Evaluating Ecosystem Services Provided By Oyster Reefs

OYSTER REEFS AS A FISHED COMMODITY

Consumer demand for oysters continues to promote wild oyster fisheries in the U.S. where populations are still viable. Increased oyster landings in the Gulf of Mexico have partly compensated for lost productivity throughout many historically productive regions such as Delaware Bay, the Chesapeake Bay, Pamlico Sound, and the south-Atlantic coast of the

U.S. Even though the precipitous decline in landings in the Chesapeake during the 1980s was partly buffered by increased oyster prices, the loss in dockside value after adjusting for inflation from 1980 to 2001 is estimated at 93% (National Research Council 2004). Given that catch per unit effort (CPUE) also decreased by 39% during this time period (National Research Council 2004) and fuel costs have increased, the erosion of profits experienced by the industry during the past couple of decades is even greater than 93%. This decline in CPUE largely reflects increased regulations restricting harvesting practices and decreased oyster biomass in the oyster habitat. For instance, Rothschild et al. (1994) divided the total harvest value by the estimated amount of total oyster bottom, which they estimated declined by 50% between 1890 and 1991, and determined that a century of overharvesting reduced the annual oyster yield in Maryland per unit oyster bottom from 550 g/m2 in 1890 to 22 g/m2 in 1991.

We calculated the commercial value of oysters per unit of reef area in two ways. First, we multiplied the oyster yields reported in Rothschild et al. (1994) by the dockside market price ($3.01/lb of oysters in 1991) in coastal Maryland. Overharvesting reduced the value of oyster yields from $36.45 per 10 m2 of oyster bottom in 1890 to $1.46 per 10 m2 in 1991 (both values are in 1991 dollars). This reduction in value may be slightly underestimated if it is partly a consequence of decreased harvesting effort rather than a decrease in the density of harvestable oysters per unit of reef. However, it is unlikely that this pattern is largely due to reduced effort given that increases in oyster prices over the past two decades would likely motivate greater harvesting effort, and fishery-independent sampling efforts have determined that the density of living oysters in the Chesapeake Bay is two to three orders of magnitude below historic levels (Rothschild et al. 1994).

Second, we calculated the harvest potential using data from oyster reef projects in which oyster reefs were restored in coastal North Carolina. Using data on oyster densities of legally harvestable sizes in the Neuse River Estuary (Lenihan and Grabowski 1998, Lenihan and Peterson 2004), we estimated that subtidal reefs in this region contain 0.6-1.6 bushels of oysters per 10 m2 worth $12.80-$32.00. The value of oysters on these reefs is roughly comparable to historic yields in Maryland a century ago and approximately an order of magnitude greater than the present oyster fishery production from reefs in Maryland. This difference is largely a consequence of experimental reefs in North Carolina having not been subjected to continual harvest because they were designated as reef sanctuaries. Furthermore, these results suggest that harvesting by traditional methods such as dredging and hand-tonging would likely result in rapidly decreasing oyster yields in subsequent years if harvesting were initiated on these reefs in North Carolina (Lenihan and Peterson 2004).

BEYOND OYSTERS: VALUING ADDITIONAL ECOSYSTEM GOODS AND SERVICES PROVIDED BY OYSTER REEFS

Although the value of oyster landings throughout the eastern U.S. has been recorded since the nineteenth century, economic evaluations of the additional ecosystem services provided by oyster reefs are limited. This paucity of economic data is partly a reflection of viewing oysters narrowly as a fishery resource to exploit rather than holistically as an ecosystem engineer that should be managed as a provider of a multitude of goods and services. Of further concern, a century of overharvesting has left most if not all ecosystems across the coastal U.S. with two to three orders of magnitude fewer oysters (Frankenberg 1995, Heral et al. 1990, Rothschild et al. 1994), and existing oyster reefs may be so degraded that they do not perform the same services as intact historic reefs (Dame et al. 2002, Jackson et al. 2001, Lenihan and Peterson 1998, Newell 1988). However, scientists have utilized oyster reef restoration over the past two to three decades to investigate how oyster reefs function (Coen et al. 1999, Dame et al. 1984, Grabowski et al. 2005, Harding and Mann 1999, Lenihan et al. 2001, Meyer et al. 1996, Newell et al. 2002, Peterson et al. 2003, Zimmerman et al. 1989). This information has assisted managers not only in shifting from an exploitable-resource to a valued-habitat view of oyster reefs but also in enhancing the ability to recover goods and services through oyster reef restoration. Incorporation of ecological data into economic models that integrate the value of each of these services will further managers' capacity to decide among management options and alternative restoration designs in order to maximize the value created by restored reefs.

