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 N2 (the Brunt‐Väisälä frequency) exceeded 1.25 × 10−4 s−2 (N = 6 cph). For 56% of the peaks, N2 exceeded 2.5 × 10−4 s−2 (N = 9 cph). Absolute abundances of aggregates increased from a mean of 38.7± 18.3 aggregates liter−1 above peaks to 59.3 ± 26.0 aggregates liter−1 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 flocs with the surrounding, higher density water. However, our observed increases in σt were insufficient to cause such slowing for flocs whose porosity was 99% or less; such flocs comprised a significant proportion of aggregates in the observed peaks. However, larger flocs 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 σt such as we observed. Equilibration times of the interstitial water of these generally larger flocs 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 flocs. 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.