Water quality - Guidance standard for the sampling of zooplankton from standing waters

This guidance standard describes general procedures for surveying zooplankton in standing waters for the purposes of water quality assessment and determination of ecological status.
Guidance on sampling procedures and the subsequent steps for preservation and storage are given. The sampling procedures provide estimate for species occurrence and their abundance (relative or absolute), including spatial distribution and temporal trends, for a given body of water. Calculation of biomass and production is made possible.
This method is restricted to the sampling of multicellular zooplankton that inhabit the pelagic and littoral regions of lakes, reservoirs and ponds. The sampling procedure may be also employed in slow running waters and canals.
NOTE   The field methods described are suitable for the collection of open-water plankton and littoral plankton species. They are inappropriate for the collection of littoral species that primarily live on or in the surface of sediments and on the surface of aquatic plants.

Wasserbeschaffenheit - Anleitung zur Probenahme von Zooplankton aus stehenden Gewässern

Diese Anleitung legt allgemeine Verfahrensweisen für die Zooplankton-Untersuchung in stehenden
Gewässern für die Zwecke der Gewässerbewertung und die Bestimmung des ökologischen Zustandes fest.
Anleitungen zur Probenahme und zu den anschließenden Schritten für die Konservierung und Lagerung
werden gegeben. Diese Verfahrensweisen für die Probenahme bieten eine das gesamte stehende Gewässer
umfassende Beurteilungsgrundlage für Artenvorkommen (relative oder absolute), einschließlich seiner
räumlichen Verteilung und zeitlicher Trends sowie die Berechnung der Biomasse und der Zooplankton-Produktion.
Dieses Verfahren gilt nur für die Probenahme vielzelliger Zooplankton-Arten, die die pelagischen und litoralen
Bereiche von Seen, Stauseen und Teichen besiedeln. Das Sammelverfahren kann auch in langsam
fließenden Gewässern und Kanälen angewendet werden.
ANMERKUNG Die beschriebenen Freilandverfahren sind für die Probenahme von Plankton-Arten der Freiwasserzone
und von Plankton-Arten des Uferbereichs geeignet. Sie sind nicht geeignet für das Sammeln der Uferarten, die primär auf
oder in der Sedimentoberfläche und auf der Oberfläche von Wasserpflanzen leben.

Qualité de l'eau - Guide pour l'échantillonnage du zooplancton dans les eaux stagnantes

La présente norme guide décrit les modes opératoires généraux pour la surveillance du zooplancton dans les eaux stagnantes a des fins d’évaluation de la qualité des eaux et de détermination de l’état écologique.
Des lignes directrices sur les modes opératoires d’échantillonnage et les étapes suivantes pour la conservation et le stockage sont fournies. Les modes opératoires d’échantillonnage donnent une estimation de l’occurrence des especes et de leur abondance (relative ou absolue), y compris la répartition spatiale et les tendances temporelles, pour une masse donnée d’eau. Le calcul de la biomasse et de la production est rendu possible.
Cette méthode se limite a l’échantillonnage du zooplancton multicellulaire qui peuple les régions pélagiques et littorales des lacs, réservoirs et étangs. Le mode opératoire d’échantillonnage peut aussi etre utilisé pour les cours d’eau et les canaux lents.
NOTE   Les méthodes de terrain décrites sont appropriées pour le prélevement du plancton de pleine eau et des especes de plancton littoral. Elles ne sont pas appropriées pour le prélevement d’especes littorales qui vivent principalement a la surface des sédiments, ou dans leur couche superficielle, et sur la surface des plantes aquatiques.

Kakovost vode - Navodilo za vzorčenje zooplanktona v stoječih vodah

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2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.DKWasserbeschaffenheit - Anleitung zur Probenahme von Zooplankton aus stehenden GewässernQualité de l'eau - Guide pour l'échantillonnage du zooplancton dans les eaux stagnantesWater quality - Guidance standard for the sampling of zooplankton from standing waters13.060.70Preiskava bioloških lastnosti vodeExamination of biological properties of water13.060.10Voda iz naravnih virovWater of natural resourcesICS:Ta slovenski standard je istoveten z:EN 15110:2006SIST EN 15110:2006en,fr,de01-september-2006SIST EN 15110:2006SLOVENSKI

