Delayed settling of marine snow at sharp density transitions driven by fluid entrainment and diffusion-limited retention

Abstract

Marine snow is central to the marine carbon cycle, and quantifying its small-scale settling dynamics in different physical environments is essential to understanding its role in biogeochemical cycles. Previous field observations of marine aggregate thin layers associated with sharp density gradients have led to the hypothesis that these layers may be caused by a decrease in aggregate settling speed at density interfaces. Here, we present experimental data on aggregate settling behavior, showing that these particles can dramatically decrease their settling velocity when passing through sharp density transitions. This delayed settling can be caused by 2 potential mechanisms: (1) entrainment of lighter fluid from above as the particle passes through the density gradient, and (2) retention at the transition driven by changes in the density of the particle due to its porosity. The aggregates observed in this study exhibited 2 distinct settling behaviors when passing through the density transition. Quantitatively comparing these different behaviors with predictions from 2 models allow us to infer that the delayed settling of the first group of aggregates was primarily driven by diffusion-limited retention, whereas entrainment of lighter fluid was the dominant mechanism for the second group. Coupled with theory, our experimental results demonstrate that both entrainment and diffusion-limited retention can play an important role in determining particle settling dynamics through density transitions. This study thus provides insight into ways that delayed settling can lead to the formation of aggregate thin layers, important biological hotspots that affect trophic dynamics, and biogeochemical cycling in the ocean.

Delayed settling of marine snow: Effects of density gradient and particle properties and implications for carbon cycling

“…The results support previous studies showing that aggregates may decrease their settling velocity when passing through a density transition (Kindler et al, 2010;Camassa et al, 2013;Prairie et al, 2013). The present study further examines the relationship between stratification and delayed settling behavior; individual aggregates were compared across multiple density gradients to isolate the effect of density gradient from the natural variation in aggregate size, shape, and composition.…”

“…NMSV ranged from less than 0 (indicating the aggregate came to a full stop) to 0.72. The few slightly negative NMSV values observed here are within video recording noise, although we note that in some cases in previous studies (e.g., Abaid et al, 2004 andLin, 2009 for solid particles; Prairie et al, 2013 for aggregates), particles have been observed to temporarily reverse direction at sharp density gradients. DSTS also displayed a large range among aggregates, spanning almost two orders of magnitude from 4.4 to 271.7.…”

“…In a recent study, both entrainment and diffusion-limited retention have been observed in the laboratory to cause delayed settling in marine aggregates . Prairie et al (2013) found that every aggregate in their study displayed a settling velocity minimum when passing through the sharp density transition, demonstrating that delayed settling behavior is relevant to natural marine aggregates, at least in a laboratory setting.…”

“…Two metrics were calculated to determine the extent to which the aggregate's settling velocity decreased within the density transition, as described in Prairie et al (2013). The normalized minimum settling velocity (NMSV) was calculated as the minimum value of the smoothed settling velocity (MSV) divided by the settling velocity in the bottom layer.…”

Scales and mechanisms of marine hotspot formation

“…This Theme Section contains 8 studies with topics ranging from hotspots of marine snow to migratory top predators. Prairie et al (2013) in a laboratory experiment demonstrated mechanisms by which particles can be temporarily retained when encountering density gradients. The density of the aggregate relative to the density of the bottom layer in the gradient is the primary determinant of the extent that marine snow will aggregate, thereby enhancing food web development at the boundary layer.…”