15.0 Studies of Oyster Reefs in New Hanover County Tidal Creeks  

Martin Posey, Troy Alphin, Heather Harwell, Bethany Noller

I. Overview

The UNCW Benthic Ecology Lab has conducted several projects to evaluate the ecosystem health of New Hanover tidal creeks, with partial support from North Carolina Sea Grant, NC Fisheries Resource Grant Program, the UNCW Center for Marine Science, and the New Hanover County Tidal Creeks Program.  Among the major areas of emphasis for these studies are oyster communities and their ecosystem functions in New Hanover County tidal creeks.  In previous studies we have examined the effect of establishing oyster reefs in small tributary creeks on water column nutrients, suspended solids, chlorophyll a and aspects of faunal use (Nelson et.al. 2004; Alphin and Posey, unpublished data).  Ongoing projects include studies of reef complexity and patch reef size effects on oyster ecosystem function. As part of a project partially supported by the New Hanover Tidal Creeks Program, we are also currently evaluating the percent cover and total cover of oysters in lower Pages, Hewletts, Howe and Whiskey Creeks. We have recently begun using archived digital images to evaluate current and historic oyster coverage among the tidal creeks (images provided by the New Hanover County Planning office), coupled with field assessment of oyster reef health based on morphological characteristics of the oyster reefs themselves. This report presents information from several years of field sampling concentrating on aspects of reef structural complexity, uniformity and oyster density that may influence their role as habitat and ecosystem function. Physical characteristics of the reefs may play an important role in determining the degree to which oysters can influence water quality and maintain habitat function, although these two functions may not always be complementary.  Characteristics of oyster reef morphology may provide a better metric to evaluate ecosystem health than simple measurements of reef coverage.

II. Background

Oyster populations along the east coast have been experiencing sharp declines throughout much of their range (Hargis 1999, Breitberg et al. 2000, Mann 2000).  This decline has been blamed on three primary factors; overfishing, disease, and loss of habitat (due in part to increases in coastal development).  As this decline has progressed over the last four decades, researchers and managers have recognized the importance of oysters as habitat for other fisheries species and for intrinsic ecosystem functions beyond their role as a harvestable resource. However, researchers and resource managers have only recently begun to understand how important oyster reefs are as a critical habitat. Oysters provide refuge and forage area for certain decapods and fish, and have broad ecosystem effects through their filtration of the overlying water (Ulanowicz and Tuttle 1992, Coen et al. 1999b).  In general, oyster reefs are a highly productive feature of the estuary that provide a greater value to the local communities than the simple market value of the harvested oysters. In the Chesapeake Bay and Pamlico Sound systems, subtidal oyster beds have historically covered a significant proportion of the bottom (Newell 1988, Hargis 1999, Lenihan 1999, Mann 2000) and are recognized as an essential fisheries habitat in the Chesapeake Bay (Coen et al. 1999b). Intertidal oyster beds are also present in portions of the Pamlico system and lower Chesapeake Bay (O’Beirn et al 2000).  From southern Pamlico Sound southward through the southeastern United States and into the Gulf coast of Florida, oysters are abundant intertidally and into the shallow subtidal (Kennedy and Sanford 1999) and may cover greater than 50% of the mid intertidal in some coastal creeks and sounds (Powell et al. 1995, Posey et. al. 1999, Grizzle and Castagna 2000, Meyer and Townsend 2000).  Because of a lack of seagrass beds from southeastern North Carolina to northern Florida, oysters represent the dominant structural habitat in the mid intertidal to shallow subtidal regions along those coasts.

