Bottom Sampling

In order to obtain a rough analysis of the bottom material in a body of water, a variety of devices have been invented. Among them are grab samplers, dredges, corers, and drills. The PONAR Grab sampler is the main bottom sampling device used on the vessels to study the composition of the bottom of a lake or river: A Phleger Corer is sometimes used on advanced cruises where the stratification of the sediment layers is studied (see Appendix D).

Dredges and grab samplers make it possible to obtain samples of material found on the bottom of a body of water (ocean, lake, or river). Dredges are weighted nets that are dragged over the bottom to scrape off samples of the surface material. This technique is satisfactory for obtaining bulk materials. Because dredges bounce and skip as they are dragged across the bottom, samples collected do not provide a well-defined or quantitative sample. The dredge is used when a bulk sample of material lying on the bottom is desired.

The grab sampler provides a means to obtain a somewhat quantitative and undisturbed sample of the bottom material. It takes a bite of known surface area and penetration depth, providing the bottom material is neither too hard or nor too soft. It is called a grab sampler because of the manner in which it obtains samples.

What is a PONAR Grab Sampler?

Early studies on Lake Michigan used oceanographic and freshwater grab samplers that were not satisfactory. Research scientists from the Great Lakes Research Division of the University of Michigan devised a new sampler, the PONAR grab sampler, that was first available for sale in 1966. The sampler is named after Great Lakes scientists, Charles E. Powers, Robert A. Ogle, Jr., Vincent E. Noble, John C. Ayers, and Andrew Robertson.

The PONAR grab sampler consists of two opposing semi-circular jaws that are normally held open by a trigger mechanism. The sampler is lowered to the bottom where contact with the bottom sets off the trigger and a strong spring snaps the jaws shut trapping a sample of the bottom inside. Fine copper screen covers the top of the jaws so that the trapped material will not wash out as the sampler is retrieved.

The deckhand normally places the PONAR grab sampler on deck at the start of the cruise. It is placed in an out of the way location until a sample of the bottom material is desired. Samples are taken while the vessel is on station and not moving through the water.

When a sample is to be taken, the PONAR grab sampler is taken to the hero platform where the deckhand attaches it to the hydrographic wire (winch line). The sampler is "cocked", that is, the jaws are opened and the trigger is set. The sampler is then swung over the side and lowered to the bottom. The jaws snap shut upon reaching the bottom and a sample of material is obtained.

As long as the PONAR sampler is hanging freely from the hydrographic wire, the trigger mechanism will keep the jaws open. However, as soon as there is slack in the winch line, the trigger will be released. When the winch starts to raise the PONAR grab sampler, the jaws will close thus taking a "bite" (sample) from the bottom of the lake. Sometimes when lake waters are rough and the rocking action of the vessel may cause the winch line to become slack enough to release the trigger prematurely thus allowing the jaws to close before the sampler has reached the bottom. In such instances the PONAR grab sampler must be brought back aboard the vessel to reset the trigger and a second sampling attempt is carried out.

When a successful PONAR grab sample is brought aboard, the sampler is lowered into a rectangular stainless steel box that has a very fine screen on the bottom side. The deckhand will empty the contents of the PONAR grab sampler into the stainless box and rinse the grab sampler with a hose to make sure that all of the sample is rinsed into the stainless steel box. The bottom sample is now ready for examination

NOTE: The PONAR grab sampler is a piece of equipment that is operated by the deckhand. It is very heavy. Please stay out of the way when it is being used.

How is bottom material studied?

The material brought up from the bottom can be examined in several ways. A quick visual inspection can give a qualitative description of the kind of material retrieved: sand, silt, clay, mud, decayed organic, or a combination. In many cases, the sample will reveal the presence of small animals. These can be found by washing the fine sediments through the fine mesh screen and leaving the organisms on the screen where they can be picked from the screen and placed in a plastic Petri dish. When all of the organisms have been collected in the Petri dish the dish can be taken into the main cabin and examined under the stereo microscope. With the video camera attached the entire group will be able to see these bottom (benthic) organisms on the color monitor. Students may be able to identify some of the organism by checking the laminated diagrams of typical benthic organisms that are on display in and around the microscope area.

The composition of bottom sediment can also be studied by separating the samples through the use of a graded series of fine-mesh brass sieves. The sediment particles sort out by size:

Sediment that is sand will have distinct grains that are easily seen and felt. Silt will form a cast when moist but will not form a ribbon when moist. Clay is sticky and plastic when wet and forms a ribbon when squeezed. Some sediment samples may have high concentrations of organic matter (muck) indicating slow decomposition rates and low oxygen conditions.

What is found in the bottom material?

Samples taken from Spring Lake or Muskegon Lake provide the possibility of observing anaerobic decay. This is especially true in August when biological oxygen demand depletes oxygen in the water above the bottom. If anaerobic decay is present, the odor of hydrogen sulfide (hard boiled or rotten egg odor) can be detected. The material from the bottom of these two lakes seldom has a great diversity of easily detected life forms. Chironomid (midge fly) larvae are sometimes found. They can be identified by their blood red color leading to the common name "bloodworms." The presence of erythrocruorin in their blood causes the red color in these organisms. The erythrocruorin may enable them to withstand lower oxygen levels as the chemical has a high affinity for whatever oxygen is present. Bloodworms live head down in tubes on soft bottoms where they feed on bottom organic matter. They have a complex life cycle (Figure 12).

Lake Michigan provides many possibilities for bottom material study. Near shore, the material is basically sandy. Oligochaetes (segmented worms related to earthworms), fingernail clams, and scuds (small shrimp-like arthropods) are commonly found. Some samples will have zebra mussels. Farther out from the shore, the sand is mixed with silt and/or clay. At greater depths, the sediments are a mixture of clay and fine grained sediment. The profile of the bottom sediment in Lake Michigan (Figure 13) illustrates this point.

Samples taken from the bottom of other bodies of water may show a greater species diversity. In a river where a strong current is present, the bottom material will most likely be sandy or rocky. In still water, silt is commonly found. Shells are usually found in these samples. Streams can harbor many benthic macroinvertebrates including a great variety of immature insects, sponges, flatworms, roundworms, annelids, and mollusks.

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