SEDIMENT TRAP PROTOCOLS
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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).
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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
Nuclepore 25 mm 0.2 µm membrane filters
petri dishes and pre-cut foil squares
vacuum filtration apparatus and glassware
Cahn electronic microbalance or equivalent
graduate cylinders
sediment trap array (spar buoy, radiotransmitter, strobe light,
floats, trap supports, collector tubes)
forceps
335 µm NitexR screen
ammonium formate solution (1 M)
7. References
Knauer, G. A., J. H. Martin and K. Bruland. 1979. Fluxes of
particulate carbon, nitrogen and phosphorus in the upper water
column of the Northeast Pacific Ocean. Deep-Sea Research, 26,
97-108.
McCave, I. N. 1975. Vertical flux of particles in the ocean.
Deep-Sea Research, 22, 491- 502.
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