15.0 BENTHIC FAUNA OF NEW HANOVER COUNTY TIDAL CREEKS

Troy Alphin and Martin Posey
UNCW Benthic Ecology Laboratory

Introduction

            Over the last decade the UNCW Benthic Ecology Lab has evaluated the tidal creek estuaries in the New Hanover County with a multi-disciplinary approach, considering benthic faunal communities (the critical link between primary production and the health of fisheries in the tidal creeks), primary production (an evaluation of the energy available to the first order consumers), ambient nutrient levels (a measure of natural and anthropogenic inputs), and select water quality parameters.  This work, supported mainly from North Carolina Sea Grant and the New Hanover Tidal Creeks Program, has shown the importance of predation (Posey et. al. 1999) in structuring benthic communities and the importance of scaling (Posey et. al. 2002) in determining nutrient impacts in the tidal creeks systems of New Hanover County.   These tidal creek systems are dynamic, meaning that the factors that influence the system (and its function) change in relative importance based on seasonal factors and event-driven watershed impacts.  For example the amount of nutrients that run into the watershed varies among years with the change in the amount of impervious surface within the watershed, but also on shorter time scales with interactive effects between rain events and ongoing development, having a large impact on the system (at least for some parameters).  For this reason it is imperative that we begin to understand the function of organisms such as oysters that are considered ecosystem engineers and have been proposed as effective modifiers of ecosystem function.

Oyster Reefs as Modifiers of Water Quality

Since 2001 the Benthic Ecology lab has focused on the function of oyster populations in the tidal creeks.  Specifically, we have studied the potential interactions between oyster reefs and water quality and the influence oyster reef morphology has on the utilization of oyster reefs and on the development of the reefs themselves.  General surveys of oyster reefs in the tidal creek show that they cover roughly one-third of the intertidal area within the lower sections of each tidal creek.  However, this is somewhat misleading because much of the reef surface is actually dead shell and live oyster density varies greatly within and among the creeks (Mallin et. al. 2004).  As part of a collaborative project with the UNCW Aquatic Ecology Lab and the UNCW Coastal Hydrology and Sedimentology Lab, we conducted several studies in 2002 specifically evaluating the water quality impacts that live oyster populations can have on the small-scale fringing tributaries of the tidal creeks.  These fringing creeks tend to be small (1-2m deep at high tide and 3-4m wide) but represent the actual source of much of the run-off from communities within the watershed.  This work showed that relatively small additions of live oyster (12 sq m) could reduce levels of total suspended solids and chlorophyll downstream of the oyster patches (Nelson et. al. 2004).  Another study focused on the effect of naturally formed oyster reefs within the main stem of the Hewletts Creek system.  This study specifically tested the upstream/downstream impact of live oyster filtration, it also tested the idea that varying live oyster density may influence oyster impacts on water quality.  Specific findings indicate that during summer periods oysters can significantly (and consistently) reduce chlorophyll levels by 10-25% and that they can have similar effects on fecal coliforms in the spring, although the amount removed was much more variable.  Moreover, rough modeling indicated that the chlorophyll reduction was not solely due to filtration but may also be affected by hydrodynamic changes related to reef structure (shell) - in other words the structure may enhance settling of particles in  and near the reef. Overall this work indicated that both feeding of live oysters and the influence of oyster structure work to reduce levels of chlorophyll and bacteria in the water column (Cressman et. al. 2003).

