Sampling the seafloor with van Veen

Tracking the health of Puget Sound doesn't stop when we get to the bottom of the sea. The sediments below the water and the critters that live in it, known as benthic organisms, are critical components of the whole system.

Benthic organisms in Puget Sound, such as crabs, sea stars, worms, hydroids, and clams, are important food sources for larger organisms. These critters are affected by the conditions in the sediment, too, like pollutant levels and the proportions of gravel, sand, mud, and clay that make it up.

The scientists on our marine sediment monitoring team take sediment samples all around the bottom of Puget Sound once a year to keep an eye on how things are doing out there.

A tool of the trade

While benthic research has its rewards, it is not an easy job. For instance, it's a major inconvenience for us that these sediments are covered by water - sometimes hundreds of feet of the stuff!

Since we can't easily go down there to collect sediment samples ourselves, we send a special tool on this odyssey of discovery. We call this tool the van Veen grab sampler. The grab sampler is named for its Dutch inventor, Johan van Veen, who created the device in 1933.

Out on the research boat, we work long days in varied conditions. In the bustle of our many tasks at sea, "van Veen grab sampler" is often shortened to "grab sampler." Even more commonly, we refer to it as "the grab."

Pulling it together

A grab sampler has two tightly fitted bucket scoops with arms at the top that close the scoop when pulled. We use a winch with a very long, thick cable to lower the grab and send it down on a deep, dark journey to bring sediments to light. The winch cable holds a cross brace that suspends both the set of chains that attach to the closing arms and four shorter cables that hold the buckets open as the grab descends to the seafloor.

When the grab hits the bottom, the tension on the short cables releases, causing them to decouple and fall loose. As the grab is winched back to the surface, the large cable pulls up on the chains, causing the buckets to close together, capturing a shallow sample of sediment, water, and the things living in it.

The grab that the sediment team uses is a double van Veen grab sampler that takes two samples at once. We use one sample to take an inventory of the organisms and the other to study the sediment and the chemical pollutants in it.

An option from the grab bag

There are lots of ways to collect benthic samples, and they all have benefits and drawbacks. For example, a box corer captures a nice undisturbed surface layer, but doesn't cover enough surface area for our needs. It also digs deeper than we are interested in.

A sediment trap that captures material as it falls to the bottom would ensure that we only capture sediment from the most recent year. That could be useful, but it would exclude the benthic critters from the sediments.

A sample of our struggles

There is no remedy for this problem other than trying multiple grabs at a station until we achieve success or, eventually, we give up and move elsewhere. Also, since it can take several minutes for the grab to travel to the bottom in deep water - and several more for the return trip - and several more for the return trip - making multiple grab attempts can significantly increase the time needed at a station.

Sometimes the released cables will snag on one of the buckets, and this can also prevent the buckets from fully closing. Finally, in rough conditions, a wave can hit the grab near the surface of the water, briefly lifting it and releasing the cables prematurely, requiring a reset.

Dialing it in

The van Veen grab can be used for a range of sediment types with some minor adjustments to the equipment and deployment technique to get the sample just right.

Sandier sediments, for example, are resistant to the buckets digging in, so we might add weights and lower the grab more rapidly to hit the sediment harder to get sufficient sample depth.

In contrast, very muddy or silty sediments may be easily disturbed, making it difficult to collect the surface sediments that we are most interested in. In these conditions, we can remove the weights and lower the grab slowly so the device does not plunge too deeply into the sediment.

On balance, the van Veen grab sampler is a simple but effective piece of equipment that meets the needs of many benthic researchers worldwide.

The van Veen grab sampler helps us take consistent and repeatable samples from station to station and from year to year, letting us make meaningful comparisons and look for trends over time. Major changes in the benthos can have cascading effects throughout the food web, so monitoring these populations is important. To learn more about the benthic invertebrates of Puget Sound, have a look at our Puget Sound Benthos Toolbox.