Sediment traps are the only tool for directly collecting the rain of sinking particles in the ocean. They are largely uncalibrated in the field and there are significant unresolved questions on the accuracy and precision of sediment traps. Any investigators that decides
to use sediment traps should become aware of all facets of this controversy and make their own decisions about the appropriate methods to use. The U.S. JGOFS Planning Report #10 provides an overview of these issues and there have been significant published papers on trap accuracy since that report.

2.0 Definition
2.1 Total particulate mass flux is defined as the amount of sinking particulate matter passing through a depth level as:
        Total Mass Flux= mg dry weight / m² / day

2.2 Total particulate carbon flux is defined as the amount of sinking particulate organic carbon passing through a depth level as:
        Total Organic Carbon Flux= mg carbon / m² / day

2.3 Total nitrogen mass flux is defined as the amount of sinking particulate organic nitrogen passing through a depth level as:
        Total Organic Nitrogen Flux= mg nitrogen / m² / day

3.0 Principle of Analysis
Fluxes of sinking material are measured using sediment traps (Knauer et al. 1979). In this case, these are simple cylinders suspended at various depths from surface and subsurface floats. These cylinders collect sinking particles. It is assumed that the collection of particles is linearly related to the aperture area of the sediment trap and that this collection is an accurate estimate of the mass of sinking particles at that depth and the particle sinking speeds. Hydrodynamic and other biases influence the collection of material by
sediment traps and the interpretation of trap data should be approached with caution.

4.0 Apparatus
4.1 Particle Interceptor Traps (PITs). The particle collection device central to the Multi-traps is a polycarbonate cylinder (cross-sectional collection area = 0.0039 m² ). The cylinder is equipped with a base which holds a 90 mm Poretics polycarbonate membrane filter. A PVC drain valve is mounted under the base of the filter holder. At the surface of the cylinder, plastic baffling consisting of circular openings 1.2 cm in diameter provide turbulence reduction at the trap opening. The cylinder also possesses two rings around its center which allow for mounting of the cylinder onto the PVC cross described below.

4.2 Cross. A PVC cross with cutouts to fit the PITs allows for mounting of up to 12 PITs at each depth. The cross is secured to the premeasured 1/2 inch polypropylene line by means of U-bolts which clamp onto the line and by 1/4 inch safety lines secured to the trap line below the cross with hose clamps. The prepared PIT cylinders are held in place on the cross by bungi cord retainers. Crosses with PITs are attached at
3 depths: 150, 200 and 300 meters.

4.3 Flotation Gear. At the surface the polypropylene line is attached to a stainless swivel, which is attached to a stainless steel chain with two 17 inch diameter glass floatation spheres covered by a polyethylene “hard hat” housing. The floats are attached to a 10 m double length of 1/2 inch bungi cord connected to a 5/8 inch double  raided Duralon line with 8 orange polypropylene A2 floats. The entire flotation array is secured to the surface spar.

4.4 Surface Spar. The surface spar consists of a styrofoam core float with a central mast on which is mounted a VHF radio beacon (Novatech), strobelight (Novatech), and ARGOS transmitter.


4.5 Current Meter. The ambient flow at the trap mouth should be monitored for every trap depth. Any of a variety of commercial or custom built flowmeters can be used. At a minimum, the package should measure the current speed and direction, pressure and tilt. It should be able to resolve the high frequency variability in flow, pressure and tilt that might be transmitted down the line from surface wave motions. If only one flow package is available, it should be placed at the depth of optimum interest (usually the base of the euphotic zone).

5.0 Reagents
5.1 Hydrochloric acid (12N: Baker Instra-Analyzed): For making cleaning solutions

5.2 Formalin (reagent grade)

5.3 Sodium chloride (reagent grade)

5.4 Density Gradient Solution. A density gradient solution is used to reduce advective-diffusive exchange of trap contents with ambient seawater during deployment. The density gradient solution is prepared by adding 1 l formalin and 2.5 kg NaCl to 50 l seawater yielding a 2% formalin and 50g/l NaCl solution. The solution is gravity filtered through a 0.5 mm cartridge membrane filter (Millipore). A 1 liter portion of this gradient is saved for subsequent processing steps (see below). The PITs are filled with the density gradient solution and covered until deployment. All of these steps are controversial. Arguments persist about the amount of salt to add, the type of fixative, the height in the tube to fill with brine, etc.