Oyster Reefs as Habitat

Oysters serve a variety of functions within the estuarine ecosystem.  Clearly they act as modifiers of water quality, but they also provide critical refuge for a number of organisms, including commercially important finfish such as those targeted by recreational anglers.  Based on the previously mentioned work, Sea Grant has supported a new study to evaluate the influence that oyster reef morphology has on the various ecosystem functions that oysters play - such as water quality impacts and habitat for finfish and crustaceans.  While the impact that oysters have on water quality is significant and critical to understanding their ecosystem impacts, it has become clear that we must also understand the factors that influence oyster reef utilization and reef development from a habitat perspective.  This study has evaluated utilization of oyster reefs based on reef edge (convoluted vs. circular) and vertical complexity.  In this case low vertical complexity was represented by a reef constructed of flat unarticulated shell, with only a couple of centimeters of relief, and high vertical complexity was simulated by reefs with both patches of flat shell and by patches of oyster matrix. A third treatment evaluated small fragmented reefs of high vertical complexity that represent reefs as they begin to experience fragmentation.  (Fragmentation is the process where habitat patches become degraded and large patches are reduced to several small patches).  All of these reef types were compared for utilization by finfish and crustaceans as well as for aspects of reefs development such as settlement and survivorship of oysters.

            This work indicated interesting patterns related to aspects of oyster reefs development.  Total oyster spat settlement and initial survivorship was greater on oyster reefs made of shell hash that had lower vertical complexity compared to reefs of higher vertical complexity, fragmented reefs or natural reef controls regardless of the edge treatment (circular vs convoluted) (Figure 1) (Posey et. al., in prep.).  A possible mechanism for these differences may be that greater abundances of predatory crabs live in high-relief reef systems. As the reefs developed, the initially low vertical complexity treatments became identical to the high vertical complexity treatment in year two.  These finding have significant implications for future restoration projects and for resource managers that are tasked with enhancing the harvestable oyster populations. 

            In 2002 and 2003 we evaluated oyster populations within the various tidal creeks of New Hanover County based on the coverage of oyster reefs and the density of live oyster populations within the various reefs, as well as categorize the reefs by the amount of vertical relief seen in the various creeks.  These studies illustrated the general difference among the various creeks, with Hewletts and Pages creeks consistently showing higher values for live oyster density and coverage (Mallin et. al. 2004).  

Given the demonstrated differences in oyster characteristics among the creeks, in 2004 the focus shifted to evaluating the oyster spat settlement and subsequent survivorship based on high and low vertical complexity within the natural reefs of each creek.  The objective in this study was to determine if the oysters that settled in natural reefs demonstrated similar settlement and survivorship, based on relative complexity as seen in the previous study of oyster reefs. Creek differences in reef complexity indicate broad-scale implications for any complexity effects. 

Methods

Oyster settlement bags were placed within areas of high and low vertical complexity within Hewletts, Bradley, Howe, and Pages Creeks and in a clam lease area at the mouth of Hewlettes Creek.  Each bag contained 10 unarticulated shells.  All shell used in the settlement experiments was clean shell that had been dried for at least 6 months.  Within each creek three sites representing low vertical complexity and three areas representing high vertical complexity were sampled.  It should be noted here that based on previous findings, the low vertical complexity treatments in the natural reefs within each creek had a greater amount of surface rugosity (complexity) than the low vertical complexity treatments previously described in the reef morphology study (Mallin et. al. 2004).  Settlement bags were placed in the oyster reefs representing each treatment in April and retrieved in July and a second set was deployed in July and retrieved October.  All live oysters, scars, and recently dead oysters were counted.  All three of these measurements allow us to calculate the total number of oysters that settled within each creek.  

Results

            Overall total settlement was an order of magnitude lower in the tidal creeks in 2004 then was previously seen in the Sea Grant study.  We believe this represented interannual variability rather than differences among locations because settlement was lower in all plots monitored, even those outside the creek environments. No significant differences were detected between high and low relief treatments in any of the tidal creeks (Figure 2), though there was a trend towards higher recruitment within low complexity plots in Hewletts Creek (consistent with previous Sea Grant work at the mouth of that creek). There was also no detectable difference for total abundances of oyster spat between natural high and low relief areas (within the same reef) when data from all creeks is combined (Figure 3).  Evaluation of survivorship does show some very different patterns with twice as many initial survivors in Pages and Hewletts creeks compared to Howe and Whiskey creeks (Figure 4).

