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Zooplankton Measurements

EOSC 473-573 Biology 2

Plankton = organisms unable to swim against currents. Drifters.

(Hensen 1887) Zooplankton = heterotrophic plankton

What are zooplankton

Importance of Zooplankton • Hold key position in pelagic food web • Transfer energy from phytoplankton to higher

trophic levels • Affect fish recruitment • Mediate the removal of anthropogenic CO2 into the

deep ocean • Fuel the benthic community • Nano & microzooplankton: key players of microbial

loop

Zooplankton taxonomic diversity unparalleled

“in the plankton we may find an assemblage of animals more diverse and more comprehensive than is to be seen in any other realm of life.” (Hardy 1965)

Size range from 2.0 µm to 200 cm (> 30,000 species)

Zooplankton size spectrum

“Net” plankton - 20 µm to 200 cm

Holoplankton: spend their entire life as plankton

Meroplankton: spend

only part of their lifecycle, usually larval stage, as plankton

Systematic Overview Protozooplankton-protozoans

Metazooplankton-metazoans (> 20 μm) Flagellate Ciliates Foraminifera Radiolaria

Cnidaria

Siphonophora Hydromedusae

Scyphomedusae

Ctenophora (comb jellies)

Mollusca

Heteropoda (swimming snail)

Cephalopoda Veliger larvae

Pteropoda (sea slugs or sea angels)

Chordata

Polychaeta

Thaliacea (salps) Appendicularia

Chaetognaths (arrow worm)

Fish larvae

Annelida

Chaetognatha

Crustacea

Amphipoda

Euphausiacea Mysidacea

Isopoda Cladocera

Ostracoda

(seed shrimp)

Crustacea

Copepoda

Cirripidia (barnacle) larvae

Decapoda

Echinodermata

Echinoderm larvae

(starfish, sea urchins, sand dollars, and sea cucumbers)

Zooplankton Sampling Problems

1. Zooplankton in highly dynamic environment 2. Hard to sample the same population 3. Fixed station = different zooplankton

populations passing by Accompany every plankton sampling program

by hydrographic documentation

Sampling Design 1. Define purpose of zooplankton scientific

study • Sampling design will depend on purpose • Identify research questions,

hypotheses, objectives

Example Assessing relationship b/w size & vertical migration vs. determining zooplankton community grazing rate

Sampling Design 2. Identify potential dominant processes that

affect the zooplankton in study • Maybe physical, chemical, biological • Supporting information on the environment of

zooplankton • Collaborative effort between physical, biological, and

fisheries oceanographers

Examples Hydrography, currents, light, fluorescence, phytoplankton biomass, production, composition, fish distribution

Sampling Design 3. Define the temporal and spatial scales these

forcing processes are operating at • Physical, chemical, biological processes might be

occurring at different spatial/temporal scales, & are interlinked

• This will determine the appropriate scale of investigation

Stommel Diagram

Sampling Design

4. Prepare a detailed work plan • Define sampling locations • Determine sampling frequency • Establish sampling size (think about statistical

analyses of your data) • Decide which variables to measure • Decide which sampling methods to use

Collecting Zooplankton • Water bottles - small volume, few L • Underway pumping - 10’s L - 10’s m3 • Nets of all shapes and sizes - 10-1000’s m3

• Continuous Plankton Recorder (CPR) • Laser Optical Plankton Counter (LOPC) • Acoustics • In situ camera system

Research requirements and species of interest dictate the sampling method used

Net Sampling

Bongo Net

Closing Net

Multi Net

Conical-cylindrical

Conical Conical w/ mouth reducing cone

OUR Opening-closing net

Net Sampling

Advantages: low cost, towed from any type of vessel, ease of use

E.G. for sampling mesozooplankton, use a conical net: 200 μm pore size net, with an R = 6 & 0.5-0.75 m mouth diameter

Cod ends examples

Net Sampling Issues • Extrusion • Avoidance • Clogging A given net sample is representative of a

limited size range. This is dependent on mesh size and avoidance

Extrusion (net escape)

• Individuals smaller than diameter of mesh opening • Water pressure can extrude organisms > mesh

opening - Influenced by tow speed (0.7-1 m/s best) - Mesh size of 75% of carapace width of organism, this

catches 95% organisms at high speeds of 9-10 m/s (Nichols and Thompson 1991)

What minimum size of organism would a 200 µm net catch?

