Sorting a zooplankton sample

The laboratory analysis should also be guided by what the investigator requires, and by the budget. Sorting and identifying zooplankton to a reasonable degree of accuracy is arduous and may take 1-4 hours per sample. Could your question be resolved by zooplankton biomass or by identifying to the level of phylum, family or genus? Perhaps only the Crustacea - the greatest phytoplankton consumers - need to be identified. Or is a size analysis sufficient? Will you sort two or three sub-samples, or do you plan to sort the entire sample for fish larvae only? You should prepare a sorting data sheet to complement the field data sheet (Figures 4.10, 4.11).

The sample should first be rinsed in a sieve (of the same or smaller mesh of the net) to remove formaldehyde solution, and to remove/rinse grass and sticks. Rinsing with cold fresh water is perfectly adequate for preserved plankton. Gelatinous zooplankton should be counted and removed at this stage, and recorded on your field data sheet (Figure 4.10). Then carefully rinse the plankton from the sieve into a beaker or a 100 mL volumetric cylinder (if necessary make up the volume to 100 mL). With bulky samples, especially with detritus, a 200 or 500 mL cylinder may be necessary. Allow a uniform time period for the plankton to settle (about 1 hour), and read off the approximate displacement volume (that is, the approximate volume in

FIELD DATASHEET

Crew: Date:

Location/GPS: _ Station:_

Depth:

Weather:

Wind speed/direction:

Air temp:_

Water @ start:

Temperature/Salinity:

Comments:

Secchi depth:

Sampling gear:_

Flowmeter: Flowmeter: Flowmeter: Flowmeter:

Temperature/Salinity: pH:_

Comments:

_ Secchi depth:

Figure 4.10 A typical plankton field sampling data sheet.

millilitres of zooplankton - normally zooplankton is added to the water). Detritus tends to sink slower than zooplankton, while any sand grains will sink faster, enabling you to estimate the actual zooplankton biomass.

After you have recorded the displacement volume, thoroughly mix the zooplankton in the volumetric cylinder, and while still swirling remove an accurate 2 or 4 mL sub-sample with a pipette (with the fine tip cut off, Figure 4.12). Thus you have removed 2 or 4% of the total sample, such that

LABORATORY DATA SHEET

Sorter's name:_Date:_

Gear and mesh: _Tow duration/speed:

LABORATORY DATA SHEET

Sorter's name:_Date:_

Gear and mesh: _Tow duration/speed:

Sample#

Comments: (sub-sample?)

Sample#

Comments: (sub-sample?)

copepods calanoid cyclopoid harpacticoid

bivalved crustaceans ostracod cladoceran

crab larvae

amphipod isopod

nauplii

elongate crust. krill mysids penaeids Jaxea

polychaetes

chaetognaths

pelagic snails

bivalve molluscs

Obelia

larvaceans

salps

other gelatinous

fish eggs

fish larvae

large jellies, ctenos, algae?

Figure 4.11 Possible laboratory data sheet.

Figure 4.11 Possible laboratory data sheet.

you multiply your counts by 50 or 25 to get an estimate of total number. The volume of the sub-sample should be determined by the density of zooplankton and the time it takes to sort. It is better to take two or three 1 mL sub-samples, rather than one 3 mL sub-sample, as the variance due to sub-sampling error can be incorporated into your analysis. (Remember to account for the fact that the second and third sub-samples are not the

Figure 4.12 Typical plankton sorter's equipment showing a) volumetric cylinders for determining settlement volume, b) a blunt-ended pipette with a deliverer to take a quantitative sub-sample from the well-mixed sample thoroughly suspended in 100 mL or 250 mL of clean tap water (a nonquantitative Pasteur pipette is included), c) a plankton splitter for dividing a plankton sample into half, thence a quarter, and eighth, and so on, d) an 'S' tray for counting samples, e) a series of stacked home-made sieves to size-sort plankton with 300, 200 and 100 ^m mesh.

Figure 4.12 Typical plankton sorter's equipment showing a) volumetric cylinders for determining settlement volume, b) a blunt-ended pipette with a deliverer to take a quantitative sub-sample from the well-mixed sample thoroughly suspended in 100 mL or 250 mL of clean tap water (a nonquantitative Pasteur pipette is included), c) a plankton splitter for dividing a plankton sample into half, thence a quarter, and eighth, and so on, d) an 'S' tray for counting samples, e) a series of stacked home-made sieves to size-sort plankton with 300, 200 and 100 ^m mesh.

same proportion of the total as the first - although the error introduced by ignoring it is minor compared with other factors).

The sub-sample is best sorted and identified in a Bogarov tray or an S-tray (a perspex square with a 1 cm deep trough milled into it, Figure 4.12d), or in a plankton ring (a perspex ring that can be rotated under the microscope). Your laboratory data sheet should be beside you (Figure 4.11). Some fine probes are useful in turning individuals to identify them (Box 4.8). Your counts could be dictated onto tape if you wish, and thence transferred onto the spreadsheet, where you can insert the necessary formulae to correct for sub-sampling and the total volume filtered (below). The remaining sample may be scanned for any large or interesting plankton, before storing it in 2% formaldehyde in fresh water.

BOX 4.8 FABRICATION OF TUNGSTEN WIRE PROBES

Tungsten wire probes are very fine and firm needles for sorting tiny plankton. The wire may be sharpened by electrolysis (Conrad et al. 1993). A mild electric current is passed between a 3 cm length of wire and an electrode immersed in al.OM solution of sodium hydroxide (20gof NaOH pellets in a litre of water). With an electric current, the tungsten tip is delicately dissolved only as it is dipped into the solution. You will need a source of magnification to observe and regulate the sharpening. The rate of electrolysis is proportional to the surface area of the wire, the amount of current and the concentration of NaOH. A microscope's AC light source can provide a variable current, with alligator clip leads. Once the wire is sharpened, the other end may be glued or fixed onto handles. You need to exercise usual care with all aspects of the process, including handling the caustic solution, using the electric current and handling the sharp needles.

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