SUMMARY: Passively sinking particulate matter is collected using a free-floating sediment array and, after prescreening (335 µm) to remove zooplankton and micronekton carcasses, the sample materials are analyzed for C, N, P and mass flux (mg m-2 d-1).

1. Principle

Although most of the particulate matter both on the seafloor and in suspension in seawater is very fine, recent evidence suggests that most of the material deposited on the benthos arrives via relatively rare, rapidly sinking large particles (McCave, 1975). Therefore, in order to describe adequately the ambient particle field and to understand the rates and mechanisms of biogeochemical cycling in the marine environment, it is imperative to employ sampling methods that enable the investigator to distinguish between the suspended and sinking pools of particulate matter. This universal requirement for a careful and comprehensive analysis of sedimenting particles has resulted in the development, evaluation and calibration of a variety of in situ particle collectors or sediment traps. The results, after nearly a decade of intensive field experiments, have contributed significantly to our general understanding of: (1) the relationship between the rate of primary production and downward flux of particulate organic matter, (2) mesopelagic zone oxygen consumption and nutrient regeneration, (3) biological control of the removal of abiogenic particles from the surface ocean and (4) seasonal and interannual variations in particle flux to the deep-sea. Future sediment trap studies will, most likely, continue to provide novel and useful data on the rates and mechanisms of important biogeochemical processes.

At Station ALOHA, we presently deploy a free-drifting sediment trap array with 12 individual collectors positioned at 150, 300 and 500 m. The deployment period is generally 72 hours. The passively sinking particles are subsequently analyzed for a variety of chemical properties, including: total mass, C, N and P.

2. Precautions

Because particle fluxes in oligotrophic habitats are expected to be low, special attention must be paid to the preparation of individual sediment trap collector tubes so that they are clean and free of dust and other potentially contaminating particles. Traps should be capped immediately after filling and immediately after retrieval. Pay particular attention to airborne and/or shipboard particulate contamination sources. In addition, the time interval between trap retrieval and subsample filtration should be minimized in order to limit the inclusion of extraneous abiotic particles and the post-collection solubilization of particles.

3. Field Operations

3.1. Hardware

Our free-floating sediment trap array is patterned after the MULTITRAP system pioneered by Knauer et al. (1979) and used extensively in the decade-long VERTEX program. Twelve individual sediment trap collectors (0.0039 m2) are typically deployed at three depths (150, 300 and 500 m). The traps are affixed to a PVC cross attached to 1/2" polypropylene line. The traps are tracked using VHF radio and Argos satellite transmitters and strobelights. Typically we deploy our traps for a period of 72 hours each cruise.

3.2. Trap solutions

Prior to deployment, each trap is cleaned with 1 M HCl, rinsed thoroughly with deionized water then filled with a high density solution to prevent advective-diffusive loss of extractants and preservatives during the deployment period and to eliminate flushing of the traps during recovery (Knauer et al., 1979). The trap solution is prepared by adding 50 g of NaCl to each liter of surface seawater. This brine solution is pressure filtered sequentially through a 1.0 and 0.5 µm filter cartridge prior to the addition of 10 ml 100% formalin l-1. Individual traps are filled and at least 10 l of the trap solution is saved for analysis of solution blanks (see sections 4.1 and 5.1).

3.3. Post-recovery processing
3.3.1. Upon recovery, individual traps are capped and transported to the shipboard portable laboratory for analysis. Care is taken not to mix the higher density trap solutions with the overlying seawater. Trap samples are processed from deep to shallow in order to minimize potential contamination.
3.3.2. The depth of the interface between the high density solution and overlying seawater is marked on each trap. The overlying seawater is then aspirated with a plastic tube attached to a vacuum system in order to avoid disturbing the high density solution. Because some sinking particulate material collects near the interface between the high density solution and the overlying seawater, the overlying seawater is removed only to a depth that is 5 cm above the previously identified interface.
3.3.3. After the overlying seawater has been removed from all the traps at a given depth, the contents of each trap is passed through an acid rinsed 335 µm NitexR screen to remove contaminating zooplankton and micronekton which entered the traps in a living state and are not truly part of the passive flux. Immediately before this sieving process, the contents of each trap are mixed to disrupt large amorphous particles. The traps are rinsed with a portion of the <335 µm sample in order to recover all particulate matter, and the 335 µm NitexR screen is examined to determine whether residual material, in addition to the so-called "swimmers", is present. If so, the screens are rinsed again with a portion of the 335 µm filtrate. After all traps from a given depth have been processed, the 335 µm screen is removed and placed into a vial containing 20 ml of formalin- seawater solution, and stored at 4 °C for subsequent microscopic examination and organism identification and enumeration.

4. Determination of Mass Flux

4.1. Three of the 12 traps deployed at each water depth are used for the determination of mass flux. At our shore-based laboratory, triplicate 250 ml subsamples of the time-zero high density trap solution (blank) and equivalent volumes individual traps (start with the deepest depth and work up), are vacuum filtered through tared 25 mm 0.2 µm Nuclepore membrane filters (see Chapter 18, sections 4.1.4 to 4.1.3). The tared filters are prepared as follows:
4.1.1. Rinse filters three times with distilled water. Place rinsed filter on a 2.5 cm2 foil square (to reduce static electricity) in a plastic 47 mm petri dish.
4.1.2. Fold the foil in half over the filter and place the petri dish in a drying oven with the lid ajar for 2 hours at 55 °C. Remove and cool in dessicator for 30 minutes.
4.1.3. Weigh filter to constant weight (i.e., repeat oven drying, cooling and weighing until a relative standard deviation of <0.005% is achieved), on a microbalance capable of 0.1 µg resolution. Record weights (to the nearest 0.1 µg) on label tape placed on top of the petri dish.
4.2. After the last of the sample has passed through the filter, the walls of the filter funnel are washed with three consecutive 5 ml rinses of an isotonic (1 M) ammonium formate solution to remove seawater salts. During each rinse, allow the ammonium formate solution to completely cover the filter.
4.3. Return the processed filter to its petri dish, record sample number (on the dish and data sheet), and place in a drying oven at 55 °C for 8 hours. Alternately, store in a dessicator, if an oven is not immediately available. Dry to constant weight (as in Chapter 18, section 4.1.3).
4.4. Mass flux is calculated as follows:
                                   [(Wa-Wb)-Wbl] * Vt
         mg (dry wt.) m-2 d-1 =  ----------------------
                                 Vf * 0.0039 * 1000 * t
where: Wa = filter weight after filtration (µg)
Wb = filter weight before filtration (µg)
Wbl = net weight of blank solution (µg)
Vt = volume of trap (l)
Vf = volume filtered (l)
0.0039 = cross-sectional area of trap (m2)
1000 = conversion factor (µg mg-1)
t = deployment period (d)

5. Determination of C, N and P Flux

5.1. The quantities of particulate C, N and P in the prescreened trap solutions are determined using methods described in Chapters 10 and 11. Six replicate traps are used for C/N determinations and three additional traps for P. Typically, 1.5-2 liters are used for a single C/N or P measurement. An equivalent volume of the time-zero sediment trap solution, filtered through the appropriate filters is used as a C, N or P blank.
5.2. C, N and P flux is calculated as follows:
                                    [(Cs-Cb)] * Vt
          mg C (or N, P) m-2 d-1 =  ---------------
                                    Vf * 0.0039 * t
where: Cs = carbon (mg) in sample
Cb = carbon (mg) in blank
Vt = volume of trap (liters)
Vf = volume filtered (liters)
0.0039 = cross-sectional area of trap (m2)
t = deployment period (d)

6. Equipment/Supplies/Reagents

7. References