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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