Oysters as a biofilter

Oysters are filter feeders that feed upon suspended particles in the water column, pumping such a high rate of water flow that they are considered an important biofilter that helps maintain system functioning (Baird and Ulanowicz 1989, Grizzle et al. 2006, Newell 1988). The decline of oyster populations in estuaries along the eastern U.S. has coincided with increased external nutrient loading into these coastal systems (Paerl et al. 1998). Collectively these ecosystem perturbations have increased bottom-water hypoxia and resulted in restructured food webs dominated by phytoplankton, microbes, and pelagic consumers that include many nuisance species rather than benthic communities supporting higher-level consumer species of commercial and recreational value (Breitburg 1992, Jackson et al. 2001, Lenihan and Peterson 1998, Paerl et al. 1998, Ulanowicz and Tuttle 1992).

Perhaps one of the most compelling examples of the consequences of loss of filtration capacity is Newell's (1988) estimate that oyster populations in the Chesapeake Bay in the late 1800s were large enough to filter a volume of water equal to that of the entire Bay every 3.3 days, whereas reduced populations currently in the Bay would take 325 days. Two other examples of the filtration capacity of bivalves include the introductions of the clam (Potamocorbula amurensis) in San Francisco Bay and zebra mussels (Dreissena polymorpha) in the Great Lakes, which have demonstrated how dramatically suspension feeding by bivalves can remove suspended solids and nutrients from the water column (Alpine and Cloern 1992, Carlton 1999, Klerks et al. 1996, MacIsaac 1996). Although the decline in oyster populations undoubtedly contributed to the decline in water quality in the Chesapeake over the past century, the application of quantified changes in water quality as a consequence of small-scale restoration studies to larger-scale, estuarine-wide management of water quality presents some significant challenges.

Experimental manipulation of oyster populations has demonstrated that oysters can influence water quality by reducing phytoplankton biomass, microbial biomass, nutrient loading, and suspended solids in the water column. Other potential water quality benefits could result by concentrating these materials as pseudofeces in the sediments, stimulating sediment denitrification, and producing microphytobenthos (Dame et al. 1989). For example, Porter et al. (2004) manipulated the presence of oysters in 1000-l tanks and found that oysters increased light penetration through the water column by shifting algal production from phytoplankton to microphytobenthos-dominated communities. Micro-phytobenthos biomass subsequently reduced nutrient regeneration from the sediments to the water column. Cressman et al. (2003) determined that oysters in North Carolina decreased chlorophyll a levels in the water column by 10-25% and fecal coliform levels by as much as 45% during the summer. Grizzle et al. (2006) developed a method to measure seston in situ and subsequently demonstrated that this method more precisely identifies differences in seston than traditional techniques conducted in a laboratory, suggesting that studies relying upon laboratory analyses may underestimate the effects of oysters on seston. Nelson et al. (2004) transplanted oyster beds in small tributaries in coastal North Carolina and noted that some small reefs reduced total suspended solids and chlorophyll a levels. Laboratory studies also have found that bivalves influence local plankton dynamics and reduce turbidity levels (Prins et al. 1995, 1998). On the other hand, Dame et al. (2002) removed oysters from four of eight creeks in South Carolina and noted that the presence of oyster reefs explained little of the variability in chlorophyll a, nitrate, nitrite, ammonium, and phosphorous. In general, bivalve control of phy-toplankton biomass is thought to be most effective when bivalve biomass is high and water depth is shallow (Officer et al. 1982), so that small-scale restoration efforts or restorations in deeper water may not necessarily achieve detectable gains in water quality.

Attributing a single value per unit of oyster reef restored may be inappropriate if the relationship between the spatial extent of oyster reef habitat and water quality is nonlinear (Dame et al. 2002). For instance, in some estuaries large-scale restoration efforts may be necessary before water quality is measurably improved because nutrient and suspended solid loading rates currently far surpass the filtration capacity of the oyster populations present in these bays. Future research efforts that provide empirical data on this functional relationship will greatly benefit attempts to model the economic services provided by oyster reefs. Because these inherent difficulties exist in generalizing the economic benefits associated with oyster filtration, one alternative approach would be to quantify the cost of providing a substitute for this service. As a natural biofilter that removes suspended solids and lowers turbidity, oyster reefs are analogous to wastewater treatment facilities. Thus the filtration rate of an individual unit of oyster reef can be quantified and compared to the cost of processing a similar amount of suspended solids and nutrients with a waste treatment facility.

