Accumulation of marines now at density discontinuities in the water column

Abstract

The vertical distribution of marine snow-aggregated particles > 0.5 mm in diameter-was strongly correlated with density discontinuities in the upper 100 m of the water column at 33 stations off central California. Eighty-seven percent of peaks in aggregate abundance were associated with density discontinuities in which . . . . . . . N* (the Brunt-Vaisala frequency) exceeded 1.25 x lop4 s-~ (N = 6 cph). For 56% of the peaks, N2 exceeded 2.5 x 1O-4 s2 (N = 9 cph). Absolute abundances of aggregates increased from a mean of 38.7+ 18.3 aggregates liter' above peaks to 59.3k26.0 aggregates liter-' within peaks. Accumulations of aggregates could result from sinking into denser water if sinking rates were reduced by at least 20% due to slow equilibrium of either interstitial water or mucus within the floes with the surrounding, higher density water. However, our observed increases in ct were insufficient to cause such slowing for floes whose porosity was 99% or less; such floes comprised a significant proportion of aggregates in the observed peaks. However, larger floes of moderate to high porosity were likely to decelerate as they encountered denser water and could even become neutrally or negatively buoyant for changes in ur such as we observed. Equilibration times of the interstitial water of these generally larger floes ranged from hundreds of seconds up to 3 h. Results of a random walk simulation of particle motion indicated that turbulence was likely to cause localized accumulations of aggregates and to increase particle aggregation. Because we found subcritical values of the Richardson number (an indicator of turbulent mixing) at the top of regions of peak abundance but not at the base, for 68% of the peaks observed, we infer that turbulence may contribute to the accumulation of the smaller floes. Occurrence of strong shear and deviations of temperature and salinity profiles from typical patterns suggest that at least 23% of these aggregate peaks were associated with horizontal intrusions.

Role of macroscopic particles in deep-sea oxygen consumption

“…On very rare occasions, deep sea medusae, copepods, and some worm-shaped plankton were observed; however, they occurred at vanishingly small numbers below 2,000 m. Most of the particles were either too small to discern a specific shape or were amorphous marine snow particles. Only in 4 of 17 profiles were particle peaks associated with the lower part of the permanent pycnocline as described by MacIntyre et al (15). However, we were not able to resolve small-scale features such as thin layers <5 m (16) by our method because of the roll of the ship at high seas and the transfer of its motion to the instruments.…”

Feeding by the euphausiid Euphausia pacifica and the copepod Calanus pacificus on marine snow

“…The patchy distribution of marine snow may also lead to underestimates of its abundance. Marine snow tends to accumulate at density discontinuities and its abundance varies vertically in the water column (Lampitt et al 1993, MacIntyre et al 1995. Euphausiids can be vertically stratified based on prey concentration (Sameoto et al 1987) or water column structure (Barange 1990), suggesting that euphausiids may be able to cue in on regions of high marine snow abundance.…”

Recipe for 1‐D Lagrangian particle tracking models in space‐varying diffusivity

“…As Visser (1997) demonstrates, not every random walk model is suitable for environments where the diffusivity is spatially nonuniform and the choice of the wrong model can lead to misinterpretations. This has been pointed out several times (e.g., Hunter et al 1993;Dimou and Adams 1993;Spagnol et al 2002) but erroneous models continue to appear in the literature (e.g., MacIntyre et al 1995, Lizon et al 1998Kim et al 2000). The following equation can be derived (Hunter et al 1993;Visser 1997) to calculate the particle displacement:…”