Saturday, 15 July 2006

Subaqueous Landforms and Soils of Chincoteague Bay, Maryland, USA.

Danielle M. Balduff and Martin C. Rabenhorst. Univ of Maryland, 1112 HJ Patterson Hall, Dept. NRSL, College Park, MD 20742

Demas and Rabenhorst (1999) demonstrated that pedogenic processes occur in shallow water mineral substrates resulting in the formation of soil horizons. Previously these substrates were only mapped as geological sediments, but they are now considered subaqueous soils and are accommodated under the pedological paradigm. Jenny's factors of subaerial soil formation (climate, organisms, relief, parent material, and time), and Folger's factors for estuarine sediment genesis (source geology, bathymetry, and hydrologic condition), were synthesized and enhanced to develop the state factor model for the formation of subaqueous soils: Ss = f(C, O, B, F, P, T, W, E) (Demas and Rabenhorst, 2001), where Ss is Subaqueous soil, C is climatic temperature regime, O is organisms, B is bathymetry, F is flow regime, P is parent material, T is time, W is water column attributes, and E is catastrophic events. In this study the subaqueous landforms of Chincoteague Bay, Maryland, USA were identified and the associated soils were described, mapped, characterized and classified. Chincoteague Bay (depth ranges mostly from 1.0 to 2.2 meters) is the largest (18,855 ha) of Maryland's inland coastal bays (coastal lagoons) bounded by the barrier Assateague Island to the east and by the Maryland mainland to the west. It is connected to the Atlantic Ocean by the Ocean City inlet to the north and the Chincoteague inlet to the south. Chincoteague Bay contains submerged aquatic vegetation (SAV) beds, which are indicative of relatively clean estuaries, having sufficient water clarity for benthic photosynthesis. Subaqueous soils have a controlling influence on the physical and chemical characteristics of estuarine ecosystems, especially their benthic communities. Thus, mapping the soils of Chincoteague Bay provides a valuable resource inventory for use in ecological research and management. Bathymetric data collected by the Maryland Geological Survey in 2003 was used to generate a digital elevation model of Chincoteague Bay. The digital elevation model was used, in conjunction with false color infrared photography, as a base map to aid in the identification of subaqueous landforms. Due to the visual obstruction by water, the subaqueous landscape units were also identified by landscape shape, depositional environment, proximity to fresh water, and geographical setting. Eight subaqueous landforms were identified in Chincoteague Bay: barrier cove, lagoon bottom, mainland cove, paleo-flood tidal delta, shoal, storm-surge washover fan flat, storm-surge washover fan slope, and submerged headland (examples will be demonstrated). Previously established soil-landscape models were evaluated for their applicability in Chincoteague Bay. The appropriate models were then utilized to create a draft soil map of Chincoteague Bay. The association of soils with particular landforms further supports the applicability of the soil-landscape paradigm in subaqueous environments. Soils were initially classified using Soil Taxonomy (Soil Survey Staff, 2003). The major soils found in Chincoteague Bay include Typic Sulfaquents, Haplic Sulfaquents, Typic Psammaquents, Typic Hydraquents, and Thapto-Histic Sulfaquents. A new suborder of Entisols (wassents) has been proposed for use in Soil Taxonomy to better recognize subaqueous soils. The differentiating criterion for these Entisols is a positive water potential at the soil surface for 90% of each day. Using this proposed modification, the subaqueous soils of Chincoteague Bay would be classified as Typic Sulfiwassents, Haplic Sulfiwassents, Thapto-Histic Sulfiwassents, Typic Hydrowassents, and Typic Psammowassents. Using the World Reference Base for Soil Resources (WRB) subaqueous soils would be classified as Gleysols (saturated by groundwater long enough to allow reducing conditions to occur). Classification of these soils as Gleysols, however, does not identify the permanent saturation of these soils. Therefore, the WRB should be modified to better recognize the unique properties and processes associated with subaqueous soils. Interpretations to assist management decisions can also be developed for subaqueous soils based on physical and chemical properties, and two examples will be presented. Interpretive maps highlighting potential hazards associated with dredging activities and dredge material disposal will be developed. In addition, subaqueous soil data collected in Chincoteague Bay will be used to assess the suitability of soils for SAV restoration (replanting) projects.

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