Larger-scale restoration efforts within shallow coastal embayments designed to achieve improvements in water quality could have substantial indirect benefits of great economic value. First, by decreasing water turbidity (i.e., by filtering suspended solids) and suppressing nutrient runoff, oyster reefs can promote the recovery of SAV in polluted estuaries (Peterson and Lipcius 2003). Newell and Koch (2004) modeled the effects of oyster populations on turbidity levels and found that oysters, even at relatively low biomass levels (i.e., 25 g dry tissue weight m-2), were capable of reducing suspended sediment concentrations locally by nearly an order of magnitude. This reduction would result in increased water clarity that would potentially have profound effects on the extent of SAV in estuaries such as the Chesapeake Bay.

Recognized as extremely important nursery grounds for many coastal fish species (Thayer et al. 1978), vegetated habitats such as SAV have been reduced in estuaries such as the Chesapeake Bay by agricultural runoff, soil erosion, metropolitan sewage effluent, and resultant N loading from all of these sources as well as atmospheric deposition. Nitrogen loading at levels of 30 kg N ha-1 yr-1 within Waquoit Bay, Massachusetts, resulted in the loss of 80 to 96% of the total extent of seagrass beds, and seagrass beds were completely absent in embayments with loading rates that doubled this amount (Hauxwell et al. 2003). They also found that nitrogen loading increased growth rates and standing stocks of phytoplank-ton, which likely caused severe light limitation to SAV by reducing light penetration through the water. Kahn and Kemp (1985) created a bioeco-nomic model to estimate damage functions for commercial and recreational fisheries associated with the loss of SAV in the Chesapeake Bay and determined that a 20% reduction in total SAV in the Bay results in a loss of 1-4 million dollars annually in fishery value. If improvements in water quality from oyster reef habitat increase the amount of SAV in the estuary, then the value of augmented fishery resources created by this additional SAV should be attributed to oyster reefs.

Second, improvements in water quality in general are valued by the general public who use estuarine habitats for activities such as swimming, boating, and sportsfishing. For instance, Bockstael et al. (1988, 1989) surveyed residents in the Baltimore-Washington area in 1984 and determined that their annual aggregate willingness to pay in increased taxes for moderate (i.e., ~20%) improvements in water quality (i.e., decreased nitrogen and phosphorous loading and increased sportsfish-ing catches) was over $100 million. The National Research Council (2004) used the consumer price index to adjust estimates reported in the preceding studies to 2002 price levels and reported that a 20% improvement in water quality along the western shore of Maryland relative to conditions in 1980 is worth $188 million for shore beach users, $26 million for recreational boaters, and $8 million for striped bass sportsfishermen. Although there are several potential sources of error in these estimates, they may be underestimated given that improvements in the Chesapeake Bay water quality will likely result in increased recreation in the Bay and these analyses did not include the value that U.S. residents outside of the Baltimore-Washington area place on Bay resources despite the nation-wide recognition and utilization of the Chesapeake Bay (Bockstael et al. 1988). Evaluation of ecosystem services provided by oyster reefs should include assessment of this suite of benefits if larger-scale oyster restoration efforts achieve measurable improvements in water quality in estuaries along the Atlantic and Gulf coasts of the U.S.

Oysters as habitat for fish

Several studies have used restoration efforts to assess the role of oyster reefs as critical habitat for commercially and recreationally important fish species (Coen et al. 1999, Grabowski et al. 2005, Harding and Mann 1999, Lenihan et al. 2001, Meyer et al. 1996, Peterson et al. 2003, Zimmerman et al. 1989). Our ability to quantify the value of fish provided per unit area of oyster reef will be dependent upon several factors, such as (1) whether oysters successfully and regularly recruit to the reef and create vertical relief that provides habitat for important prey species;

(2) the amount of existing oyster reef habitat already available locally;

(3) whether other habitats are functionally redundant to oyster reef habitat and consequently compensate for oyster reef degradation; (4) an oyster reef's location (or landscape setting) within the network of oyster reefs and other important estuarine habitats that already exist; and (5) the biogeographic region (or ecosystem) where it is located.

Given the context dependency of oyster reef community processes, assessments of economic benefits for commercial and recreational fisheries must incorporate knowledge of the life history and ecology of local fish species. For instance, while water quality improvements in the Chesapeake Bay would generate value for striped bass fishermen if catches increased, improvements in water quality in the estuaries of the southeast would not provide this particular value because striped bass do not extend south of Cape Lookout, North Carolina. Peterson et al. (2003) reviewed existing data on oyster reef restoration efforts from the southeast U.S. and determined that each 10 m2 plot of restored oyster reef habitat produces an additional 2.6 Kg yr-1 of production of fish and large mobile crustaceans for the functional lifetime of the reef. Because these efforts were focused on the southeast U.S., species that utilize oyster reef habitat located in other estuaries in the U.S. such as striped bass but are not indigenous to this region were excluded from the analyses. On the other hand, those species utilizing an oyster reef in the southeast U.S. clearly include ecological equivalents for species restricted to other bio-geographic regions, so the degree of enhancement of fish production may be similar.