EUROPEAN STANDARDNORME EUROPÉENNEEUROPÄISCHE NORMEN 15110May 2006ICS 13.060.45 English VersionWater quality - Guidance standard for the sampling ofzooplankton from standing watersQualité de l'eau - Guide pour l'échantillonnage duzooplancton dans les eaux stagnantesWasserbeschaffenheit - Anleitung zur Probenahme vonZooplankton aus stehenden GewässernThis European Standard was approved by CEN on 13 April 2006.CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this EuropeanStandard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such nationalstandards may be obtained on application to the Central Secretariat or to any CEN member.This European Standard exists in three official versions (English, French, German). A version in any other language made by translationunder the responsibility of a CEN member into its own language and notified to the Central Secretariat has the same status as the officialversions.CEN members are the national standards bodies of Austria, Belgium, Cyprus, Czech Republic, Denmark, Estonia, Finland, France,Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania,Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.EUROPEAN COMMITTEE FOR STANDARDIZATIONCOMITÉ EUROPÉEN DE NORMALISATIONEUROPÄISCHES KOMITEE FÜR NORMUNGManagement Centre: rue de Stassart, 36
B-1050 Brussels© 2006 CENAll rights of exploitation in any form and by any means reservedworldwide for CEN national Members.Ref. No. EN 15110:2006: E

EN 15110:2006 (E) 2 Contents Page Foreword.3 Introduction.4 1 Scope.5 2 Normative references.5 3 Terms and definitions.5 4 Principle.7 5 Equipment.7 6 Preserving solutions.10 7 Preliminary stages.11 8 Sampling procedure.11 9 Identification and records.16 10 Preservation and storage of samples.17 11 Quality assurance.18 Annex A (informative)
Preservation.19 Annex B (informative)
Example of a field data sheet.22 Bibliography.23

EN 15110:2006 (E) 3 Foreword This document (EN 15110:2006) has been prepared by Technical Committee CEN/TC 230 “Water analysis”, the secretariat of which is held by DIN. This European Standard shall be given the status of a national standard, either by publication of an identical text or by endorsement, at the latest by November 2006, and conflicting national standards shall be withdrawn at the latest by November 2006. According to the CEN/CENELEC Internal Regulations, the national standards organizations of the following countries are bound to implement this European Standard: Austria, Belgium, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.

EN 15110:2006 (E) 4 Introduction Zooplankton community structure provides information on a range of physico-chemical and biotic causative variables. These include pH- and acidification-related variables, toxic chemicals, phytoplankton structure and abundance (i.e. lake productivity), and intensity of fish predation. The effects of size-selective predation are well known and the size-structure of zooplankton communities can give valuable information of the fish community. Metazoan zooplankton (metazooplankton) constitute a large number of species within a range of total lengths of about 0,05 mm to 20 mm, but mostly < 2 mm. The main groups are the rotifers (Rotatoria), the cladocerans (Cladocera) and the copepods (Copepoda). Some shrimps (Natantia; e.g. Mysidae) and larvae of dipterans (Diptera, e.g. Chaoborus) may also be considered as part of the zooplankton fauna. Rotifers and crustaceans inhabiting the littoral of standing waters can also be grouped with the more strictly planktonic forms. Fish larvae, hemipterans (Heteroptera, e.g. Corixidae) and coleopterans (Coleoptera) are occasionally recorded in the plankton samples but are not considered as part of the zooplankton fauna. Procedures for sampling of protozooplankton (Protozoa) are not included in this standard.
Surveys of zooplankton have provided valuable information for the environmental monitoring of standing waters, because this group includes species which: a) occur in a wide range of standing waters over a large geographical area and at the same time have specific environmental requirements; b) are well known with regard to their geographical distribution and environmental requirements; c) have a generally high capacity for dispersal enabling them to respond rapidly to remedial actions; while d) sampling requires only a modest expenditure of time and equipment. WARNING — Working in or around water is inherently dangerous. This standard does not purport to address all of the safety problems, if any, associated with its use. It is the responsibility of the user to establish appropriate health and safety practices and to ensure compliance with any national regulatory conditions. NOTE According to the classification by Fryer [5] the assemblage long known as the Cladocera is split into four orders; Ctenopoda, Anomopoda, Onychopoda and Haplopoda. Cladocera is however used in this standard as a general descriptive term.

