Sampling Methods For Zooplankton 471 Mesh size extrusion and avoidance

Zooplankton is typically collected with a fine mesh net, but using buckets or dip nets around bright lights is also possible. The appropriateness of mesh size can be determined through the trade-off between the net avoidance of zooplankton and net extrusion of zooplankton. With towed plankton nets, the smallest mesh size will never sample all the zooplankton, because larger and better swimming zooplankton will sense the pressure wave in front of a small mesh net and dodge it (this is known as net avoidance). If you use larger mesh, then the smaller zooplankton will be extruded through the mesh. We must accept that our sample is a selective view of plankton, but it will be a consistent view. The standard UNESCO mesh size for sampling zooplankton is 200 ^m mesh (Harris et al. 2000) (Figure 4.4), but we have found that a 100 ^m mesh is useful in estuaries as small zooplankton respond to environmental variability more rapidly than larger zooplankton (see Sections 3.7 and 4.8, 4.9). Many larval fish biologists use 500 ^m mesh, knowing full well that fish eggs and small, unidentifiable larvae will be extruded through the mesh. Ultimately, net size should be determined in accordance with the objectives of your study.

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Figure 4.4 Plankton nybolt mesh at the same magnification. a) 15 pm, 10% free area, b) 48 pm, 31% free area, c) 150 pm, 51% free area, d) 250 pm, 44% free area, e) 500 pm, 39% free area, f) typical design for a 40 cm diameter ring net, g) mouth of a plankton net showing the bridle and attachments, h) the cod-end of the net, showing the thread made to suit the sampling jars.

Figure 4.4 Plankton nybolt mesh at the same magnification. a) 15 pm, 10% free area, b) 48 pm, 31% free area, c) 150 pm, 51% free area, d) 250 pm, 44% free area, e) 500 pm, 39% free area, f) typical design for a 40 cm diameter ring net, g) mouth of a plankton net showing the bridle and attachments, h) the cod-end of the net, showing the thread made to suit the sampling jars.

Vertical hauls provide a depth-integrated plankton sample, and are useful for broad-scale spatial surveys of microplankton (less than 200 ^m, small zooplankton and phytoplankton). The vessel must be stationary, and the net is either hauled up from a specified depth (an up-cast), or a heavy

Figure 4.5 Types of plankton net, bridles and deployment. a) A standard plankton net configuration, with a two-point bridle and a depressor (note flow meter); a high speed plankton net with a sampling cone is illustrated, b) a bongo net sampler with no effects of the bridle, c) two neuston net samplers illustrating the robust four-point bridle and box neuston net sampler, d) gear for vertical hauls using a drop net or lift net (that is, down-cast and up-cast).

Figure 4.5 Types of plankton net, bridles and deployment. a) A standard plankton net configuration, with a two-point bridle and a depressor (note flow meter); a high speed plankton net with a sampling cone is illustrated, b) a bongo net sampler with no effects of the bridle, c) two neuston net samplers illustrating the robust four-point bridle and box neuston net sampler, d) gear for vertical hauls using a drop net or lift net (that is, down-cast and up-cast).

Figure 4.6 Some plankton collection gear. a) A square surface neuston net, b) a successful phytoplankton collection, c) deploying a ring net over the stern, d) beginning to tow the net in a circle to avoid sampling the propeller wash, e) retrieving a plankton light trap after a night's sampling.

Figure 4.6 Some plankton collection gear. a) A square surface neuston net, b) a successful phytoplankton collection, c) deploying a ring net over the stern, d) beginning to tow the net in a circle to avoid sampling the propeller wash, e) retrieving a plankton light trap after a night's sampling.

metal ring (10-20 kg) carries the net down to a specified depth (a drop net or down-cast; see Figure 4.5d).

Zooplankton is collected horizontally by slowly towing the net at a constant speed - around 1-2 metres per second (Figure 4.6). Any faster will increase the extent of extrusion, and any slower may increase the incidence of avoidance. Nets may be fitted with a flow meter to determine the volume of water filtered (Figure 4.7), to then determine the number or biomass of zooplankton per cubic metre. For plankton sampling, you should be concerned with speed through the

Figure 4.7 a) two types of flow meter and b) reading the flow meter before and after each tow (note that the flow meter is located to one side of the opening).

water, rather than speed over the sea floor. You should tow for a constant period of time (between 3 and 10 minutes, depending on mesh size and the amount of debris in the water) for a number of practical reasons. A constant sampling interval reduces potential sources of error such as sleepiness or sampling by a variety of personnel. Sometime flow meters break during the tow, orjam or become tangled with debris and, rather than dumping an un-metered sample, the volume filtered can be estimated with reasonable precision from the tow duration.

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