Using 2001-2004 dockside landing values from the southeastern U.S. and Gulf of Mexico (National Marine Fisheries Service 2006), we converted the amount of augmented production per each of the 13 species groups that were augmented by oyster reef habitat in Peterson et al. (2003) to a commercial fish landing value (Table 15.2). We then calculated the streamline of cumulative benefits provided by a 10 m2 oyster reef for the functional lifetime of the reef (Figure 15.1). Future landings values were discounted at a rate of 3% to adjust for the opportunity cost of capital adjusted for inflation. Our estimates suggest that a 10 m2 reef that lasts 50 years would produce finfish valued at $98.06 in 2004 dollars, whereas harvesting this same reef for oysters destructively after 5 years would reduce this finfish value to $17.45 in 2004 dollars. Although a

TABLE 15.2 The commercial fisheries value of augmented fish created by oyster reef restoration in the southeast U.S.

Augmented Fish

Commercial

Augmented

Productiona

Fish Priceb

Fish Valuec

(Kg/10 m2)

(S/Kg)

(S/yr/10 m2)

Sheepshead Minnow

G.GGG

S -

S -

Bay Anchovy

G.G19

S -

S -

Silversides (3 spp.)

G.GG2

S -

S -

Gobies

G.644

S -

S -

Blennies

G.GSG

S -

S -

Sheepshead

G.S86

S 1.17

S G.69

Stone Crabd

G.6S3

S 6.7S

S G.88

Gray Snapper

G.114

S 3.43

S G.39

Toadfish

G.G22

S 4.9S

S G.11

Gag Grouper

G.293

S 4.82

S 1.41

Black Sea Bass

G.G46

S 2.9G

S G.13

Spottail Pinfish

G.GGS

S 1.14

S G.G1

Pigfish

G.13S

S G.6G

S G.G8

Total (S/yr/10 m2): $3.70

a Estimates of annual augmented fish and crustacean biomass produced per 10 m2 of restored oyster reef are from Peterson et al. (2003).

b Individual fish landing prices were derived from the National Marine Fisheries Service commercial landings online database (National Marine Fisheries Service 2006). c Augmented fish values were calculated by multiplying augmented fish production values by the commercial fish price for each species group.

d Because Peterson et al. (2003) estimated the total biomass of stone crabs but commercial landings price is derived from only the weight of the claws, we divided our fish value estimate by 5 (i.e., we estimated that the claws account for 20% of the total weight of stone crabs).

a Estimates of annual augmented fish and crustacean biomass produced per 10 m2 of restored oyster reef are from Peterson et al. (2003).

b Individual fish landing prices were derived from the National Marine Fisheries Service commercial landings online database (National Marine Fisheries Service 2006). c Augmented fish values were calculated by multiplying augmented fish production values by the commercial fish price for each species group.

d Because Peterson et al. (2003) estimated the total biomass of stone crabs but commercial landings price is derived from only the weight of the claws, we divided our fish value estimate by 5 (i.e., we estimated that the claws account for 20% of the total weight of stone crabs).

50-year life span may seem extremely long given that oyster diseases have hampered many recent restoration efforts, reefs historically persisted for centuries prior to mechanical harvesting began and some recent reef sanctuaries in North Carolina that are currently intact were constructed over 2 decades ago (Powers et al. unpublished data). This difference illustrates that how a restored reef is managed (i.e., whether destructive harvesting of oysters is permitted or if the reef is protected) will largely influence the streamline of ecosystem goods and services that it provides.

Comparing the value of augmented fish production with oyster harvests revealed that consideration of ecosystem services provided by oyster reefs more broadly could enhance the value derived from oyster reef habitat. Specifically, the value of oyster harvests from 10 m2 of reef

$- i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i—i 0 10 20 30 40 50 60 70 80 90 100

Functional life time of oyster reef (yr)

FIGURE 15.1 The long-term projection of cumulative value of enhanced fish and mobile crab production per 10 m2 of restored oyster reef habitat for various hypothetical alternative lifetimes of functionality of a restored reef. Values were discounted at an annual rate of 3% to adjust for the opportunity cost of capital. The functional lifetime of the reef is influenced by management of the reef (i.e., whether it is set aside as a sanctuary or destructively harvested), oyster recruitment levels, and the incidence of oyster diseases that can decimate living oyster populations prior to establishing the vertical relief and structural complexity associated with intact reef habitat.

