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A Sampler for Quantifying the Vertical Distribution of Macroinvertebrates in Shallow Wetlands

Introduction


Interest in quantifying aquatic macroinvertebrates in wetlands is increasing because they are consumed by waterfowl (Drobney and Fredrickson 1979, Swanson et al. 1979, Euliss and Harris 1987) and shorebirds (Hicklin and Smith 1979, Baldassarre and Fischer 1984, Boates and Smith 1989). They are also important in the epizootiology of avian botulism (Bell et al. 1955, Jensen and Allen 1960, Duncan and Jensen 1976) and are valuable as biological indicators of pesticide residues in wetlands (Grue et al. 1989). Consequently, there is a need to develop a variety of quantitative sampling gear for aquatic invertebrates to address broad ecological questions. In this paper, we describe a tube sampler that separates the water column and benthos of shallow wetlands into distinct vertical profiles. We developed the device to describe vertical distribution patterns of invertebrates in wetlands for research on avian botulism epizootiology and microhabitat selection by wetland macroinvertebrates in relation to vertical abiotic gradients.

The described sampler (Fig. 1) is similar to the multiple tube sampler described by Euliss et al. (1992) in that benthic and pelagic invertebrates are quantified simultaneously. Thus, bias from double sampling does not occur because separate devices are not required to sample benthic and pelagic organisms (Euliss et al. 1992). The portion of the tube that samples the water-column is partitioned into individual compartments to facilitate sampling of specific profiles in the water-column.

Figure 1
Figure 1.  Schematic view of sampler in open and closed position. Numbered parts are:  1) Radiator-type hose clamps; 2) Closure cable handle storage hooks; 3) Closure cable handle; 4) Sampler handle; 5) Braided nylon line; and 6) Sediment tube section door (in open position).

The sampler is constructed by stacking several interconnected 10.1-cm ID (11.3-cm OD) clear acrylic-tube water-column samplers on a bottom tube that samples benthos (Fig. 1). We used 15.2-cm long tubes, but tube length may be modified for individual needs. Several acrylic doors isolate water-column samples into separate vertical profiles. The leading edges of the doors are sharpened to cut minor vegetation as they are closed. The bottom of the tube is also sharpened to facilitate penetration of sediments. Acrylic doors (flat 6-cm-thick acrylic) are integrated into the base of each compartment (Fig. 2) to physically block the movement of invertebrates among profiles. A 9-cm-diameter 0.5-mm-mesh stainless steel screen is attached with clear silicone adhesive to the center of each door. The doors are sanded to reduce their thickness and to facilitate rapid closure. A laminated acrylic sheet is attached flush with the top of each compartment to form a firm attachment plate with the adjacent compartment. Acrylic cement was used to bond door housing units and joint plates.

Figure 2
Figure 2.  Expanded view of water-column compartment and door closure unit. Numbered parts are:  1) Tube compartment; 2) Screen; 3) Sliding door; 4) Cable line clamp; 5) Barrel swivel; 6) Nylon tie; 7) Snap; 8) Bolt; 9) Joint plate (top section); 10) Wood dowel; 11) Plastic coated wire cable; and 12) Wing nut.

The top tube section of the sampler must be long enough to facilitate placement of sampler handles and brackets used to lock sampler doors. Sampler handles and the door closure handle brackets are attached with standard hose clamps to the top tube. Handles may be obtained from hardware stores and the two brackets were constructed from 16 mm × 7 mm (0.25 inch) rods welded to a metal plate.

The water-column compartments and the sediment tube are stacked vertically and coupled together at the corners with 7 mm bolts. Wing nuts are used to facilitate breakdown and assembly of the sampler in the field.

Compartment doors are secured to the closure cable with a loop of small-diameter braided nylon line. Nylon line is inserted through two holes in the rear of each door housing and then into two holes drilled in the leading edge of the door. A knot in the nylon line prevents the line from pulling out of the doors when they are closed. A snap swivel attaches the loop of nylon line to the closure cable. The closure cable is constructed with 1.5 mm plastic-coated braided wire cable, a 7 mm OD wooden dowel, and a handle of 25 mm OD wooden dowel. The 90 cm wooden dowel is attached to the wire cable to prevent tangling.

A sample is collected by lowering the sampling device into the water with doors in the open position. The bottom tube section is forced into the sediment to the joint plate of the sediment compartment. Immediately after the sampling device is seated into the sediments, the closure cable is raised to simultaneously close all doors. The sampler is then rocked back and forth to break the sediment seal and lifted to the surface. Individual water-column compartments are disassembled and the samples washed into specimen jars. Re-assembly of the sampler averages 2 minutes per individual water-column sample.

Benthic samples are removed from the sampling device by hand. The core is forced through the top of the tube by hand until the protruding core is the desired depth from the benthic-pelagic interface. The door is then closed to sever the sample from the remaining core. Individual sediment cores are then placed in a modified self-cleaning screen (Euliss and Swanson 1989) to concentrate sample residues and placed in specimen jars.

We used the described sampling device for two field seasons to collect macroinvertebrates in shallow wetlands. Our study sites contained minimal submergent and emergent vegetation that would interfere with the operation of this device. The sampler withstood rigorous use and required only occasional minor maintenance of the cable used to close the doors. Materials for the sampling device cost about $175.00. Total construction time was about 8 hours.

We thank H. R. Murkin, F. A. Reid, and G. A. Swanson for critical review of the manuscript and D. M. Mushet for preparing figures.


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