EN 15110:2006 (E) 5 1 Scope This guidance standard describes general procedures for surveying zooplankton in standing waters for the purposes of water quality assessment and determination of ecological status. Guidance on sampling procedures and the subsequent steps for preservation and storage are given. The sampling procedures provide estimate for species occurrence and their abundance (relative or absolute), including spatial distribution and temporal trends, for a given body of water. Calculation of biomass and production is made possible. This method is restricted to the sampling of multicellular zooplankton that inhabit the pelagic and littoral regions of lakes, reservoirs and ponds. The sampling procedure may be also employed in slow running waters and canals. NOTE The field methods described are suitable for the collection of open-water plankton and littoral plankton species. They are inappropriate for the collection of littoral species that primarily live on or in the surface of sediments and on the surface of aquatic plants. 2 Normative references The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. prEN 14996, Water quality — Guidance on assuring the quality of biological and ecological assessments in the aquatic environment. EN 25667-1, Water quality — Sampling — Part 1: Guidance on the design of sampling programmes (ISO 5667-1:1980). EN ISO 5667-3, Water quality - Sampling - Part 3: Guidance on the preservation and handling of water samples (ISO 5667-3:2003) 3 Terms and definitions For the purposes of this document, the following terms and definitions apply. 3.1 anoxic condition in which the concentration of dissolved oxygen is so low that certain groups of micro-organisms prefer oxidized forms of nitrogen, sulphur, or carbon as an electron acceptor NOTE As the oxygen concentration approach zero the concentration of hydrogensulfide (H2S), released by bacterial anaerobic (no oxygen present) activity, is increasing. The anoxic conditions markedly affect the cycling of other nutrients, ecosystem productivity, and the distribution of biota. 3.2 body of surface water discrete and significant element of surface water such as a lake, reservoir, stream, river or canal, part of a stream, river or canal, a transitional water or a stretch of coastal water [EC Directive 2000/60/EC] 3.3 dimictic lake lake with spring and autumn turnovers (temperate lake)

EN 15110:2006 (E) 6 3.4 epilimnion water above the thermocline in a stratified body of water 3.5 fixation protection from disintegration of the morphological structure of organisms 3.6 impact study investigation of the physical, physico-chemical and biological consequences of a given intervention in a body of water NOTE A study of consequences should be capable of forming a basis for the subsequent remedial measures. 3.7 habitat locality in which a plant or animal naturally grows or lives NOTE It can be either the geographical area over which it extends, or the particular station in which a specimen is found. 3.8 hypolimnion water below the thermocline in a stratified body of water 3.9 littoral zone shallow marginal zone of a body of water within which light penetrates to the bottom; usually colonised by rooted vegetation 3.10 metazoan multicellular animals that develop from embryos 3.11 metazooplankton multicellular zooplankton (see 3.21) 3.12 pelagic zone free body of water beyond the littoral zone 3.13 plankton organisms drifting or suspended in water, consisting chiefly of minute plants or animals, but including larger forms having only weak powers of locomotion 3.14 preservation protection from (bio)chemical degradation of organic matter 3.15 sampling site (sampling station) general area within a body of water from which samples are taken NOTE A station is defined in terms of its location (geographical position, depth) and invariant conditions (e.g. type of bottom in shallow-water areas) and is delimited on the basis of the accuracy with which these are given. In cases of doubt when sampling stations have to be re-identified, most weight should be placed on depth and type of bottom.

EN 15110:2006 (E) 7 3.16 stratified water standing water with temperature gradients resulting in an upper, warmer, isothermal layer floating on cooler, denser, usually also isothermal water NOTE Between the upper layer, the epilimnion, and the lower layer, the hypolimnion, is a transitional zone, the metalimnion (see thermocline). The thermal stratification may be very short-lived or persist for all of the warmer part of the year. Lakes with ice-cover during the cold season may show inverse stratification; an upper, cooler (< 4 ûC) layer floating on warmer water. Water has its highest density at 4 ûC and during stratification and inverse stratification the deeper water has a temperature of approximately 4 ûC. 3.17 subsampling collection of a sub-sample that consists of a known fraction of the total sample and that is representative of the quantity and species composition of the latter 3.18 thermocline (metalimnion) layer in a thermally stratified body of water in which the temperature gradient is at a maximum 3.19 trend monitoring study intended to reveal any changes in the ecological status of a body of water over time 3.20 turbidity reduction of transparency of water caused by the presence of undissolved matter 3.21 zooplankton animals present in plankton 4 Principle The sampling strategy adopted provides information on the current status of the metazooplankton community. The selection of sampling sites (numbers and location), sampling depth, time and frequency of sampling, size of samples and type of sampling gear is of great importance for the evaluation of the data collected. As a general guidance EN 25667-1 should be consulted. 5 Equipment There exist several overviews of the most widely used zooplankton sampling techniques and their advantages and drawbacks (e.g. [1], [2], [3], [7], [8] and [13]). This standard provides some general recommendations.

