Dataset: aggregates

   PI:              Ian Walsh, W. Gardner, M. Richardson
   of:              Texas A&M University
   dataset:         Aggregates with equivalent spherical diameters gt .5 mm
   dates:           October 04, 1992 to October 21, 1992
   location:        N: 0.1077  S: -0.1623  W: -140.1713  E: -139.778
   project/cruise:  EqPac/TT012 - Fall Time Series
   ship:            Thomas Thompson
   Equatorial Pacific Protocols Document (1993)
 

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.