square, measured by the length along their sides. For larger zooplankton (euphausiids,
adult copepods) 200 or 333 μm mesh is typically used. To catch copepod nauplii, you
need 50 μm mesh. Finest meshes available are 10 μm or less, but the fabric is delicate
and hard to push much water through. Plankton nets in common use (Fig. 6.1) have
mouth areas from 0.2 (a circular, half-meter net ring) to 10 m^2 (“MOCNESS-10”; a 1
m^2 “MOCNESS” is shown). A 0.4 m^2 net (a 70 cm “bongo” net, for example) towed
at 1 knot will filter 12 m^3 per minute. A 15-minute tow will filter a volume equivalent
to a small swimming pool. A variety of mechanisms have been developed for opening
and closing nets at depth so as to catch only the plankton in vertically restricted
layers. MOCNESS systems are widely used for that at present (Frost & McCrone,
1974; Wiebe et al. 1985). They are sets of nine nets attached by their tops and
bottoms to bars held by cables at the top of a frame. The cables (and bars) are dropped
one after another by a computer-controlled motor, closing and opening nets in
sequence as the system is raised through the water. Sensors on the frame report depth,
temperature, distance traveled, and other variables, via the conducting tow cable to a
computer and its operator on deck. Smaller zooplankton are often filtered from water
collected by pumps or in bottles closed at depth. Pumps of reasonable size for
deployment aboard ships cannot filter as much water as nets, but they allow for
precise control of depth of sampling and use of fine mesh for small, resilient forms
like copepod eggs and nauplii.
Box 6.1 Mesh sizes for filtering zooplankton
(^) Nets for capture of plankton are usually cones of loosely woven fabric. Originally made from silk,
modern netting is precisely woven of nylon melted together at the thread crossings. The holes are
square and mesh size is specified as the length along the sides of the holes (Box Fig. 6.1.1). Smallest
holes are about 5 μm; the largest in common use are 1 mm. Since fabric can twist and stretch, mesh
sizes should be selected so that the diagonal measure is shorter than the narrowest axis of the target
organism. Phytoplankton nets are usually 20–60 μm; zooplankton nets are commonly 50–1000 μm.
Box Fig. 6.1.1 Diagram of fibers surrounding a mesh hole in a plankton net.
Fig. 6.1 (a) A simple ring net rigged with a bridle, and in this case weighted at the cod
end for vertical hauling. (b) Retrieving 70 cm bongo nets after an oblique haul, that is,