habitat using our preceding estimates (in the section "Oyster Reefs as a Fished Commodity") of $36.45 in year 5 and $1.45 in each subsequent year discounted at a rate of 3% totals to $63.97 for a reef with a 50-year life span. This estimate is 34.8% less than the value of fish produced by a similar amount of reef habitat during this time span. Scaling the estimate of finfish value up to a 1-acre reef sanctuary that lasts 50 years would result in ~$40,000 in additional value from commercial finfish and crustacean fisheries. If this value can be extrapolated to entire estuaries, the total value of augmented fish production from oyster reef habitat far surpasses the economic value of oyster landings over the past decade in many estuaries throughout the eastern U.S. This estimate undoubtedly is subject to potential sources of error due to the unpredictability of future fishery resources. For instance, the provision of ecosystem goods and services by oyster reefs may be dependent upon the amount of oyster reef habitat already in the system, such that the marginal value of each unit of restored oyster reef may vary as more and more reefs are restored in the system. In particular, restoration of extensive amounts of reef habitat within some estuaries may result in reef-related species that are limited by factors other than habitat availability. However, this initial estimate may be conservative because the abundances of many fish and crustacean species that utilize oyster reef habitat also have been dramatically reduced from decades of overfishing, so that recent studies investigating fish use of restored reef habitat likely underestimated the potential abundance of these species. Enhancing our understanding of these processes to regional scales will be especially important if restoration efforts ever begin to approach historical levels of intact oyster bottom given that the amount of shell bottom in 1884 in just the Maryland portion of the Chesapeake Bay was estimated at 279,000 acres (Rothschild et al. 1994).

Other ecosystem services provided by oysters

Oysters create biogenic structure by growing in vertically upright clusters that provide habitat for a wide diversity of densely aggregated invertebrates (Bahr and Lanier 1981, Lenihan et al. 2001, Rothschild et al. 1994, Wells 1961). Although few of these species (i.e., mollusks other than oysters, polychaetes, crustaceans, and other resident invertebrates) are of commercial or recreational value to fishermen, they are consumed by many valuable finfish and crustacean species and thus indirectly benefit fisheries (Grabowski et al. 2005, Peterson et al. 2003). Given that we have already calculated the benefit of oyster reef habitat to fish in the previous section, we did not ascribe additional value to these benthic invertebrates that reside on oyster reefs. However, evaluations of estuarine biodiversity and its maintenance should include consideration of oyster reef habitats given that they can contain one to two orders of magnitude more macro-invertebrates than adjacent mud bottom (Grabowski et al. 2005).

Oyster reefs attenuate wave energy and stabilize other estuarine habitats such as salt marshes (Meyer et al. 1997). Oyster reefs also promote sedimentation, which potentially benefits the establishment of SAV (Henderson and O'Neil 2003). Oyster reefs are a living breakwater that can and will rise at rates far in excess of any predicted sea-level rise rate and thus help stabilize shoreline erosion and habitat loss, which are otherwise predicted to be dramatic in many coastal estuaries if left unprotected by natural buffers (Reed 1995, 2002; Zedler 2004). Although there are currently insufficient data with which to quantify the generality of these processes and assess their economic value, these services are indicative of the integrated mosaic of estuarine habitats. Thus a more complete evaluation of the ecosystem services provided by oyster reef habitat will require not only a greater understanding of its role in influencing coastal geology within the estuarine landscape but also evaluations of the services provided by the habitats oyster reefs promote.

Investigation of how the landscape setting of restored oyster reefs influences these ecological processes will be pivotal to future assessments. For instance, an oyster reef located in between a salt marsh and SAV may be an important corridor for predators moving among habitats (Micheli and Peterson 1999, Peterson et al. 2003). Conversely, some ecosystem services provided by oyster reefs in these vegetated landscapes may be redundant. For instance, Grabowski et al. (2005) found that restored oyster reefs in vegetated landscapes do not affect juvenile fish abundances, whereas oyster reefs restored on mudflats isolated from SAV and salt marshes augment juvenile fish abundances and potentially increase fish productivity within estuaries. The landscape setting of an oyster reef will also influence other processes such as oyster recruitment and survivorship (Grabowski et al. 2005), which in turn could affect filtration rates and subsequent removal of seston from the water column. Ecological studies that quantify landscape and ecosystem-scale variation in these processes will enhance our ability to model spatial variability in the value of services provided by oyster reefs.

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How To Have A Perfect Boating Experience

How To Have A Perfect Boating Experience

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