P.G. Ross, M.W. Luckenbach and A.J. Birch Characterization of an Oyster Population in an Urban Landscape: Comparisons of Man-made and Natural Habitats The College of WILLIAM & MARY P.G. Ross, M.W. Luckenbach and A.J. Birch Eastern Shore Laboratory, Virginia Institute of Marine Science, College of William and Mary Introduction Traditional oyster habitat in the lower Chesapeake Bay is mainly comprised of subtidal (and to a lesser extent intertidal) biogenic reefs. With increasing coastal development, manmade habitats used to armor shorelines (e.g. bulkheads and granite or concrete rubble) are becoming prevalent in some areas. These intertidal structures are often colonized by oysters creating “non-traditional” oyster habitats. Such habitats are found in the urban landscape of the Lynnhaven River basin, a small and relatively closed tidal sub-tributary of the lower Chesapeake Bay. We characterized the oyster population in this system in terms of overall abundance, density and size distribution for different habitat categories. Differences in oyster density and size structure were observed across habitat types. While we do not suggest that shoreline armoring is ecologically advantageous nor a prudent restoration technique, we believe comparisons of these habitats may be helpful for improving restoration reef design, especially with respect to alternatives to shell as a reef substrate. FIGURE 2. Top row, an urban coastal landscape Middle row (l to r), fringing reef, marsh, & patch reefs Bottom row, riprap on the shoreline (note zonation) Results Despite the Lynnhaven’s urban landscape and predominance of man-made structures, marshes actually dominate the shoreline and intertidal patch reefs dominate the shell-type reef habitats (Table 1). Nevertheless, there are substantial amounts of manmade shoreline structures (Table 1). Approximately, 17.8 million oysters inhabit this area. We estimate that 57% (9.8 million) and 32% (5.7 million) of these oysters are found on intertidal shell patch reefs and marsh, respectively. Approximately 6% (1.1 million) live on riprap and bulkhead structures (Table 2). Riprap and intertidal patch reefs had higher oyster densities relative to bulkhead and marsh, with fringe and subtidal patch reefs being intermediate (Figure 5). A similar pattern was observed in terms of dry tissue biomass, with the exception of fringe reefs which tended to have larger oysters (Figure 6). When shell reefs and riprap were examined in more detail (i.e. dividing patch reefs into subcategories), there appeared to be a trend of lower oyster density as habitats were more subtidal in nature (Figure 7). Oyster population size structure indicate the presence of several age classes for most habitats. However, there appeared to be less recently settled oysters (<20 mm) on manmade structures (Figure 8). Table 1. Extent of oyster habitats mapped in the Lynnhaven River basin. Habitat Type Extent Relative Prop. (%) Marsh 152,419 m 78.4 Bulkhead 21,735 m 11.2 Riprap 11,403 m 5.9 Sand 8,685 m 4.5 Other 283 m 0.1 Intertidal Patcha 41,323 m2 84.2 Subtidal Patch 4,500 m2 9.2 Fringing 3,277 m2 6.7 a Some patch reefs categorized as intertidal also have a subtidal component Table 2. Estimated oyster abundance by habitat. Habitat Type Overall Abundance Intertidal Patch Reefs 9.84 x 106 Marsh 5.71 x 106 Riprap 0.86 x 106 Fringe Reefs 0.68 x 106 Subtidal Patch Reefs 0.37 x 106 Bulkhead 0.28 x 106 Sand 0.01 x 106 Other Total 1.78 x 107 Reefs Shoreline Features Natural Intertidal Patch Reefs Riprap Bulkhead Marsh Shell Height (mm) % of Oysters FIGURE 8. Size frequency distribution (%) of oysters based on shell height (plotted in 5 mm bins) for several habitats. A A,B,C C B GLM, p<0.0001 # Live Oysters · m-2 Habitat Type FIGURE 5. Mean (+ SE) oyster density for habitats. Means with different letters are significantly different (p<0.01). Methods Mapping – Sub-meter accuracy surveying Global Positioning System (GPS) data integrated with an ArcView-based Geographic Information System (GIS) were utilized to precisely map potential oyster habitat along emergent shorelines and intertidal flats (e.g. Figure 3). Oyster Sampling –Individual features within each habitat type were randomly selected and sampled using standard quadrate techniques (Figure 4). All oysters within quadrates were counted and shell height (longest lip-hinge distance) of the first 50 individuals encountered was measured to the nearest mm. Density and size distribution data were delineated by habitat type across all regions of the Lynnhaven Basin; including the densely populated areas near the confluence of the various basin branches as well as the much less densely populated areas further upstream. Only features with >1% oyster coverage were used in this analysis. Statistics – Statistical comparisons for Figures 5 & 7 utilized ANOVA (Proc GLM in SAS). Differences were considered significant when p<0.01 and