The potential ecosystem and economic importance of oyster reefs as habitat directly relates to effects on smaller prey species as well as indirect and direct effects on larger fish and decapods that may be intermediate or top predators within these smaller estuarine systems. The presence of a structural refuge has been shown to reduce predation on smaller fish and invertebrates and is often associated with higher faunal abundances (Peterson 1979). The shell matrix of oyster reefs provides refuge habitat for species living on the sediment surface or among the shells, including several species of fish, crabs, shrimp and other small crustaceans (Larsen 1985, Castel et al. 1989, Meyer 1994, Breitburg 1999, Posey et al. 1999, Coen et al. 1999b). The shells also provide hard substrate for the attachment of species such as sponges. Elevated densities of panaeid shrimp, grass shrimp, xanthid and blue crabs, and bottom-oriented fish have been noted in some reefs (Wilber and Herrnkind 1986, Meyer 1994, Breitburg et al. 1995, Eggleston et al. 1998, Coen and Luckenbach 2000, Harding and Mann 2000). Effects on species burrowing into the sediment are more variable. Some studies have shown enhanced infaunal abundance or biomass within or adjacent to oyster reefs (Castel et al 1989, Larsen 1985), while other studies have indicated lower infaunal abundances under certain conditions (Powell 1994, Iribarne 1996), possibly reflecting indirect trophic effects. Consequences of enhanced invertebrate populations for commercially and recreationally important species may be simple direct effects of increased available food resources (Luckenbach et al. 2000) or more complex indirect effects as sources of larvae that enhance prey recruitment to adjacent areas. Oyster reefs may serve as protected source habitats helping to support prey numbers in open sandflats where predatory fish and crabs have greater access. Enhancement of epifauna and infauna may not only occur from increased refuge, but may also result from greater food availability. Oysters remove particulates from the overlying water and deposit material as feces or psuedofeces that tend to be high in organic content and are likely associated with locally enhanced nitrogen levels (Dame and Libes 1993). Locally enhanced nutrients or organics may stimulate bacterial and benthic microalgal production as well as other deposit feeder resources.

Subtidal reefs may serve as permanent forage sites for species such as striped bass, weakfish and bluefish (Harding and Mann 1999). Many commercially important species, such as blue crabs, panaeid shrimp, striped bass, sheepshead, and flounder, utilize intertidal oyster reefs as transients, coming and going with the tide (Posey et al. 1999, Coen et al. 1999b). For species not resident on oyster reefs, these habitats may provide important ephemeral foraging areas or refuges, with many fish and decapods moving into shallow water to feed as the water covers the tideflat. Intertidal reefs in the Chesapeake Bay and South Carolina are characterized by higher densities of transient fish such as pinfish, bluefish, blue crabs and seabass compared to adjacent open areas (O’Beirn et al. 2000, Coen et al. 1999a).  Diver observations of a tideflat region in southeastern North Carolina indicated greater densities of fish moving onto the edge of oyster reefs and feeding along the edge of those reefs compared to adjacent unstructured tideflat areas (Powell 1994, Posey et al. 1999).

The habitat value of oyster reef patches lies not only in the use of single reef patches, but also in their potential role as part of a series of habitats in a larger system. When combined with other structural habitats and/or present in a series of patches, subtidal reefs may enhance movement of fish across estuarine or sound systems (Breitburg et al. 2000). In such instances, a given individual may utilize a specific oyster patch for only a short period, but may move between oyster patches or among oyster, seagrass, marsh or debris patches on a regular basis utilizing different food resources.  On a smaller scale, a combination of oyster and seagrass patches was shown to enhance the foraging extent and possibly increase the accessibility of clam prey resources for blue crabs foraging in a North Carolina system (Micheli and Peterson 1999). In this case, the location of several habitat types in proximity was associated with greater foraging extent, probably related to greater protection from bird predators. Similarly, oyster reefs may provide refuge for only certain life stages, but their presence may have important population implications (Ray 1997) and restoration of reefs in some areas may augment juvenile fish production (Grabowski et al. 2000).

Aside from their role as habitat, oysters may also have important ecosystem functions through their high filtering capability. Their high filtration rates and ability to remove particulates has led to the suggestion that oyster reefs may significantly impact water quality, at least at historically high densities (Ulanowicz and Tuttle 1992, Dame and Libes 1993, Mann 2000). Some researchers have suggested that oysters may have significantly reduced suspended particulates, water column chlorophyll and water column nutrients before reefs were decimated by disease and over harvesting. Recent efforts have concentrated on re-establishing oyster reefs in areas where they have been historically present (and where anthropogenic pressures have now lessened). Several state and local governments have begun oyster restoration programs with the specific objective of improving coastal water quality.

Although there is growing data indicating the potential ecosystem and fisheries habitat roles of oyster reefs, the function of oyster reefs appears to vary with landscape characteristics. Critical characteristics of oyster reefs that may affect their habitat functions include shell cover within a reef, vertical complexity within the bed, vertical relief, and edge characteristics (Breitburg 1999, Lenihan 1999, Griffitt et al. 1999, O’Biern et al. 2000). Greater vertical complexity (presence of a mix of high relief and low relief areas) and vertical relief may provide greater quality habitat for fish and decapods utilizing the reef as refuge. Greater density of live oyster, larger oysters and greater vertical relief are among the reef architecture factors that may enhance water quality effects (enhance removal of material from the water flowing over a reef). In this project, we assessed the following reef characteristics on a per reef basis in Pages, Howe, Hewletts and Whiskey Creeks: % shell cover within a reef (relates to complexity of the shell habitat), occurrence of shell hash (habitat complexity), density of live oysters, and vertical relief of shells (maximum height above the underlying substrate). Understanding how these characteristics of oyster reefs vary among the New Hanover County tidal creek systems is critical for assessing current health of oysters in these systems as well as for management planning related to oyster restoration.

