Particle flux was measured at a standard reference depth of 150 meters using multiple cylindrical particle interceptor traps deployed on a free-floating array for approximately 60 hours during each cruise. Sediment trap design and collection methods are described in Winn et al. (1991). Passively sinking particulate matter is collected, prescreened (335 \u00b5m) to remove zooplankton and micronekton carcasses, then the sample materials are analyzed for C, N, P and mass flux (mg m-2 d-1).\u00a0Samples were analyzed for particulate carbon (PC), nitrogen (PN), phosphorus (PP), and silica (PSi). Typically six traps are analyzed for PC\u00a0and PN, three traps for PP, and three additional traps for PSi.\u00a0
\nA summary of methodology is listed below.\u00a0Full details can be found at the HOT\u00a0Field & Laboratory Protocols page\u00a0or below in Related Publications section (Karl et al., and HOT Program Sediment Trap Protocols: Chapter 18)
\n1. Principle
\nAlthough much\u00a0of the particulate matter both on the seafloor and in suspension in seawater is very fine, recent evidence suggests that material deposited on the seafloor arrives via relatively rare, rapidly sinking large particles (McCave, 1975, Sharek et al., 1999). Therefore, in order to describe\u00a0the 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 three decades 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 these important biogeochemical processes.
At Station ALOHA, we initially deployed a free-drifting sediment trap array with 12 individual collectors positioned at 150, 300, and 500 meters. The deployment period was approximately 72 hours. The passively sinking particles were subsequently analyzed for a variety of chemical properties, including C, N, P, biogenic-Si and total mass. From HOT-64 to present, the traps were deployed at a single reference depth (150 meters) for a period of approximately 48 to 60 hours. Routine mass-flux measurements were discontinued at HOT-68.
\n2. Precautions
\nBecause particle fluxes in oligotrophic habitats are expected to be low, special attention must be given 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 until deployment, and again 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 to reduce the chances of post-collection solubilization of particles.
3. Field Operations
\nHardware
\nThe HOT program free-floating sediment trap array is patterned after the MULTITRAP system\u00a0(Knauer et al., 1979) and used extensively in the decade-long VERTEX program. Twelve individual\u00a0sediment trap collectors (0.0039 m2 mouth opening) are deployed at 150 meters depth. The traps are affixed\u00a0to a PVC cross that is attached to 1/2\" Spectra line. The traps are tracked using XEOS and Argos\u00a0satellite transmitters, VHF radio, and strobe lights. Since HOT-71 (April 1996) both the trap array configuration and the deployment period were altered to conserve ship time.
Trap solutions
\nPrior 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/L\u00a0NaCl to a sample of surface seawater. This brine solution is gravity filtered\u00a0through a 0.2\u00a0\u00b5m filter cartridge after the addition of 10 ml per liter 100% formalin solution. Individual traps are filled and at least 10 liters of the trap solution is saved for analysis of solution blanks.
Post-recovery processing
\nUpon recovery, individual traps are capped and transported to the shipboard\u00a0laboratory for initial processing. Care is taken not to mix the higher density trap solutions with the overlying seawater.\u00a0The depth of the interface between the high density solution and overlying seawater is marked on each trap and a second mark is made 5 cm above the interface. The overlying seawater is then aspirated with a plastic tube attached to a vacuum system to the upper mark in order to avoid disturbing the high density solution below. *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.*
After the overlying seawater has been removed from all the traps,\u00a0the contents of each trap is gently mixed to disrupt large amorphous particles and then passed through an acid rinsed 335 \u00b5m NitexR screen to remove contaminating zooplankton and micronekton that may have\u00a0entered the traps in a living state and that, therefore, are not part of the desired passive flux.\u00a0The traps are rinsed with a portion of the <335 \u00b5m sample in order to recover all particulate matter, and the 335 \u00b5m 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 \u00b5m filtrate. After all traps from the same depth have been processed, the 335 \u00b5m screen is removed and placed into a vial containing 20 ml of formalin-seawater solution, and stored at 4\u00b0C for subsequent microscopic examination and organism identification and enumeration.
\n4. Determination of C, N, P and biogenic-Si Flux
\nThe quantities of particulate C, N, P, and biogenic silica in the screened trap solutions are determined using methods described in the chapters for Particulate Carbon and Nitrogen, Particulate Phosphorus, and Particulate Biogenic Silica in the HOT protocols document (see Related Publications section below). Six replicate traps are used for C/N determinations and three additional traps for P. Typically, 1.5 to 2 liters are used for a single C/N or P measurement, and a subsample of 250 mL for particulate Si.\u00a0An equivalent volume of the time-zero sediment trap solution, filtered through the appropriate filters is used as a C, N, or P blank
sediment trap array: spar buoy, radio and satellite transmitters, strobe light, floats, 12 place sediment trap crosstrap supports, Particle Interceptor Trap (PIT) collector tubes and baffles)
\nmicrobalance or equivalent
\npetri dishes and pre-cut tin foil squares
\nvacuum filtration apparatus and glassware
\nmembrane filters (25 mm diameter, 0.8 \u00b5m Nuclepore)
\nmicrobalance or equivalent
\ngraduated cylinders
\nforceps
\n335 \u00b5m Nitex screen
Addendum - PPO4 protocol (April 7, 2015)
\nBoth the previous and modified procedures were tested in paired analyses on samples collected over one year (12 cruises). The modified procedure resulted in higher yields by approximately 50% for water column samples (integrated 0-100 m: old method 1.00\u00b10.27 mmol P m-2, versus 1.56\u00b10.14 mmol P m-2) and approximately 30% for P-flux estimated from sediment trap samples (old method: 0.31\u00b10.07 mg P m-2 d-1 versus 0.40\u00b10.09 mg P m-2 d-1).
\n\u00a0
\n\u00a0
Monthly measurements of particle flux were collected at Station ALOHA as part of the HOT program.\u00a0Passively sinking particulate matter is intercepted using a free-floating sediment trap array. After screening\u00a0the collected materials to remove zooplankton and micronekton carcasses, portions of the remaining sample are analyzed for C, N, P and biogenic-Si flux.
Calculations
\nC, N, P and biogenic-Si flux is calculated as follows:
\n\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 [(Cs-Cb)] * Vt
\n\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 mg C (or N, P) m-2 d-1 =\u00a0 ----------------------
\n\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 Vf * 0.0039 * t\u00a0 \u00a0 \u00a0 \u00a0\u00a0
\nwhere:
\n\u00a0