EN 15110:2006 (E) 8
Key a conical plankton net b Schindler trap c modified Ramberg sampler d Clarke-Bumpus sampler Figure 1 — Examples of different zooplankton sampling equipment 5.1 Qualitative sampling 5.1.1 Nets Nylon plankton nets of various dimensions and mesh sizes can be used for sampling (Figure 1a). It is important that nets should have a large filtering surface relative to their opening in order to ensure that filtering is as efficient as possible. A net with an opening diameter of 30 cm, for example, should have a length of about one metre as a minimum. A cylindrical net section above the conical part increase the filtering area compared with a conical plankton net with the same opening diameter and length. If both rotifers and crustaceans are to be analysed, a net with a mesh of about 40 µm to 50 µm should be utilised. Nets with meshes smaller than 40 µm will readily become clogged and their use should normally be avoided, although they may be useful in oligotrophic waters. If only crustacean plankton are to be analysed a mesh of 90 µm (max. 100 µm) can be used. If both rotifers and crustaceans, including large predatory species, are to be sampled with a reasonable degree of efficiency, the use of three nets with different mesh sizes are

EN 15110:2006 (E) 9 recommended: 45 µm for rotifers, 90 µm for most of the crustaceans, and 150 µm or more for the predatory species. NOTE All the mesh sizes mentioned in this standard should be regarded as for guidance only. Mesh sizes will also vary somewhat from manufacturer to manufacturer. 5.1.2 Other field equipment Winch with line-length counter or line with length markings fitted with a shackle or similar device to enable the line to be joined to the net. Draining cup with nylon netting, which is capable of being attached to the net either by means of a tightening strip or tape sewn into the net. The netting of the draining cup should have the same mesh size as the net. A draining cup with hose and hose clamp can also be utilised. Weight, e.g. a standard sounding lead weight. Spray bottle with water for rinsing out the net and draining cup. A small plastic funnel may be needed to transfer the sampled material to the sample bottle. 5.1.3 Optional equipment Portable echo sounder or depth meter to estimate the water depth at a sampling site. The former may also be used to quantify large zooplankton species. Global Positioning System (GPS) to define site location.
5.2 Quantitative sampling 5.2.1 Sampling equipment For this purpose various types of volume samplers (bottles/traps/tubes) can be used (e.g. Schindler-Patalas trap or modified Ramberg sampler) (Figure 1b and 1c). Plankton nets with a closure mechanism and built-in water flow-through meter (e.g. Clarke-Bumpus sampler) (Figure 1d), plankton pumps (e.g. [10] or [14]) and flexible tubes (e.g. [9], [11]) can be used for obtaining vertically or horizontally integrated quantitative zooplankton samples. Echo sounder records can be used in the field to quantify large zooplankton species (e.g. Chaoborus). Several of the most widely used quantitative zooplankton samplers have been described in detail by [2] and [3].
Where volume samplers are concerned, a sampler should be chosen that allows a free flow of water when the sampler is not closed. It should also be possible to close the sampler rapidly, and the sampler should be as transparent as possible (plexiglas walls) in order to prevent avoidance by large plankton species with good vision and mobility. For the same reason, it is advantageous to use a sampler that is not too small (minimum 5 l). In locations with large populations of algae (nutrient-rich lakes), however, it may be advantageous to use a smaller model of volume sampler (e.g. a tube sampler of 3 l). Such samplers may also be suitable for use in lakes in locations that require equipment to be carried over long distances. Motorised plankton pumps with continuous flow-through are recommended rather than hand-powered plankton pumps, because motorised pumps provide a regular flow, thus providing better estimates of the quantity and composition of the plankton. Large active plankton species are liable to be sampled less efficiently using a plankton pump than by other types of quantitative samplers. The opposite can be the case for small species. A large plastic funnel (diameter about 50 cm) at the end of the sampling tube may be useful in order to prevent escape of “jumping” copepods. For practical and safety reasons, when deep lakes are being sampled, it may be more appropriate to use sampling equipment that allows efficient sampling of the whole water column (e.g. a plankton net with a closure mechanism or a plankton pump) rather than a volume sampler.