III. Methods

Two to three intertidal oyster reefs randomly selected in the lower portion of Pages, Howe, Hewletts and Whiskey Creeks were sampled in 2001 and 2003. All reefs selected had a diameter of at least 3m (but not more that 6m) and were separated from sources of disturbance such as channels by at least 10m. Regions of the creeks with active marinas were also avoided.

Percent shell cover, presence of shell hash, and live oyster density were measured in replicate 30 cm x 30 cm quadrats. Ten randomly placed quadrat samples were taken within each of the selected reefs. All quadrat samples were at least 0.5m within the reef to reduce edge effects.  Percent oyster cover was determined by the point intercept method. Five monofilament lines were strung at right angles over the quadrat creating 25 intersecting points. The presence or absence of shell under each point was then noted. Shell hash was defined as broken shell matrix underlying live shell or shell culms. Information on the presence and type (broken shell vs 3 dimensional culms) of shell hash may be important as habitat for cryptic fish, crabs and shrimp. A quadrat was recorded as having shell hash if there was greater than 10% of the underlying area covered. The number of live oysters in each of these quadrats was also recorded. Culms were not destroyed (broken apart) in this process, so there is some possibility that the complexity of culm structure may have led to some under representation of new recruits.

During the 2001 sampling period vertical relief of the oyster bed was defined as the height of shells (culms) above the underlying sand substrate. Ten 50 cm X 50 cm quadrats were selected on each reef. Each quadrat was divided into 25 equal squares creating 16 intercept points defined by the intersection of four equally spaced lines set perpendicular to each other.  The vertical height of shell underlying each of the 16 points was measured from the substrate surface to the highest point within 1 cm diameter of the intersection point.  During subsequent sampling events in 2003, vertical complexity of oyster reefs was measured using the chain method (a ratio of the straight line distance to vertical contour).  Ratios approaching zero indicate extreme complexity while ratios approaching 1 indicated virtually no complexity.  Vertical complexity values for this region are commonly in the 0.6-0.7 range, whereas values of 0.9 are low.  The chain method gives a better estimation of the amount of vertical complexity within the reef structure and has been applied in other reef habitats (e.g. coral reefs).  In addition to the chain measurement, absolute height of the oyster culms was also measured in 2003.  This data was collected by measuring from the substrate surface to the highest point on the oyster culms.  Qualitative data on the presence of algal cover (if algal growth covered greater than 25% of the total area of the reef, it may negatively impact the development of healthy oyster reefs), sedimentation (sediment coverage of the > 10% of the total reef area), percent shell hash coverage, and percent open area (percent cover determined as defined above). 

IV. Results

Here we present data from both 2001 and 2003.  During the 2001 sampling year we collected data from 4 of the tidal creeks (Pages, Howe, Hewletts, and Whiskey) as a broad survey of the tidal creeks within the county.  During 2003 we focused on collecting data from two target creeks (Hewletts and Pages) that were the subject of other studies evaluating utilization of oyster reefs based on physical characteristics. During 2001, percent shell cover varied among creeks. Reefs within Pages Creek had the highest cover of shell, with 92.5% of the reef surface area having shell (Figure 1). Reefs within Whiskey Creek were more variable, having large amounts of open sand patches within the reefs. Whiskey Creek oyster reefs had only 52.9% actual shell cover within the definable reef areas. Both Howe and Hewletts Creeks had intermediate levels of shell cover, with 69.3% of the reef area actually covered by shell in Howe Creek and 75.8% shell cover for reefs in Hewletts Creek (Figure 1). Shell hash presence did not strictly correlate with overall shell cover. Greatest shell hash was observed in Hewletts Creek, with 50% of quadrats having 10% or more shell hash present. This was followed by Howe (40%), Pages (30%), and Whiskey (21%) Creeks (Figure 1).  During 2003 percent shell cover at Hewletts and Pages Creeks showed a disconcerting trend towards loss of well-developed structure, with shell cover comprised primarily of shell harsh with very few 3 dimensional culms (Figure 1).

