Sampling Protocols for Marine Snow Aggregates

Vernon Asper

The fragile nature of marine organic aggregates (marine snow) precludes the use of normal hydrocasting techniques for assessing aggregate abundance through the water column and requires the application of non-contact photographic methods as described in Honjo et al. (1984). The camera system consists of Lobsiger Deepslope, dual-head strobe lights mounted at the focal point of a compound Fresnel lens, producing a collimated `slab' of illumination. Mounted at approximately 90 from this slab, a Lobsiger Deepslope underwater camera using T-Max 400 film photographs an intersecting volume of water (600 liters). Objects above or below the slab are not illuminated and thus will not appear in photographs. The system is lowered slowly (10--20 m/min) through the water column on a trawl wire, exposing frames at a time interval (7--20 sec.) calculated to yield 700--800 frames between the surface and the sea floor (1.2--5.6 m/frame). Depth is monitored and recorded using a pinger and the ship's precision depth recorder. Lowering is halted when the frame is within 2--10 meters of the sea floor. Portions of each film will be processed immediately on board the ship to assure that all systems are functioning and to assist in planning further deployment and sampling strategies.

Images will be analyzed directly from the film negatives using a PC compatible computer with Data Translation frame grabber and Java software. Total image area occupied by aggregates, average diameter, and the number of particles in 11 size classes (0.5--5.0 mm in 0.5 mm increments and larger than 5 mm) will be counted in representative portions of each image. Large zooplankton or nekton will be manually excluded from all images. Because this method relies on optical contrast between large aggregates and relatively clear water, results are more predictable when acquired at night when ambient light levels are lowest.

Also included in this instrument package are a self-contained CTD and transmissometer. Data from the CTD are used to calculate a density structure profile for the water column to determine the role of pycnoclines on the vertical distribution of aggregates and to follow water masses to evaluate the role of advective processes in distributing aggregates. The transmissometer is used to determine the concentration of fine suspended particles for comparison with the abundance of aggregates. These data will be used to evaluate whether or not scavenging of fine particles by sinking aggregates is a significant process.

The CTD will be calibrated by the manufacturer prior to shipment to Seattle and the transmissometer will be cleaned and calibrated according to the protocols established by the optics recommendations. The aggregate profiling camera system has been calibrated in the past by placing objects of known size in the field of view and will be re-calibrated during the cruise to preclude the possibility that a misalignment has occurred.