The location, composition, and origin of oyster bars in mesohaline Chesapeake Bay
Introduction
Oyster reefs have been identified as critical estuarine habitat supporting oysters themselves, other macro-invertebrates, and finfish (Coen, Luckenbach, & Breitburg, 1999). But Chesapeake Bay oyster stocks are in decline for a variety of reasons including: over harvesting (Kennedy and Breisch, 1983, Rothschild et al., 1994), high mortality due to disease (Andrews, 1988), and loss of habitat (Kennedy and Breisch, 1981, Seligier and Boggs, 1988). Estuarine ecosystem components created and sustained by oyster reef habitat may be in jeopardy because reef longevity relies on successful oyster recruitment. Recent efforts at establishing a unified framework for habitat restoration (Luckenbach, Mann, & Wesson, 1999) has led to heightened interest in the processes and materials that influence oyster bar morphology and distribution.
Both biotic and abiotic processes contribute to oyster reef structure. The general model is a platform shaped shell accretion with steep sloping sides that initially formed from the settlement and growth of individual oysters on some hard substrate (Bahr & Lanier, 1981). Continual settlement and growth of oyster larvae on surface shell increases vertical reef dimensions, and incremental lateral growth may result from colonization of detached shell fallen from reef edges (Bahr & Lanier, 1981). Larger scale horizontal reef dimension and spatial distribution are regulated by water current patterns, salinity, tidal height, hypoxia (Kennedy & Sanford, 1999), and the presence of non shell substrates for initial oyster settlement.
Reef morphology varies within the eastern oysters (Crassostrea virginica) range and various nomenclatures have been described (Graves, 1905, Stenzel, 1971, Haven and Whitcomb, 1983), but three general patterns emerge: (1) reefs separated from shore and parallel to the prevailing current with a low width to length ratio (fringing reefs), (2) reefs extending from shore against current axis with a low width to length ratio (string reefs), and (3) reefs in deeper water with a width to length ratio approaching unity (patch reef).
Descriptions of reef morphology generally come from observations of intertidal reefs. This model, however, may not be universal because intertidal reefs are generally absent from estuaries where freezing temperatures and ice scouring are common (Kennedy & Sanford, 1999). Hargis (1999) suggested that the Chesapeake Bay oyster reef is generally an ‘upthrusting reef’ that protrudes upward from the bottom, or a ‘fringing reef’ extending outward from and usually attached to adjacent, exposed coastal formation or shorelines. Others describe a form common to Virginia's James River called a Point Bar. An example, Wreck Shoal, was examined in detail by DeAlteris (1988a).
Fine scale morphology of sub-tidal bars is not well documented (Hargis & Haven, 1999, Chap. 23) and original reef conditions are difficult to determine in most areas because of extensive harvest disturbance and sedimentation. Limited stratigraphic information indicates that existing oyster reefs are comprised of a veneer of living oysters and other fauna covering a layer of dead shells and organic and inorganic sediments (Bahr and Lanier, 1981, DeAlteris, 1988a, Kennedy and Sanford, 1999, Hargis and Haven, 1999). Thick deposits of old ‘fossil’ oyster shell have been observed from cores taken up to 21 m below the seabed (Bouma, 1976), suggesting that reefs may persist over millennia. The composition of sub-bottom material forming the foundation of existing reefs is not well documented (Hargis & Haven, 1999), but has been reported as being either mud (Bahr & Lanier, 1981) or a coarse ‘lag deposit’ (DeAlteris, 1988a). Acoustic sub-bottom profiling surveys, bathymetric comparisons and sediment coring have been used to assess the composition and model historic changes associated with oyster reefs (Seligier and Boggs, 1988, DeAlteris, 1988b). In Virginia's James River, reef sediments were of biogenic origin and are composed of fecal and pseudofecal deposits, along with shell material overlying a coarse lag deposit. The biodepositional processes of the oysters create sediments and shell that have accumulated vertically at approximately 0.5 cm year−1 (DeAlteris, 1988a).
The spatial distribution of oyster bottom in the Maryland portion of the Chesapeake Bay was initially charted by Yates (1911), and summarized by Smith (1997). The Maryland Bay Bottom Survey, 1975–1983 (Smith, Greenhawk, Bruce, Roach, & Jordan, 2001) was conducted by the Maryland Department of Natural Resources and charted the distribution of shelled bottoms. Although both surveys focused on the presence of oyster shell, they did not address the quality of benthic habitat or describe reef morphology. These surveys did not provide information on what makes a location suitable for oyster bar development, nor did they assess fine scale morphological characteristics.
Our objectives are to understand how geological factors may influence the spatial distribution and morphology of oyster reefs in mesohaline Chesapeake Bay. We (1) described the sub-bottom features associated with existing oyster habitat; (2) identified general spatial relationships between specific geologic features and the boundaries of charted oyster bars; (3) documented oyster bar stratigraphy; and (4) assessed temporal changes in bathymetry to hypothesize processes of oyster bar formation and succession.
There have been many criteria evaluated for prospective site location for oyster rehabilitation such as sediment types, currents, salinity patterns, disease areas, historical productivity, brood stock abundance, water depth and exposure to storms (Marshall et al., 1999, Wesson et al., 1999). We believe the features and processes described here are intrinsic to mesohaline Chesapeake Bay oyster habitat and should be considered when evaluating sites for future habitat restoration. Furthermore, our findings may explain changes in oyster habitat condition as it faces ongoing shoreline erosion, sedimentation, and rising sea levels.
Section snippets
Acoustic sub-bottom profiling
Acoustic sub-bottom surveys were conducted in 1996 and 1997 in the Choptank River system and Herring Bay in the Maryland Chesapeake Bay (Fig. 1) in order to discriminate sub-bottom features associated with previously charted oyster bars or cultch (exposed oyster shell) areas. Initial survey tracks were planned to traverse several Yates oyster bar boundaries (Yates, 1911) (Fig. 2). Some sub-bottom profile survey transects were designed to perpendicularly cross the cultch bottom types of the
Feature characterization from acoustic sub-bottom profiling
Sub-bottom profiles can be grouped into seismic facies indicative of the acoustic characteristics of sediment distribution and depositional history. Our examination and interpretation of the sub-bottom profiles revealed four principal bottom and sub-bottom feature types that are ubiquitous throughout the Choptank River and nearby regions. Below are detailed descriptions of each feature.
Oyster bar genesis and morphology
Within the study region, oyster bar configuration appears to be a product of channel dissection of exposed Pleistocene and Tertiary terrace structure. Where sub-bottom profiling transects crossed oyster bar boundaries there often is evidence of paleochannel features. Rivers and streams present during the last ice age low sea level stand, carved deep channels into the Pleistocene (alluvial) or Tertiary (generally shallow marine) terrestrial sediment terraces of the Choptank River region (Colman
Acknowledgements
The Authors wish to thank Debra A. Willard, United States Geological Survey (USGS) for providing core analysis. Assistance in generating and interpreting field data was generously provided by: Don Walters, Stennis Space Center, Mike Czarnecki, U.S. Naval Research Laboratory; Tom Waddington, U.S. Army Corps of Engineers; Jeff Halka, Randy Kerhin, and Richard Ortt, Maryland Geological Survey (MGS); as well as Richard Younger for commanding the RV Kerhin. We also wish to thank Lt. Shepard Smith
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