6.0 Sampling
6.1 Pre-sampling preparation:

6.1.1 Filter Preparation. Poretics polycarbonate membrane filters (90 mm diameter, 0.8 mm pore size) are soaked overnight in 1.2N HCl (Baker Instra-Analyzed), rinsed with 1.2N HCl, rinsed three times with Milli-Q water, and then put in individual plastic petri dishes. The cleaned filters are oven dried (65° C for a couple of days), allowed to cool in a desiccator, and tared to constant weight on an analytical balance (Sartorius R160P).

6.1.2 Trap Cleaning Procedure. The porous polyethylene filter frit is rinsed in Milli-Q, soaked for 24 hours in 1.2N HCl, and rinsed with Milli-Q three times. All other trap parts are soaked overnight in a 5% dilution of Aquet Manostat detergent, rinsed thoroughly in tapwater to remove the detergent, soaked 24 hours in 0.6N HCl, and then rinsed in Milli-Q. The PITs are assembled while wearing latex gloves. The prepared Poretics filters are attached to the base of the polycarbonate cylinders together with the porous filter frit and covered by the filter holder with the PVC drain valve. Polyethylene tape is used to provide a leaktight fit of the filter holder to the cylinder. The assembled PITs are stored in plastic bags until used.

6.2 Deployment and Recovery:

6.2.1 Deployment. The trap array is deployed for a minimum of 72 hours. Triplicate PITs are deployed at each of three depths (150, 200, 300 m). A non-functioning fourth PIT serves as a counterweight to balance the cross. Generally the array is deployed as the first cruise procedure (see Chapter 2). The location of the trap is checked periodically during the deployment.

6.2.2 Recovery. The traps are covered with polyethylene gloves before they are removed from the cross. The seawater at the top of the trap is siphoned off to just above the level of the visible density interface using acid-rinsed (0.6N HCl) Teflon tubing. The density gradient solution is drained through the bottom
of the trap and discarded. The Poretics filter is removed, returned to its petri dish, sealed with Parafilm and labeled. The filters are stored in the refrigerator until analyzed.

7.0 Sample Processing Procedures
7.1 Picking Swimmers. The “swimmers” (recognizable zooplankton) are removed using forceps under a dissecting microscope (12–50 power magnification). The filters are kept wet during this period by adding small volumes of the saved density gradient solution (see above). The zooplankton (down to less than 100 mm in size) should be removed with very fine-tipped forceps and placed into small vials with some of the
reserve trap preservative. They can later be used to assess the effectiveness of swimmer removal (see below). Manual removal of swimmers is a time-consuming process and still may leave significant swimmer material behind (e.g. see Michaels et al., 1990). It is superior to screening or other indirect methods. Screening can remove very large passively sinking particles and will not remove swimmers that are smaller
than the mesh. Picking swimmers is also a subjective exercise. Some labs remove only the largest zooplankton and some attempt to pick the samples at sea where the ship motion reduces the ability to discern the smaller zooplankton. As there is no absolute standard to compare sediment traps with, there is no absolute way to determine the effectiveness of the swimmer removal by any lab. In the BATS deploy-ments,
it generally takes 1-12 hours to remove the swimmers from each PIT tube after a three day deployment in that oligotrophic regime. (see below for additional techniques to assess the swimmer problem).

7.2 Mass Flux. The material on the filter is scraped into a bolus at the center of the filter with a scalpel and salts are removed by rinsing with Milli-Q water adjusted to pH 9 with ammonium hydroxide. The filter with the sample bolus is oven dried (65°C), placed in a dessicator and weighed daily until weight is constant for 2 consecutive weighings.

7.3 Particulate Carbon and Nitrogen Analysis. Carbon and nitrogen analyses are performed using a Control Equipment Corporation (CEC) 240 XA elemental analyzer calibrated with acetanilide. The bolus is scraped off the filter with a scalpel and ground in an agate mortar. The whole sample (50-300 mg) is transferred to a silver boat and weighed on a CAHN Electrobalance (Model 4400). The silver boats are put in wells drilled in a Teflon block, and fumed with concentrated HCl for 36 hours to volatilize inorganic carbon. The fumed boats are desiccated overnight and then analyzed for total nitrogen and organic carbon. The results from the C/N analysis yield %C and %N.

