Annex C-7 Mesozooplankton

1. Introduction
2. Purposes
3. Sampling
4. Preservation
5. Sub-sampling
6. Analysis
    6.1 Abundance
    6.2 Biomass
7. Data reporting
8. Quality Assurance
References 

1. Introduction

The mesozooplankton (0.2-20 mm) constitutes an important part of the pelagic food web, since they form the link between primary producers and higher trophic levels. Changes in phytoplankton biomass and species/size composition change mesozooplankton community structure and productivity. Such changes potentially influence fish stock recruitment and sedimentation (i.e. oxygen concentration in the bottom water).

2. Purposes

The sampling of mesozooplankton serves, inter alia, the following purposes:

• to describe the species composition and the spatial distribution of mesozooplankton abundance and biomass;
• to describe temporal trends (i.e. over several years) in mesozooplankton biomass and community structure.

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3. Sampling

Mesozooplankton should be sampled by means of vertical hauls using a WP-2 net of 100 µm mesh size. The WP-2 net should be hauled vertically with a speed of about 0.5 m/s.

The nets should always be equipped with flowmeters. They should be mounted at 1/4 of the diameter of the ring (UNESCO 1968). If the filtration capacity of the net is lower than 70% the sample should be discarded and a new sample taken after the rinsing of the net.

The weight to keep the wire vertical should be 25 kg (40 kg when the wire angle tends to exceed 25º, UNESCO, 1968).

The wire angle should always be reported. A correction table for sampling depth is given in Table C.7.1. If the wire angle exceeds 40º, the sample should be discarded. Records of wind speed should be kept.

For fractionated hauls the following intervals should be considered (Fig. C.7.1):

• bottom to halocline (included)
• top of halocline to thermocline (included)
• top of thermocline to surface

If there is no thermocline, a standard haul of 25-0 m should be made.

If there is no halocline, there should be a standard haul of 75 m to the thermocline (included) or to 25 m if there is no thermocline either.

If an anoxic bottom layer is present, sampling should be conducted above the anoxic zone.

No hauls shorter than 5 meters should be made.

In the Kattegatt and Belt Sea a standard haul of 25-0m should be made.

After collection of each sample the net shall always be rinsed by use of a gentle flow from a hose. When fractionated hauls are taken, only the part below the strap should be rinsed. After emptying, the whole net shall be rinsed with the cod-end open.

After each cruise, the net shall be washed in warm freshwater with a detergent to secure optimum filtration capacity.

When jelly-fish appear in the sample, it is recommended to discard the sample and take a new. When it is impossible to avoid jelly-fish, they should be rinsed from other mesozooplankton and then discarded. When applicable, these procedures should be recorded.

Table C-7.1. Correction of depth from wire angle

Figure C-7.1.

e. g.         haul 1:      5 – 61 m

               haul 2:    61 – 30 m

               haul 3:     30 –   0 m

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4. Preservation

The samples should be preserved in 4% formaldehyde solution (1 part 40% formaldehyde solution and 9 parts water). The formaldehyde has to be buffered to pH 8-8.2 with disodiumtetraborate (borax) (Na₂B₄0₃ . 10 H₂0). The samples should be stored until the subsequent assessment is completed.

5. Sub-sampling

A calibrated Stempel-pipette or a Kott Splitter is recommended. The Kott Splitter is somewhat better in precision but is more time-consuming to handle ( G. Behrends, A. Korshenko, pers.comm.).

For the work with Stempel-pipette the sample is concentrated by sieving or diluted with tap water as necessary. The volume of the sample is measured in a graduated glass/plastic ware.

A few drops of a detergent should be added to allow the cladocerans to mix in the sample

The sample should be mixed intensively until all organisms are distributed randomly in the sample volume. Non-random distribution in the sample during sub-sampling is the most important source of errors. Aggregations of organisms should be taken out of the sample and the organisms counted.

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6. Analysis

The microscopes used should have magnifications to at least 125 X.

