MapCoast

Introduction to Subaqueous Soils

Adapted from Bradley and Stolt, Soil Sci. Soc. Am. J. 67:1487-1495 (2003). More information on the classification of subaqueous soils can be found on NESoils.

Recently, the USDA-Natural Resources Conservation Service (NRCS) established that sediment in shallow water may be considered soil. With MapCoast (www.mapcoast.org), the NRCS strives to map subaqueous soil areas in Rhode Island. Estuarine subaqueous soils are permanently flooded soils that occur immediately below the intertidal zone to shallow water depths (generally <5 m) in protected coves, bays, inlets, and in back-barrier coastal lagoons. This new designation is the result of work in Maryland in which estuarine substrates in shallow water were shown to undergo soil forming (pedogenic) processes. Processes operating in subaqueous soils include additions of biogenic CaCO3 and marine humus from benthic biota, bioturbation from shellfish and worms, and chemical transformation of sulfur and iron in anoxic environments; all of which differentiate surficial sediment into soil horizons.

The correlations between shallow water estuarine sediment and the classic tenets of soil formation support the inclusion of these substrates within the realm of soil science (Pedology). One of the principle components of the definition of terrestrial soils is the ability to support rooted plants in a natural environment. Dense beds of submerged aquatic vegetation (SAV, or seagrass) are often found in subtidal estuaries. Unlike macro-algal species, which anchor themselves to a substrate, SAV species are rooted vascular aquatic plants in which roots serve both structural and nutrient uptake purposes. A highly diverse benthic faunal community also depends on subaqueous soils for nutrients, structure, and habitat. The actions of these marine animals are similar to those inhabiting terrestrial soils. Marine animals mix grain sizes, diffuse oxygen to the subsurface layers, decompose organic matter, and concomitantly supply organic carbon from decaying organisms, fecal pellets, and excretion of mucus. Finally, numerous studies have emphasized the importance of landscape components for predicting and explaining soil distributions. Submerged landscapes are fundamentally the same as terrestrial systems and have a discernable topography from which subaqueous landforms and landscape units may be identified.

Considerable research has focused on many components of estuarine and coastal ecosystems including hydrology, vegetation and floral and faunal interactions. However, the substrate, which supports a wide variety of benthic invertebrates and supports dense areas of SAV, has been largely ignored. Geologic studies have focused on this realm of the ecosystem, but the information provided by these studies is not detailed enough to be of ecological significance and most of these studies focused on a single parameter (e.g. grain size). An advantage of using the soil science approach to study shallow water sediments is that soils are studied as a collection of horizons that are linked with depth across the landscape. These horizons are studied and characterized by examining a combination of properties and characteristics, instead of a single component or parameter. As such, the NRCS, along with MapCoast partners, recognize the need for a new model of mapping shallow water substrates that incorporates traditional soil survey methods along with established geologic oceanography techniques.

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