Sampling at Discrete Depths

Often surface or oblique tows get the variety needed for general examination. At times, you might want to compare surface and near-bottom collections or samples from above and below a thermocline or pycnocline. Nets that open and close at specific depths are widely used in research, but are expensive and a bit more involved to use. The method below is a quick and easy way to control the depth of a net that gives good results for qualitative studies of relative abundances. This design works in depths to 6 m (~20 feet) and perhaps somewhat deeper with sufficient weight. The amount of weight and the size of the float depend on the size and resistance of the net and the depths of the samples. A little trial and error will determine the correct balance.

Materials:

  • A plankton net.
  • A float- a piece of Styrofoam or a small plastic jug with a hand grip.
  • A weight- a 8-16 oz fishing weight or small dive weight will do. larger nets require more weight.
  • Light rope to tie the weight and float to the net. 1/8” cord works well.

Construction

Near-surface tows. (float at the surface)

  • Make sure the float has plenty of buoyancy to remain at the surface as the net is towed.
  • Adjust the length of the rope from the float to the center of the net to the depth you want to sample, usually < 3 m.
  • Weight, if any, should be relatively light and attached within 10 cm of the bottom of the net, opposite of where the float line is attached. The purpose of this weight is to keep the line to the float relatively vertical as the net is towed so that the depth of the sample is close to the length of that line.
  • HINT: Click on the illustration for better resolution.

 

NET FROM TOP 1

Near-bottom tows (weight on the bottom,  float subsurface

  • The float should be just buoyant enough to cause the net and its frame to rise, without the weight attached. It should be attached within 10-20 cm of the net.
  • The weight should be heavy enough to sink to the bottom with the net and float attached and remain there throughout the tow.
  • Adjust the distance between the weight and the net to set the distance above the bottom that you desire. Do not try to get closer than 10 cm or your risk filing the net with bottom sediment.
  • HINT: Click on the illustration for better resolution.TOW surface

 

Sampling Instructions

  • Surface samples: Samples within a meter or so of the surface, require no special techniques.
  • Deeper samples:
  •      To minimize contamination with plankton from shallower depths, raise and lower the net quickly. If using a boat, try the following.
    • Let the net descend quickly with a minimum of forward motion from the boat, while taking care not to foul the net in the prop. Then put the boat in gear for the tow.
    • At the end of the tow, take the boat out of gear and let the motion slow. Pull the net in with dispatch.

Hint: Keeping the net out of the prop is easier if the tows are in a down-wind direction.