6.1. Abundance

All specimen should be identified and counted until one has reached 100 individuals of each of the three dominating taxonomic groups (excluding nauplii, rotifers and tintinnids). If this figure is not reached in one subsample, additional subsamples must be counted. The taxonomic group(-s) that reached 100 individuals in the previous subsamples, need not be counted in the next subsample(-s). The precision of calculated abundance for organisms of the first three groups, that will be counted up to 100 specimens, amounts to 20% (Tables C-7.1 and C-7.2). The estimation of abundance for other groups ("tail") will be less precise (Cassie 1971, HELCOM 1988).

The term "taxonomic groups" includes species, genera, families and different developmental stages of copepods.

The abundance of nauplii, rotifers, tintinnids and meroplankton larvae can be estimated semi-quantitatively from the first subsample. The presence of macrozooplankton organisms and rare species can be noted after an overview of the whole sample.

Although macrozooplankton, nauplii, rotifers and tintinnids fall outside the size range of mesozooplankton, as do many of the meroplankton, there is a considerable amount of historic data on these groups. Thus they could be reported.

Table C-7.2. Lower and upper 95% confidence limits (in units and as a percentage) for number of counting specimens lower than 17 [4]

Table C-7.3. Lower and upper 95% confidence limits (in units and as a percentage) for number of counting specimens more than 17 (see also [2, 3, 4])

6.2 Biomass

The biomass factors for the different taxonomic groups and developmental stages should be used (Hernroth, 1985). The method of Standard Size Classes (Witek Z., G. Breuel, M. Wolska-Pyś, P. Gruszka, A. Krajewska-Sołtys, L. Ejsymont, D. Sujak 1996. Comparison of different methods of Baltic zooplankton biomass estimations. Proceedings of the XII BMB Sympozjum, Institute of Aquatic Ecology, University of Latvia: 87-92)** should be used if appropriate factor is missing. The improvement of present factors taking into account the seasonal and geographical differences in individual volume is an urgent QA task.

Direct measurements of ash free dry weight (AFDW) of ½ sample should be used. Samples, which have been deep frozen (- 18 C) on pre-weighted glass fibre filters (Whatman GF / C, d = 47 mm), should be dried at 60 C in an oven (Lovegrove, 1962, 1966) and ashed at 500 C.

**Reference to Standard Size Classess has been added instead of reference to table C.7.2. which is not available.

7. Data reporting

Data should be reported according to ICES data reporting formats.

8. Quality Assurance

[The quality assurance instructions should be according to the recommendations given in the SGQAB Report of 1998, section 10, part B. This work should be done by the BMB Zooplankton Working Group.

Additional items that should be included:

• participation in ring - tests;
• intercalibration of equipment;
• participation in scientific workshops when e.g. recent taxonomical changes can be implemented.]

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References

Cassie, R.M. 1971. Sampling and statistics. In: A manual on methods for the assessment of secondary productivity in fresh water. Ed. By W.T. Edmondson and G.G. Winberg. IBP Handbook No. 17, Oxford, 181 pp.

HELCOM 1988. Guidelines for the Baltic Monitoring Programme for the Third Stage. Part D. Biological Determinands. Balt. Sea Environ. Proc. No. 27D, 161 pp.

Hernroth, L. 1985. Recommendations on methods for marine biological studies in the Baltic Sea. Mesozooplankton assessment. BMB Publication No.10: 1-32.

Kozova, O.M. and Melnik, N.G. 1978. Instruction for plankton samples treatment by counting methods. Eastern Siberia Pravda, Irkutsk, 52 pp. (In Russian).

Lovegrove, T. 1962. The effect of various factors on dry weight values. Rapp.P.-v.Reun. Cons. int. Explor. Mer 153, 86-91

Lovegrove, T. 1966. The determination of dry weight of plankton and the effect of various factors on the values obtained. In: Some contemporary studies in marine science, pp. 462-467. Ed. by H. BARNES. London: Allen and Unwin Ltd.)

UNESCO, 1968. Monographs on oceanographic methodology. 2. Zooplankton sampling. Paris. 174 pp.

Veldre, S.R. 1961 Statistical verification of counting methods used for quantitative analysis of plankton samples. In: Application of mathematical methods in biology. Collected papers, Leningrad State University, Vol. 2: 124-131. (In Russian).

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