Dataset: aggregates

See Platform deployments for cruise specific documentation

Large Aggregate Profiling System Protocol

Ian D. Walsh, Wilford B. Gardner and Mary Jo Richardson

Camera systems have been developed to characterize millimeter size particle distributions in the water column (Honjo etal, 1984; Asper, 1987; Gardner etal, 1988). It is conjectured that the millimeter size class range of particles, thought to be primarily composed of aggregates (``marine snow'') may dominate the total mass flux because of their abundance and high settling rates (Asper, 1987). Camera systems integrated with a CTD and transmissometer (such as the Walsh/Gardner Large Aggregate Profiling System (LAPS)) have the advantage of simultaneously collecting data on the distribution of suspended particles and aggregates along with the physical structure of the water column. This is important as previous work has shown that the distribution of aggregates at depth does not reflect the Suspended Particulate Matter (SPM) distribution, particularly in the case of intermediate depth layers of high aggregate abundance (Gardner and Walsh, 1990; Walsh 1990; Walsh and Gardner, in press). The continuous nature of the LAPS profile allows for the identification of mid-water aggregate nepheloid layers which might be missed by sediment traps or pumping because of low sampling density. This is particularly important to the success of the EQPAC program as the previous sediment trap moorings deployed in the area have shown mid-water column flux maximums (~1000--2000 m) on a yearly and seasonal basis except for a three month period at 11� N, 140� W during which the flux was dominated by a diatom bloom (Walsh et al., 1988; Dymond and Collier, 1988).

The LAPS system as configured for the EQPAC program consists of a Deep-Sea Power and Light AVCS-101 Autonomous Video Camera (Sony CCD V801) synchronized with a high power strobe, and a Sea-Bird Seacat CTD coupled with a Sea Tech 25 cm pathlength deep transmissometer and a Sea Tech deep fluorometer. The strobe flash is contained and collimated using a stainless steel tube and a triple-lense Fresnel stack. PVC baffles on the lense stack can be set to produce a slab of light 5 to 10 cm thick, perpendicular to the camera. The illuminated imaging area can be varied using the zoom capability of the camera. For imaging the particle size range down to 0.5 mm, a 23 cm wide by 17.25 cm high image is acquired with a slab thickness of 10 cm. Calibration of the images is made by placing a target in the image volume during a preliminary cast and subsequent to all changes of the system parameters (e.g., image volume). Images from the camera are captured using a Data Translation frame capture board and NIH Image software on an Apple Macintosh IIci computer. Images are thresholded and particle counts made using the capabilities of the NIH Image program. Obvious zooplankton and nekton are excluded from the particle counts. Frames in the upper water column where sunlight is visible are excluded from the analysis because of potential ambiguity as to the water volume sampled (i.e., particles outside of the strobe illuminated volume may have been illuminated by sunlight). The strobe flash rate and lowering rate of the LAPS can be varied depending on the desired image density and the length of the cast. Generally the strobe interval is set for 6 seconds and the LAPS is lowered at 20 m/min yielding an image every 2 meters.

Each image is analyzed for the total number of particles and their maximum, minimum and equivalent circular diameters. The particles are binned into 0.5 mm size ranges based on the equivalent circular diameter starting at 0.5 mm. Particle volume is calculated assuming sphericity and diameters equal the means of the ranges.

The Seacat CTD will be factory calibrated prior to the EQPAC cruises. Transmissometer data reduction will be accomplished as outlined in the optics protocols.

Literature Cited

Asper, V.L. (1987).

Measuring the flux and sinking speed of marine snow aggregates. Deep-Sea Research, 34(1A):1-17.

Dymond, J. and R. Collier (1988).

Biogenic particle fluxes in the equatorial Pacific: Evidence for both high and low productivity during the 1982-1983 El-Ni�o. Global Bioceochemical Cycles, 2: 129-137.

Gardner, W.D. and I.D. Walsh (1990).

Distribution of macroaggregates and fine-grained particles across a continental margin and their potential role in fluxes. Deep-Sea Research, 37: 401-412.

Gardner, W.D., I.D. Walsh, and V.L. Asper (1988).

Comparison of large-particle camera and transmissometer profiles. Presented at the JOA Special Symposium on New Observation Methods, Acapulco, Mexico (1988).

Honjo, S., K.W. Doherty, Y.C. Agrawal, and V.L. Asper (1984).

Direct optical assessment of large amorphous aggregates (marine snow) in the deep ocean. Deep-Sea Research, 31: 67-76.

Walsh, I.D. (1990).

Project CATSTIX: Camera, transmissometer, and sediment integration experiment. Ph.D. Dissertation, Texas A & M University, 96pp.

Walsh, I.D. and W.D. Gardner (1992).

Comparison of large particle camera profiles with sediment trap fluxes. Deep-Sea Research, 39: 1817-1834.

Walsh, I.D., J. Dymond and R. Collier (1988).

Rates of recycling of biogenic components of fast settling particles derived from sediment trap experiments. Deep-Sea Research, 35: 43-58.