During 2001, densities of live oysters were greatest in Pages and Hewletts Creeks (Figure 2) and least in Howe and Whiskey Creeks. The densities for Pages and Hewletts Creeks were low but comparable to that observed in other southeastern North Carolina marine intertidal areas (T. Alphin and M. Posey, personal observation).  The densities in Whiskey and Howe Creeks were lower than we have observed in other areas of Masonboro Sound and areas between Pages Creek and the New River (Alphin and Posey, personal observation). These low densities may reflect low recruitment and/or high mortality. However, in a preliminary study conducted in 1995, we observed high mortality of oyster spat transplanted to Howe and Hewletts Creeks relative to Pages Creek (unpublished data). Pages Creek was also characterized by high variability in density of live oyster among quadrats.  Evaluation of live oyster density in Hewletts and Pages during 2003 indicated a decline since 2001 (Figure 2).  This decline in live oyster density coincides with a decline in the amount of 3 dimensional oyster culms (Figure 1), such that most of the shell coverage for this sampling period was nearly all broken shell hash.

Assessment of vertical height of oyster reefs conducted in 2001 was greatest in Howe Creek (Figure 3) and intermediate in Pages Creek. Average vertical relief was low in both Hewletts and Whiskey Creeks. Vertical height presented here is the greatest height of oysters above the substrate and not average reef height overall. As such, it is a measure of the presence of well-developed oyster culms. 

In 2003 assessments of oyster reefs for both Pages and Hewletts Creeks were conducted in much the same way as earlier assessments.  However measurements of vertical relief using the chain method were collected in addition to measurements of absolute height.  The chain method provides a better estimate of vertical complexity and so allows for a better understanding of the habitat quality of the oyster reefs in question.   In addition to the relief measurements we also collected qualitative data on coverage (% open mud, % actual shell hash), sedimentation, and presence of significant algal cover (>25% of the total reef area).  All of which are factors that may impact the overall function of the oyster habitat and will allow inferences as to whether the reefs are in a state of growth or decline. 

Measurements of vertical complexity in 2003 show that reefs in both Pages and Hewletts Creeks seem to have similar vertical profiles with average complexity measurements in Hewletts Creek ranging from 0.71 to 0.85 (Table 1) and Pages Creek ranging from 0.52 to 0.77 (Table 2).  (Note that a complexity value of 1 would indicate the reef had no vertical relief).  There was high variability in the amount of shell hash present among the various reefs for both creeks, although algal cover did not seem to be a significant problem and sedimentation was only noted at two reefs in Hewletts Creek (Table 1 and Table 2)

Table 1. Shell cover, oyster density and physical parameters of oyster reefs in Hewletts Creek during 2003.  

Table 2.  Shell cover, oyster density and physical parameters of oyster reefs in Pages Creek during 2003.

Conclusions  

These results suggest considerable variability in oyster reef characteristics among the New Hanover tidal creeks examined.  In 2001 Pages Creek was described as having “high shell coverage, relatively high densities of live oysters (compared to other creeks examined), and intermediate vertical relief”, while Hewletts Creek was defined as intermediate among the four creeks overall with “relatively high live oyster densities, but low relief”.  As noted previously live oyster densities in Pages and Hewletts Creeks were greater than measurements in Howe or Whiskey Creeks during 2001 but declined in the 2003 measurements.  This is a source of some concern because we also recorded an increase in the amount of shell hash coverage compared to total oyster shell coverage (culms + shell hash) (Figure 1).  Our concern here is that this change may reflect a decline in the oyster populations within these areas.  Data from an associated project looking at settlement within the Hewletts Creek system (during late 2002 and early 2003) shows that larvae did recruit to that system, suggesting that factors other than larval supply may be primarily responsible for this pattern.  Measurements of reef complexity were moderate for both Pages and Hewletts Creeks (Tables 1-2) indicating the presence of some high relief culms.  Although these same data showed that the amount of open space within reefs at these two creeks was variable, with some reefs having >50% of the area within the reef as open space.  These data would seem to indicate that reefs in these two systems demonstrate a high degree of variability among years and give some indication of the dynamic nature of oyster reef formation.  In many cases the most complex oyster habitats may be those that provide both oyster structure and open patches within the reef proper.  The question that we must now focus on is at what point do oyster reefs provide the greatest benefits to other organisms (as habitat, refuge, and as system modifiers) while still providing the aggregate needs for healthy oysters and good settlement structure for larvae to maintain the reef integrity.

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