Submerged Aquatic Vegetation Restoration and Subaqueous Soils Mapping

Research has shown that submerged aquatic vegetation (SAV) plays an important ecological role in estuarine systems, providing shelter, habitat, and a food source for many aquatic and terrestrial organisms.   SAV also benefits organisms indirectly by producing oxygen in the water as part of the photosynthetic process, protecting shorelines from erosion, filtering and trapping sediment, and absorbing excess nutrients to stem unwanted growth of algae.

Unfortunately, because of nutrients, sediment runoff from overdevelopment, boat-generated waves and other factors, SAV beds have been severely degraded or destroyed in many estuarine regions, including the Inland Bays.  Since SAV is a primary source of food and shelter for many types of animal life, including economically important finfish and shellfish species, the thrust to restore SAV has become urgent.  Restoration efforts have recently been undertaken --- specifically eelgrass --- in various regions throughout the Inland Bays.

Eelgrass is a species of SAV found in the Inland Bays and other areas in Delaware.

SAV restoration is not only ecologically important, it is also very costly.  Therefore our restoration efforts should be targeted in those areas where they are most likely to succeed.  Recent research demonstrates that the quality of the sediments is a primary key for SAV reestablishment.

Even in healthy estuarine areas, it is well documented that there is a strong relationship between the physical and chemical characteristics of submerged estuarine sediments and the density and composition of rooted vascular SAV --- similar to the relationship between soils and vegetative composition and density on terrestrial soil systems.  In fact, both terrestrial soils and their subaqueous counterparts have been shown to undergo the same soil-forming processes, supporting the inclusion of these substrates within the realm of soil science.  Consequently, these observations provided the thrust to propose a modification in the definition of soil to include subaqueous sediments that support submerged aquatic vegetation.

The new definition of soil was officially “codified” by the Natural Resource Conservation Service (NRCS) in the 1998 edition of the Keys to Soil Taxonomy (Click here for online resource.). They redefined soils to include an allowance of up to 2.5 meters of permanent water covering the sediment surface --- considered to be the maximum depth at which submerged rooted vegetation can survive.

DNREC team members take samples of subaqueous soils for mapping.

In recognition of the importance of subaqueous sediments or subaqueous soils in the restoration of estuarine resources, a workshop entitled “A National Workshop on Subaqueous Soils” was held in Georgetown, Delaware on July 14-18, 2003.  This workshop was sponsored by three universities (University of Maryland, University of Rhode Island, and the University of Maine) and the Natural Resource Conservation Service (NRCS) with the intention of disseminating and training Federal and State personnel with the latest findings and techniques in subaqueous soils mapping in relation to SAV restoration.

A sample of subaqueous soils obtained by using an auger to remove a sample at a pre-determined depth. The sample is used to determine the soil characteristics for mapping.

Similar to terrestrial soils, subaqueous soil mapping uses a predictive landscape model on the basis of topography, land surface shape, location, and water depth.  Soil type can then be inferred by identification of the specific soil landscape unit.  However, unlike terrestrial land surfaces, a published 3-D representation (i.e., topographic mapping) is not available and must be self generated.  Using a global positioning system in conjunction with a fathometer (i.e., depth finder), a detailed 3-D representation of a subaqueous land surface (bathymetric mapping) is generated by numerous georeferenced data points.  In essence, an underwater topographic map is generated which allows us to map underwater sediments in a manner similar to that of their terrestrial counterparts.

The recent scientific evidence confirming the relationship between distribution and vigor of floral and faunal populations with the distribution of subaqueous soil characteristics will help allow us to focus our eelgrass restoration efforts in areas where the soils are most suitable.  Using a subaqueous soil-based approach gives us an important methodology from which we can manage and restore important estuarine resources.