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PRPOOS Net Deployment

Summary

Deployment of the PRPOOS (Planktonic Rate Processes in Oligotrophic Ocean Systems) net was first implemented on CalCOFI Cruise 0507NH and samples organisms > 202 µm within a vertical water column from 210 m to the surface. PRPOOS samples are primarily collected to analyze mesozooplankton within the California Current Ecosystem.

History

The original description of the method was written July 2005 by M. Ohman and S. Dovel. Changes to the method are listed below.

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Methods

1. Principle

A Planktonic Rate Processes in Oligotrophic Ocean Systems (PRPOOS) net, formerly known as the Soutar-Hemingway Animal Trap or “SHAT” (Kramer 1972), is a 202 µm mesh net that has a 50 cm diameter single frame opening and is 3 m in length. The PRPOOS net deployment is a vertical tow and is lowered into the water to 210 m. The descent rate is 40 meters per minute to depth, 20 seconds at depth and has an ascent rate of 50 meters per minute. Samples are collected for all stations on line 90.0 and 80.0, as well as stations out to and including station 70.0 on lines 86.7 and 83.3.

2. PRPOOS Net Deployment

2.1

Check with the bridge for depth at the beginning and have the winch operator deploy the PRPOOS net to 210 m. If the bottom depth is less than 210m, deploy net 10 m above the bottom.

2.2

Attach 170 lb. weight to the hydrowire and deploy weight a few meters below the surface to allow wire to steady. Attach wire clamp and PRPOOS cod end, slowly lower weight until the net is fully taut and then attach and secure frame to the hydrowire.

2.3

Check flowmeter numbers and if numbers vary from last cast then change numbers on the tow sheet or spin the flowmeter impeller forward or backward to match numbers on the tow sheet.

2.4

Have the winch operator zero the net opening at the surface and start the PRPOOS net to 210 m at 40 m/min. Start the stopwatch as soon as the net starts down and record start time. Sink time should be about 5 min 15 s for a standard 210 meter tow. When the net reaches depth, stop the watch and record sink time. After 20 seconds at depth, have the winch operator haul back the net at 50 m/min and start the stopwatch. Maintain 50 m/min until the net mouth breaks the water’s surface. Record wire angles with a hand-held inclinometer when the net is at depth, then again halfway back to the surface and finally at 10m. Angles should be 15 degrees or less during the ascent to the surface (ideally angles should be 0 degrees). If angles are too high then rinse net and repeat PRPOOS net deployment. Stop the watch when PRPOOS net reaches the surface and record total ascent time. This should be about 4 min 20 s for a standard tow.

2.5

Read and record flowmeter numbers.

2.6

Wash down net to congregate plankton sample into cod end. Only rinse from outside of the net. Never spray seawater inside the net; this might contaminate the sample or damage copepods inside the net or cod end. Once the PRPOOS net is washed down, remove it from the hydro wire and secure frame. Remove wire clamp from the hydro wire and secure the weight on deck.

2.7

Remove cod end and preserve the PRPOOS sample.

2.8

Complete interior label on waterproof stock and label for the jar lid.

3. Sample Preservation

PRPOOS samples preserved contain a 5% solution of Formalin in seawater, buffered with sodium tetraborate.

3.1

Concentrate sample using a 202 µm mesh draining sock if needed, rinse sample into a pint jar (quart jar if sample is large), and fill jar with seawater until it is about 90% full.

3.2

Add saturated sodium tetraborate (10 ml to pint jars and 20 ml to quart jars) to buffer the sample. Formalin is slightly acidic and the sodium borate will raise the pH of the sample to neutral.

3.3

Add 37% formaldehyde to each sample jar (25 ml to pint and 50 ml to quart jars). Put label in jar, tighten lid and carefully rotate jar to mix the preservative within the sample.

4. Flowmeter Calibrations

4.1

General Oceanic Flowmeter 2030R is calibrated before and after each cruise at 12 different speeds (14 – 36 seconds) over a 16 meter measured distance.

4.2

Calculate the average revs, average time in seconds, rev/sec and met/rev (total distance/rev) for each speed.

4.3

Obtain common slope and intercept for the flowmeter calibrations from pre- and post-cruise data.

x axis = 1/revs/sec (reciprocal of rev/sec)
y axis = met/rev

5. Sample Calculations

Once slope and intercept are defined, you can solve for MRev for each tow in which the specified flowmeter was used. By defining MRev for each tow, you can solve for VWS.

To calculate Revs per second:

RPS = TotalRevs / TowingAndAscentTime (s)

For calculating meters per one revolution, or MRev, (intercept and slope of regression line) for each plankton tow:

MRev = Intercept + (Slope / RPS)

To obtain the Volume Water Strained (VWS m3) through the tow:

VWS = (Revs) * (MRev) * (Area)
Revs = total flowmeter revs during the tow
MRev = meters per one revolution
Area = mouth area (m2) of frame opening (Πr2)

The PRPOOS net tow is a vertical tow from 210 m to the surface. The actual depth needs to be corrected when the hydrowire is > 0 degrees. The actual depth of the tow is calculated by:

ADT = (Cos(Radian of the Angle at Depth)) * (Wire out)

To calculate the Standard Haul Factor (10m2 / unit area):

SHF = (10) * (ADT) / VWS

For Standardizing Plankton Volumes:

(Raw Volume * (1000)) / VWS = Volume in ml/1000m3 of Water Strained

6. Equipment/Supplies

  • 170 lb. weight
  • Clipboard, stopwatch, ratchet wrench
  • Waterproof inside and outside labels
  • PRPOOS frame, hose clamps and hardware to secure flowmeter
  • PVC hard cod end, 202 µm strainers
  • 202 µm Nitex mesh, 50-cm diameter mouth opening, 3 m long tow net. This net is encased in a protective outer net made of 1000 µm nylon mesh.
  • 202 µm Nitex mesh plankton concentrating sock
  • Wire clamp
  • Pint and quart jars
  • Flowmeter: digital, mechanical, w/standard rotor (General Oceanics Cat. No. 2030R)

7. Reagents

  • Sodium tetraborate with filtered seawater for a saturated solution
  • 37% formaldehyde

8. References