Resources at the Seafloor
“Raining” Particles
(^) Because most of the seafloor is in continuous darkness, deep benthic communities,
apart from those of hydrothermal vent areas, are entirely dependent upon imported
nutrition: particulate material sinking (or transported partway by swimmers) from the
euphotic zone, deadfalls of large animals like whales and tuna, and to a minor extent
waterlogged wood. We know something about the amounts of the falling particles and
are still evaluating the importance of the latter two sources. McCave (1975) showed
some features of the particle-size distribution in ocean waters. Most of the particles
are small; there is an exponential decrease in particle abundance with increasing
particle size. Below about 200 m there is generally less than 1 particle ml−1 larger
than 20 or 30 μm. If the likely sinking rates of these particles are multiplied by their
mass, however, the relationship flips. Because the few larger particles fall faster, they
carry most of the mass. The particles carrying most of the mass would rarely be
collected at all in water bottles. To study supply to the seafloor, something else is
needed. That something, developed in the 1970s and deployed in increasing numbers
since, is sediment trapping.
(^) Traps come in a variety of shapes and sizes. The first ones were tables that sat on
the seafloor with sticky collecting sheets exposed under flow baffles. Those were not
effective. Next came various large conical collectors (PARFLUX traps, Dymond
traps, etc. (Fig. 13.23a). There are traps based on simple tubes, usually called PITs,
particle-interceptor traps (Fig. 13.23b). Tube sizes vary, but typically are small
enough to handle readily, 3.5 to 15 cm diameter. There are various schemes for
getting time-series of samples, such as carousels at the bottom of the cone which
rotate a new collecting cup into place at some interval, often every two weeks. The
effort is plagued by biases, and the treatment of that has been to do calibration studies
in flumes; to model the collection process so as to calculate correct fluxes from
measured ones; and to seek cool tricks to overcome the problems.
Fig. 13.23 (a) Design of the Honjo trap
(after Honjo 1982).
Mooring cables attach to upper and lower metal rings. Sediment falling into the cone
works its way down the sides into a cup at the bottom. A spring visible at the lower
right pulls a shutter over the top of the cup before the mooring is retrieved.
(^) (After Honjo et al. 1982.)