials, such as small Surf clamshell fragments (this study) or gravel (Soniat et al. 1991), could be placed as a base within the core of the mound, and then covered by a veneer of a material comprising larger elements (such as oyster shell, if available, or whole clamshell valves), which offers greater habitat complexity. Because the settlement of oyster larvae is often restricted to the outer layer of substrate material on a reef base (Bartol & Mann 1999), a practical construction approach would be to limit preferred substrate materials to the areas available for settlement. This layer could then provide the small-scale surface structural complexity to the peripheral strata of the mound, providing ample convolutions or tortuosity and surface area to afford settlement surface and refuge for young oysters from predation and physical stress. Thus, the choice of an appropriate construction configuration and substrate type for use as a reef base can dictate success or failure of the developing reef assemblage.
Sources and Amounts of Funding
Funding was provided by grants from the U.S. Environmental Protection Agency (EPA’s) Aquatic Reef Habitat Program and the National Estuarine Research Reserve in Virginia Graduate Research Fellowship.
Nestlerode, J.A., M.W. Luckenbach, and F.X. O’Beirn. 2007. Settlement and survival of the oyster Crassostrea virginica on created oyster reef habitats in Chesapeake Bay. Restoration Ecology 15: 273-283.
Janet A. Nestlerode