8.0 Calculation and expression of results.
8.1 Mass flux. The mass flux is calculated as follows: The mass weight minus the tare weight of the filter divided by the number of days deployed and the by the trap cross-sectional area (0.0039 m² ) equals the mass flux (mg m^-2 d^-1).
        Mass Flux ( mg/m2 /day) = (mass weight - filter weight)  /  ( days deployed * trap area)

8.2 Particle flux. C/N analysis yield %C and %N determinants. Particulate flux (mgN or mgC m -2 d -1 ) is then calculated by multiplying the %C or %N by the mass flux.
        Particle flux (mgN or mgC) = Mass flux * %C (or %N)

9.0 Quality Control and Assessment
9.1 Hydrodynamics. Although there are few field data, published reports indicate that flows above 15 cm/s at the trap mouth probably cause biases in trap collection. There is a large, but insufficient, literature on trap hydrodynamics (see U.S.JGOFS Planning Report # 10).

9.2 Swimmers. The effectiveness of swimmer removal can be determined by examining a replicate PIT sample (different tube) with different techniques. The swimmer tube(s) should be deployed in the same way as the mass flux tubes. On recovery, the entire tube contents (after siphoning the upper, exchanged solution) should be transferred to a sample bottle (about 1 l of liquid). This solution should be allowed to set-tle
for a few days, then the supernatant gently siphoned off. By repeating this process, the sample can be gently concentrated down to a manageable volume (size will depend on the amount of material). This sample can then be counting in much the same way as a plankton tow. The numbers and sizes (values that can be converted to biovolumes or carbon units) of zooplankton can be counted on both a dissecting microscope and an inverted compound microscope using quantitative techniques. The picked swimmers from each of the mass flux traps can then be counted with the same techniques (they should have been saved after removal from the filters). By comparing the zooplankton in the complete sample(s) with the zooplankton actually
removed, the biovolume of unremoved zooplankton can be calculated. Some zooplankton from each of the dominant unremoved swimmer taxa should then be measured for biovolume and carbon content to create a conversion factor for relating the unpicked biovolume to the total measured carbon. This allows a first-order correction for the residual swimmer problem. In practice it is often of similar magnitude as the passive flux in shallow traps (Michaels et al., 1990).

10.0 References and Related Literature
Baker, E. T., Milburn, H. B. and Tennant, D. A. (1988). Field assessment of sediment trap efficiency under varying flow conditions. J. Mar. Res. 46: 573-592.
Coale, K. H. (1990). Labyrinth of doom: A device to minimize the “swimmer” component in sediment trap collections. Limnol. Oceanogr. 35: 1376-1381.
Honjo, S. (1978). Sedimentation of materials in the Sargasso Sea at 5367m deep station. J. Mar. Res. 36: 469-492.
Hurd, D. C. and Spencer, D. W. (1991). Editors; Marine Particles: Analysis and Characterization. Geophysical Monograph 63. AGU. Washington DC. 472p.
Knauer, G.A., J.H. Martin and K.W. Bruland. (1979). Fluxes of particulate carbon, nitrogen and phosphorus in the upper water column of the northeast Pacific. Deep-Sea Research 26A:97-108.
Knauer, G. A., Karl, D. M., Martin, J. H. and Hunter, C. N. (1984). In situ effects of selected preservatives on total carbon, nitrogen and metals collected in sediment traps.J. Mar. Res. 42: 445-462.
Lee, C., Hedges, J. I., Wakeham, S. G. and Zhu, N. (1992). The effectiveness of various treatments in retarding bacterial activity in sediment-trap material and their effects on the collection of swimmers. Limnol. Oceanogr. 37:117-130.
Michaels, A. F., Silver, M. W., Gowing, M. M. and Knauer, G. A. (1990). Cryptic zooplankton “swimmers” in upper ocean sediment traps. Deep Sea Res. 37: 1285- 1296.
US GOFS Working Group (1989). Sediment Trap Technology and Sampling. Planning Report No. 10. November 1988. WHOI, USA.
Verardo, D. J., Froelich, P. N. and MacIntyre, A. (1990). Determination of organic carbon and nitrogen in marine sediments using the Carlo Erba NA-1500 Analyser. Deep Sea Res. 37: 157-165.