EN 15110:2006 (E) 10 When shallow lakes or littoral areas with a great deal of vegetation are being quantitatively sampled the use of a volume sampler, plankton pump or flexible tube is recommended. 5.2.2 Other field equipment The equipment listed in 5.1.2 (excluding the lead weight) and in addition a mixing vessel (e.g. plastic bucket or similar) to combine a number of individual samples into a single sample in the field. Conducting mixed samples may be necessary to reducing analysis times and costs. If a volume sampler is being used (with the exception of a Schindler-Patalas trap) filtration equipment is also required to concentrate the samples. This may take the form of either a plankton net (mesh size 45 µm, or 90 µm if only crustacean plankton are being collected) or a large funnel with draining cup fitted with a netting. 5.3 Storage Bottles (100 ml, 200 ml or 250 ml brown bottles with screw-tops) or glass vials for storing samples. Labels or tape to attach to the outside of the sample bottles. Waterproof paper for labels to put inside the sample bottles. Marker pen. If ethanol is being used, an alcohol-proof pen or pencil is recommended for both internal and external marking. NOTE Plastic vials are not suitable for storing samples if Lugol’s Iodine is used as preserving solution. 6 Preserving solutions 6.1 General A number of different preserving solutions with different areas of application are available. The advantages and disadvantages of each of these solutions are defined in Annex A. 6.1.1 Formaldehyde, 37 % by volume. This is neutralised, e.g. with hexamethyl tetramine (C6H12N4). Dilute the formaldehyde with water to 20 % (v/v) to avoid precipitation, and then add 100 g of hexamethyl tetramine and 40 g to 80 g sucrose ([6]) per litre of 20 % formaldehyde. WARNING — Formaldehyde may trigger allergies or cancers, and should therefore be handled with care. 6.1.2 Ethanol, 96 % or 99 % C2H5OH. 6.1.3 Lugol’s Iondine Acidified Lugol’s Iodine: Dissolve 100 g KI (potassium iodide) in 1 l of distilled or demineralised water; then add 50 g iodine (crystalline), shake until it is dissolved and add 100 ml of glacial acetic acid. As this solution is close to saturation, any precipitate should be removed by decanting the solution before use. Most preservatives are also commercially available. For more details on the use of different preservatives the reader is referred to Annex A. Preserving solutions for field use should be kept in small stoppered bottles and should be accompanied by a pipette for transferring the solution to the plankton samples. The bottles should be kept in a plastic box or container with lid during transportation.

EN 15110:2006 (E) 11 7 Preliminary stages 7.1 Documentation of strategies and methods The following documentation should be available before the start of field work:  description of objectives and strategy;  description of methods;  safety instructions;  personnel plan;  overview of equipment and instruments;  registration forms;  procedures for the maintenance of records and samples;  quality assurance requirements according to prEN 14996. 7.2 Preparation of sampling equipment After each day of sampling, the net should be washed in warm freshwater with detergent or in an ultrasonic water-bath in order to reduce clogging and ensure optimum filtration capacity. Check that the netting in the plankton net and draining cups is completely free of holes and tears. Check that the line is securely attached to the plankton net/volume sampler. Check that the plankton sampler’s closing mechanism is functioning well and that any seals are in order. It may be advantageous to label the sample bottles and to add the requisite amounts of preserving solution to them before the start of fieldwork. In order to prevent spreading of flora and fauna between water bodies, the sampling equipment should be disinfected between uses in the different waters. 7.3 Safety instructions Before initiating the survey notify a contact of which localities and areas are to be surveyed on a specific day. If the samples are being collected from a boat, always have a shore-based contact in case of emergencies. Check the weather forecast in order to ensure safe and effective surveying conditions. For safety reasons, it is recommended that surveys should not be undertaken by lone workers but by a minimum of two people. 8 Sampling procedure 8.1 Investigation program An investigation programme shall be developed according to the investigation aims, required precision of results, hydromorphological conditions in the area, prior knowledge of local pollution sources, results of