means were compared a posteriori using Tukey’s multiple comparison. Study Area The Lynnhaven River Basin (Figure 1) is characterized by environmental gradients, such as salinity (polyhaline to mesohaline) and tidal range (15 to 75 cm), from the Lynnhaven Inlet to upstream portions of the basin. The portion of the Lynnhaven Basin relevant to data presented here is comprised of approximately 52 km2 of surface water and over 200 km of shoreline. The Lynnhaven River Basin currently has a surrounding population of ~500,000 people and the study area contains a diversity of natural and man-made habitats, including Spartina marsh, two dimensional patch reefs, granite riprap and bulkheads made of various materials (Figure 2). Oyster Biomass, g · m-2 Habitat Type FIGURE 6. Oyster density in terms of biomass (dry tissue wt.) based on habitat-specific size distributions and shell height-biomass relationships (i.e. best-fit power functions). FIGURE 3. Example of shoreline oyster habitat features overlaid on high resolution aerial images (2002) using ArcGIS. Discussion & Conclusions Intertidal patch reefs and rip-rap support the highest densities of oysters within the Lynnhaven River; owing to their greater aerial extent the intertidal patch reefs contain the majority (55%) of the oysters within the system. Fringing reefs, which are largely intertidal and often border marsh edges, along with subtidal patch reefs support intermediate densities and total abundances of oysters within the system. Lower densities were observed on bulkheads and within marsh habitats; however, because of the large extent of marshes within the basin this habitat supports a large fraction (32%) of the oyster population. A trend of increasing oyster density with intertidal exposure was observed among the hard substrate habitats (i.e., marsh excluded, Figure 7). Throughout the Lynnhaven Basin, the oyster population tends to inhabit the intertidal zone with distinct zonation at the low intertidal/subtidal interface. While several parameters may contribute to this pattern, predation is likely an important factor. Our observation of greater biomass density in intertidal patch reef, rip-rap and fringing reef habitats suggest that these habitats may be particularly important in supporting the oyster population within the region. The lower relative abundance of oysters <25 mm for riprap and bulkhead may reflect recruitment or survival differences that have important consequences for the population. In the case of riprap, however, this may reflect sampling error due to difficulties enumerating small oysters in the interstices of complex riprap structures. State and federal agencies, along with several NGO’s, are pursuing further oyster restoration projects in the Lynnhaven River with the goals of increasing both the abundance and biomass of oysters within the system. Our results provide some guidance for that work. First, structurally complex hard substrate habitats, such as natural oyster reefs and rip-rap support greater densities and biomass of oysters than solid shorelines (bulkheads). Second, oyster abundance and size within the intertidal zone is greater than in subtidal habitats, presumably reflecting decreased predation related mortality. Third, rip-rap, properly located within the intertidal zone, can provide structurally complex habitats that support comparable densities of oysters to natural intertidal patch reefs. Finally, with limited area available for the construction of new intertidal patch reefs within this system, the extensive shoreline still bordered by marsh offers the opportunity for the construction of intertidal fringe reefs which can serve the multiple purposes of creating oyster habitat and protecting marshes in a “Living Shoreline” setting. FIGURE 1 Left: Chesapeake Bay region on the mid-Atlantic seaboard of USA (inset) Right: Lynnhaven River Basin surrounded by the City of Virginia Beach (2002 1-m high resolution aerial imagery). Virginia Del-Mar-VA Peninsula James River York River Potomac River Source: Univ. of Alabama USA Western Branch Eastern Branch Broad Bay Chesapeake Bay/Atlantic Ocean Lynnhaven Inlet First Landing State Park City of Virginia Beach Pop. ~ 0.5 million A A,B B,C C GLM, p<0.0001 # Live Oysters · m-2 Habitat Type FIGURE 7. Mean (+ SE) oyster density for several types of patch reefs (especially restoration reefs) and shoreline riprap. Means with different letters are significantly different (p<0.01). FIGURE 4 Quadrate samples: (Clockwise From Top Left) intertidal patch reef, fringe reef, and bulkhead. Project Partners NOAA - Chesapeake Bay Office Virginia Marine Resources Commission College of William and Mary - VIMS U.S. Army Corp of Engineers Chesapeake Bay Foundation Acknowledgements: We greatly appreciate the assistance provided by Edward Smith, Sean Fate, Peter Kingsley-Smith and Jamie Wheatley. This study was funded by: NOAA - Chesapeake Bay Office