Conclusions

            Settlement and survivorship are the two most important factors determining the development and stability of oyster reefs within any system.  Findings of initially high settlement and survivorship of oyster spat on low reefs constructed solely of shell hash compared to reefs constructed with a greater degree of vertical complexity show a great deal of potential for the development of oyster reef for habitat restoration and mitigation.  Low reefs lead to initial rapid colonization, while more complex reefs lead to less oyster colonization but better habitat for the entire community.  The comparison presented here between areas within natural reefs with high and low vertical complexity suggests that differences in habitat type may be due in part or whole to the habitat provided by oysters to other organisms.  The habitat function of oyster reefs is critical especially in shallow estuarine environments where oysters may represent one of the few structural habitats available (Larsen 1985, Breitberg 1999, Posey et. al. 1999, Coen et. al. 1999).  As oyster reefs develop the number of crevices and the amount of internal space within the oyster matrix increases.  These areas are colonized very quickly by small crabs and shrimp that may in turn prey on newly settled oyster spat.  Thus as oyster reefs begin to provide a more complex refuge the overall survivorship of oyster spat may decline.  This provides an excellent example of biological controls and illustrates how a healthy ecosystem operates.  As the oyster reefs develop they provide more habitat allowing a greater number of species of epifauna including crabs and shrimp, these species in turn provide food for many of the commercially and recreationally important finfish, such as drum, blue fish, spots, croaker among others.

Current Projects 

            The UNCW Benthic Ecology Laboratory currently has two projects focused on evaluating the factors that influence the development of oyster reefs within the New Hanover County tidal creeks.  1) One project evaluates the role that early colonizing xanthid crabs play in survivorship of oysters and other reef resident fauna. 2) The second project involves the detection of disease and disease burden in oyster population based on factors of reef complexity.  These two projects will evaluate how intertidal oyster reefs develop, with a goal of providing the resource managers and restoration groups with information that will increase their effectiveness as replacing and enhancing the oyster population.

Citations

Breitburg, D.L. 1999.  Are three-dimensional structure and healthy oyster populations keys to an ecologically interesting and important fish community? P.239-250. In:  M.W. Luckenback, R. Mann, and J.A. Wesson (eds.), Oyster reef habitat restoration: a synopsis and synthesis of approaches.  Virginia Institute of Marine Science Press.

Coen, L.D., M.W. Luckenbach, and D.L. Breitburg.  1999.  The role of oyster reefs as essential fish habitat:  a review of current knowledge and some new perspectives.  American Fisheries Society Symposium, Vol. 22. 438-454.

Cressman, K.A., M.H. Posey, M.A. Mallin, L.A. Leonard, and T.D. Alphin.  2003.  Effects of oyster reefs on water quality in a tidal creek estuary.  Journal of Shellfish Research. Vol. 22 (3):53-762.

Larsen, P.F.  1985.  The benthic macrofauna associated with the oyster reefs of the James River estuary, U.S.A. Int. Rev. Gesamt. Hyddrobiol. 70:797-814.

Mallin, M.A., L.B. Cahoon, M.H. Posey, V.L. Johnson, T.D. Alphin, D.C. Parsons, and J.F. Merritt.  2004.  Environmental quality of Wilmington and New Hanover County watersheds 2002-2003.  CMS Report 04-01.

Nelson, K.A., L.A. Leonard, M.H. Posey, T.D. Alphin, and M.A. Mallin.  2004.  Using transplanted oysters (Crassostrea virginica) beds to improve water quality in small tidal creeks: a pilot study.  Journal of Experimental marine biology and Ecology 298: 347-368.

Posey, M.H., T.D. Alphin, L. Cahoon, D. Lindquist and M.E. Becker. 1999. Interactive effects of nutrient additions and predation on benthic communities. Estuaries 22: 785-792.

Posey, M.H., T.D. Alphin, L.B. Cahoon, D.G. Lindquist, M.A. Mallin and M.B. Nevers. 2002. Top-down versus bottom-up limitation in benthic communities: direct and indirect effects. Estuaries. 25: 999-1014.