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Benthic Habitat Mapping

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Applying Benthic Data: Dredging and Disposal of Marine Sediments

Assessing Biological Impacts

Image of biological monitoring

Sediment profile images and grab samples are often used for biological monitoring at dredged material disposal sites.
Courtesy: Science Applications International Corporation

The impact of deposited sediments on the biological resources living in and near the disposal site is a concern for dredged material disposal. The addition of dredged material to the seafloor affects the area in two ways. First, the deposit will cause some mortality of benthic organisms due to burial. The amount of mortality caused by burial will depend on the particular organisms and on the depth of burial. The second impact is the formation of a new, uninhabited substrate that is available for colonization by organisms.

Benthic organisms living in shallow-water estuarine and nearshore environments are well adapted to frequent physical disturbance. Tides, currents, waves, and storms cause sediments to be lifted, deposited, or shifted. The resilience of benthic organisms to these environmental changes allows them to recolonize areas of the seafloor affected by dredging.

Small image of OSI map

Click image for a close-up view of disposal map. View Image

Benthic grab sampling and sediment profile imaging (SPI) are the primary tools used by the Disposal Area Monitoring System (DAMOS) developed and managed by the U.S. Army Corps of Engineers to monitor biological impacts within and near disposal sites. Both grab sampling and SPI provide a direct view of physical habitat changes, such as changes in sediment grain size and the amount of oxygen in the sediments. Unlike grab sampling, however, SPI allows the researcher to view an undisturbed sediment profile, showing the relationship between the organisms and the sediment in which they live. SPI has been most commonly used because it is more cost-effective and can provide the necessary data more quickly (on the order of weeks rather than months) than traditional grab sampling and taxonomic analysis.

How can SPI be used to measure benthic community condition? View Image

To standardize the monitoring effort, a grid of SPI stations is established over each disposal mound. Nearby reference areas (areas not affected by dredged material disposal) are also included in the sampling. Surveys are conducted every few years to test the prediction that as biological succession progresses, redox boundaries will deepen, and overall benthic habitat quality, as indicated by the calculated Organism-Sediment Index (OSI) value at each mound, will eventually be comparable to that found at the reference areas.

Example images

Side-scan and photographic images of the seafloor at the Massachusetts Bay dredged material disposal site showing rocky habitat inside (A) and outside (B) of the disposal area.
Courtesy: U.S. Geological Survey

The benthic habitat mapping data collected using DAMOS are used to identify potential negative impacts and appropriate management procedures. If survey data do not show either the expected biological succession or the projected increases in OSI values, action must be taken:

  • More intensive monitoring can be used to detect the presence of contaminants and/or toxic materials, or
  • Additional, clean sediment can be placed over the disposal mound (i.e., capping).

How are SPI data used in the DAMOS program? View Image

DAMOS is only one of many dredged material disposal site monitoring programs throughout the world. Combinations of bathymetry, side-scan sonar, SPI, and benthic grab or coring techniques have been used to monitor disposal sites in Puget Sound, offshore San Francisco, the southern California Bight, Mobile Bay, Chesapeake Bay, the New York Bight, New Zealand, and Hong Kong.

The U.S. Geological Survey has conducted seafloor mapping studies of dredged material disposal sites using photographic techniques in combination with bathymetry, side-scan sonar, sub-bottom profiling, and sediment core and grab sampling. In one such study, plan-view photography showed that piles of rock debris placed on the seafloor at the Massachusetts Bay Disposal Site were only sparsely populated by benthic organisms three years after deposition.

Regardless of which techniques are used, the role of dredged material disposal site monitoring is to determine the benthic community response to the habitat change caused by disposal. The advantage of grab and core sampling is that the response of individual species can be examined, whereas SPI allows the researcher to evaluate community response in terms of different successional stages. The use of sediment grabs and cores in conjunction with SPI provides a valuable means of verifying or reinforcing the interpretation of SPI images, thus allowing for more effective impact assessment.

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