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2010 Deep Sea Coral Cruise - east coast
Mission info 2007 Nancy Foster Cruise
 

Mission Log: Apr. 12, 2010

Technology: Observational Tools: Bottom Grabs
By Cindy Cooksey
NOAA's National Centers for Coastal Ocean Science

A grab sample taken from a coral mound results in collection of dead coral fragments that can be used for determining coral growth rates and historical oceanographic conditions.  Photo by Robin Cobb, University of Alabama.
A grab sample taken from a coral mound results in collection of dead coral fragments that can be used for determining coral growth rates and historical oceanographic conditions. (Photo by Robin Cobb, University of Alabama)

There are many types of bottom grabs currently used to sample the sediment at the bottom of the ocean.  Which type of grab is chosen for any particular study depends upon the interests of the scientists, the type of habitat, how much sediment is needed, and how the grab can be deployed.  The Van Veen grab and the Young grab are two versatile types of bottom grabs that can be used in many different habitats.  The Van Veen is the larger of the two grabs, sampling a surface area of 0.1m2, and is best suited for deployment from large research vessels.  The Young grab is actually a modified Van Veen grab which has been scaled down in size and samples a surface area of 0.04m2.  The Young grab can be deployed from both small and large research vessels.  We have brought both types of grabs on this research cruise in order to maximize our capabilities.

The Sand Beneath the Waves

People may wonder why scientists want to study the seemingly ‘barren’ sand layer that covers vast stretches of the ocean floor.  One good reason is because this important habitat is not barren at all!  The unconsolidated (loose) bottom that occupies the majority of the sea floor can be teaming with life.  The types of animals found can include polycheate worms, mollusks, crustaceans, and fish.  Some are large enough to see with the naked eye, but many are so small that you would need to use a microscope to see them.  These bottom-dwelling (infaunal) organisms play vital ecological roles in the decomposition of organic matter, nutrient cycling, and as prey to higher trophic levels.  Check out www.seamonsters.noaa.gov for more information on benthic infauna.

The important role of the infauna is most apparent around rocky outcrops or reef structures found in the open ocean.  Many varieties of fish congregate around deep-water reefs and the infauna can be an important source of food.  Certain types of fish, especially juveniles, will leave the shelter of the rocky outcrops and go foraging out over the open sand, feeding upon the animals that they find.

The ocean floor.
The ocean floor.
Because almost everything that washes out to sea will eventually settle to the ocean floor, this type of habitat also is an important place to look for potential human impacts such as the presence of chemical contaminants (like metals, pesticides, and PCBs).   Although scientists do not expect to find any high levels of chemical contamination and other human-related stressors in offshore locations, it is important to establish a baseline of information so that in the future they will be able to tell whether amounts of potential pollutants in our oceans is changing.

How the Grab Works

Both types of grabs (Van Veen and Young) consist of a set of jaws that can grab a chunk of sediment from the seafloor  The jaws are set within a metal frame that ensures the grab will not land on its side when is reaches the bottom.  The frame also has metal posts on it for adding additional weight to make sure that the grab samples properly in cases where the water current is very strong or where the bottom sediment is very coarse.  When the grab is deployed from the ship, the jaws are open.  As it hits the bottom, it’s triggering mechanism releases; and as the ship pulls the grab up, the jaws dig into the sediment.

An example of how he grab works
How the grab works.
Once the Grab is back on deck with its bite of the ocean floor, scientists can open up the doors on top of the grab to glimpse a small section of ocean life.  The sample is good if sediment fills the grab and has not washed out of the top.  The doors on top of the jaws not only allow the scientists the chance to evaluate the sample, but also allow them to scoop off the top layer, take a small core, or decide to keep the whole grab depending on what is being studied.

Usually at each station replicate grabs are taken and the entire contents analyzed for the numbers and types of species that are present.  Small subcores also are taken from additional grabs to provide material for the analysis of chemical contaminants, organic matter, and the type of sediment (e.g., percentage of sand, silt, and clay).

Grabbing Rubble
On this cruise, we will use the grab to obtain samples of dead coral rubble.  The rubble can tell us what kinds of corals have lived here in the past.  The corals lock in the chemical composition of the ocean at the time of their formation, and we can analyze dead coral fragments to determine historical oceanographic and climate conditions.

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