EN 15110:2006 (E) 12 previous investigations and any other factors that may be of significance. The choice between qualitative and quantitative methods is made according to the aims of the investigation and the required precision of results. Qualitative samples (net hauls) provide information about the species composition, number of species, size distribution and relative dominance of species and groups of zooplankton. Quantitative samples also provide information regarding the quantity of zooplankton (individual density) per unit volume. Quantitative samples also allow calculation of biomass and production for the zooplankton assemblage as a whole as well as for the individual species. In production studies (estimates of biomass and secondary productivity) it is essential to obtain quantitative samples. If the objective is to give a species list as complete as possible, net hauls are also recommended.
The vertical and horizontal distribution of zooplankton is uneven. Some species are found regularly in the shallow-water regions with stands of aquatic vegetation whereas others perform horizontal migration in the course of the day. Vertical patterns and vertical migration is also common among zooplankton species. To obtain whole-lake estimates of species composition and abundances, samples should be taken from both pelagic and littoral areas as well as from different depths. NOTE Standard plankton nets are not suitable for quantitative sampling and usually give a less accurate estimate on species composition than quantitative zooplankton sampler. 8.2 Number and location of sampling sites 8.2.1 General consideration The number and location of the sampling sites should be determined according to the aim of the study, the morphology of the water body and the level of accuracy in the provided estimates. In general, sites selected should be representative of the area under consideration. 8.2.2 Pelagic samples The samples are normally collected at the same site as used for other observations (temperature, Secchi depth, water chemistry, phytoplankton, etc.). The deepest area near the centre of the main basin of the lake is usually preferred as the sampling location, if a single location is regarded as sufficient for the purposes of the study. If this is not known, the maximum depth of the lake should be estimated, by use of a portable echo sounder or a depth-meter, prior to the sampling. In large lakes, and lakes with several more or less separate basins or with a complex morphology, it will often be desirable to have several sampling locations in order to obtain an impression of any intra- or inter-basin differences. It is recommended that a minimum of one station should be established in each basin. If, for example, the effects of point discharges are to be assessed, it may also be appropriate to select a location, which is not near the centre of the main basin of the lake, and/or to set up more than one sampling station. Generally, samples of strictly pelagic species should be collected at a good distance from the shore in order to avoid as far as possible any influence from the littoral fauna. Zooplankton are often irregularly distributed in lakes, i.e. there is often a horizontal and a vertical variation. If it is important, to the aims of the study, to obtain information regarding the horizontal distribution of the zooplankton, samples should be obtained from several locations along a gradient that represents the dominant wind direction. Other horizontal gradients may also be considered. The vertical variation is discussed in 8.3. If information regarding spatial variation or a high level of precision in the estimates is required, it may be necessary to draw up a sampling programme adapted to the lake morphology and vertical stratification (see 8.4).

EN 15110:2006 (E) 13 8.2.3 Littoral samples Such samples are collected from the lake’s shallow-water regions (usually at depths of < 1 m). Samples are taken from areas containing aquatic vegetation (protected shores) and from areas with little vegetation (exposed shores). In large lakes with a wider range of aquatic vegetation, it is recommended that separate samples should be collected from areas with different types of aquatic vegetation, prioritising the dominant types. 8.3 Sampling depth 8.3.1 General consideration Samples may be collected individually from fixed depths, or as combined samples where samples from different depths are mixed together to form a single sample. A third option is to use integrated samples, where sampling encompasses the entire water column within fixed depth intervals. It can be difficult to decide upon the appropriate sampling approach or exact sampling depths in each individual case. Zooplankton usually are distributed unevenly. More information on how to deal with spatial variation is given in 8.4. 8.3.2 Pelagic samples The samples are collected from specific depths or from the whole watercolumn, depending on the objectives of the study and the depth of the sampling station. In general, it is most important to have a relatively dense network of observations from the surface and down to the thermocline, as the majority of the plankton is usually to be found at these depths in the water column (epilimnion). In clear lakes, zooplankton is usually to be found at greater depths than in humic (“brown-water”) lakes or water with high turbidity. Vertical net hauls should cover all depths in order to include deepwater species. In locations with large quantities of algae and other particles the net will easily become clogged and an additional sample should therefore be taken from the uppermost 1 m to 5 m. In order to avoid stirring up sediments and contaminating the samples, the deepest samples should be taken no lower than 0,5 m from the sediment surface. If the greatest depths of the lake are anoxic, the samples should be collected down to this zone. If the objectives of the study make it important to obtain information regarding the vertical distribution of the zooplankton, it is recommended to collect samples at a number of depths. The samples should be taken at intervals of no more than 2 m in the epilimnion and metalimnion. In the hypolimnion samples should be taken at less frequent intervals. In de

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