Atkinson et al. (2012)

mesozooplankton

macrozooplankton

1 2 3

Schindler’s sampler

• simple & cheap • near surface and bottom

• 20-30 l of water • shallow sampling • small zooplankton fraction • discrete sampling

Avoidance • Zooplankton can actively swim out of the net path • Most serious bias for meso & macrozooplankton • Net size dependent

Individual escape velocity increases proportionally with net radius and towing speed

• Few solutions as reaction distance and escape velocities of zooplankton are poorly known

Kaartvedt et al. (2012)

Micronekton net avoidance

To prevent/reduce net avoidance Of visual zooplankton (euphausiids, mysiids, and fish

larvae): – Sample at night – Paint frame and net dull dark colour, such as grey or blue

Creation of bow wave in front of net increases net

avoidance Obstructions in net mouth lead to avoidance

– When sampling organisms > 5 mm, use no bridle. E.g. Bongo net OR multi-net

Strobe light system (blinding plankton)

Clogging

Affected by: • Density and composition of suspended

material • Mesh size • Net filtration efficiency • Shape of net

– Conical ok, conical cylindrical best for very productive waters

Problems associated w/ clogging

• Water column not sampled uniformly • More organisms extruding b. of pressure differences b/w inside and outside of net Significant clogging if filtration efficiency drops below 85%

Filtration Efficiency (need no less than 85%)

Depends on R, the ratio b/w the open area of the net mesh VS. area of mouth opening

F =V/(A*D) V = volume filtered by the net A = area of net mouth D = Towing distance

F = ratio b/w actual volume vs. theoretical volume through mouth opening

R = ratio of open area of net mesh to

area of mouth opening

R = (α*β)/A

α = total area of net mesh β = porosity, open area fraction of mesh size, from gauze

manufacturers, average (~ 0.47) A = mouth area (pi*r2)

Note: Porosity ranging from 0.15 for a 20 µm nitex mesh (15% porosity) to 0.6 for a 1000 µm nitex mesh (60% porosity)

To avoid clogging, filtration efficiency should be > 85%, and then

• Recommended R is at least: For oceanic sampling: 3.5 For coastal waters: 6 Smiths, Counts, and Clutter (1968) developed an equation

to enable choice of best net design

Smiths, Counts, and Clutter (1968) equations: Log10R=0.37*Log10(V/A) - 0.47 for blue water Log10R=0.38*Log10(V/A) - 0.17 for green water Where R = ratio of open area of net mesh to area of mouth

opening V = volume to be filtered (mouth area X depth of plankton

haul) A = mouth area You can use this equation to calculate whether your

assumption of R (either 3.5 or 6) was appropriate given your deepest sampling station (ie. 150 m)

Volume Filtered Determination To obtain a quantitative estimate of

[zooplankton], the volume filtered during a tow must be known

Measured by a flowmeter, a propeller that rotates with flow of water and records the number of revolutions

Volume Filtered (V) Determination

V = (L/F) *A

L = length of H20 column, (# revolutions) F = calibration factor (ratio of revolutions

per meter), supplied by manufacturer A = net mouth area

Filtration efficiency (F’) of net can also be computed using flowmeter directly.

Measuring filtration efficiency of a net

Measuring filtration efficiency of a net On a calm day 1. Lower the empty net frame w/ flowmeter & haul it back up at

usual haul speed. Repeat 5 times and record # of revolutions (this is your N’)

2. Repeat with net attached, and record # of revolutions (this is your N)

1. Calculate F’ = filtration efficiency = N/N’ N = avg. # of revolutions with net N’ = avg. # of revolutions without net 4. This F’ can then be used to calculate actual volume filtered

during plankton haul from F’ =V/(A*D), so V = F’ x A x D

Flowmeter Flometer should be placed halfway between

the center of the net and the frame.

Towing paths of a net tow

1. Vertical

2. Deep horizontal 3. Oblique

Towing Methods Vertical

Ship stationary 10-15 kg weight Bring up at 0.7-1 m/s

Horizontal Ship is moving Range of depths and weights

Oblique Heavy depressor weight below net Out: ship moving at 2-3 m/s, wire out at 1 m/s In: ship moving at 1m/s, wire in at 1m/s

Sampling Considerations If wind > 10 knots, to prevent wire from

going under the boat, keep wind on the side of bow you are sampling from

When windy, the wire will be at an angle, how

do you measure the actual depth of your haul?

Sample Handling 1. Carefully rinse net with seawater to concentrate

every organism in cod end

2. If closing net, rinse only lower part

3. Screw-off cod end to pass specimens into jar (250-1000 ml glass or polyethylene)

4. Use filtered seawater to concentrate sample and rinse cod end

5. Label, Pickle 6. Rinse net with freshwater

Sample Preservation

Use 4 % buffered formalin – Concentrated formalin (37-40%) – Buffered by Borax – Add borax in a ratio of 2 g per 98 ml of 40%

formalin – This results in a pH of 8.2 , suitable for a

mixture of zooplankton – If too acidic, calcareous shells dissolve – If too basic, crustacean and gelatinous tissue

is damaged

How much formalin would you add to a 500 ml sample?

Sampling for Live Zooplankton

• Handle net, cod end gently • Tow upwards at an angle at low speeds, 0.1-0.5 m/s • Choose a fine mesh relative to size of organism • Avoid direct sunlight or bright deck lights • Have filtered seawater at ambient T and S, ready for

reception of animals • Plastic 4 L jars make optimal containers • Use large bore pipettes to transfer organisms from jar

into sorting petri dish