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

Abstract

Marine snow aggregates are often a dominant component of carbon flux and are sites of high bacterial activity; thus, small-scale changes in the settling behavior of marine snow can affect the vertical locations of carbon export and remineralization in the surface ocean. In this study, we experimentally investigated the sinking velocities of marine snow aggregates formed in roller tanks as they settled through sharp density gradients. We observed between 8 and 10 aggregates in 3 different experiments, each of which displayed delayed settling behavior -that is, a settling velocity minimum -as they crossed the density transitions. Characteristics of delayed settling behavior were also compared to density stratification and aggregate density and size; aggregate settling velocity decreased more, and for longer periods of time, when density gradients were sharper and when aggregates were less dense. The observed relationships between non-dimensional parameters and aggregate settling allow for direct application of our results to the field, providing insight into the conditions under which strong delayed settling behavior is likely to occur. Activities of extracellular enzymes (the initial step in microbial remineralization of organic matter) were more than an order of magnitude higher in the aggregates compared to the surrounding water from which the aggregates were derived. Coupling measured enzyme activities with observations of delayed settling behavior demonstrates that the extent as well as the vertical location of enzyme activity is strongly affected by aggregate settling behavior: total enzyme activity within the region of the density transition increased by a factor of 18 with increasing stratification. This study, which combines direct measurements of small-scale aggregate settling and microbial enzyme activity, offers an opportunity to determine the potential implications of delayed settling behavior for local and larger-scale carbon cycling in the ocean.

Heterotrophic Extracellular Enzymatic Activities in the Atlantic Ocean Follow Patterns Across Spatial and Depth Regimes

“…This calculation in turn depends on the relative lability of the organic carbon in the sinking particles (Engel et al, 2009), the hydrolytic capacities of the microbial communities acting on them as they sink, the extent of water column stratification, and the residence time of the particle at different depths (Prairie et al, 2015). Holding all other factors constant, the biological pump would be more efficient at less stratified stations-such as stations 2, 4, and 7-where a larger proportion of the hydrolytic capacity occurs in the mesopelagic zone, below the permanent thermocline (Figure 7).…”

Effects of Phytoplankton Growth Phase on Settling Properties of Marine Aggregates

“…An important consideration is that the density gradient used in delayed settling measurements for GP1-early exponential and GP2-late exponential was sharper than that used in delayed settling measurements for GP3-early stationary and GP4-late stationary, so the delayed settling time for aggregates between these pairs of growth phases may not be comparable. However, delayed settling time for aggregates is typically longer when passing through sharper density gradients [22], and so if the delayed settling measurements in GP3-early stationary and GP4-late stationary were conducted with an equally strong density gradient as the earlier growth phases, the delayed settling times would likely be even greater. The increased delayed settling time at later growth phases is expected given a previous experimental study that found longer periods of decreased velocities through sharp density gradients for aggregates of lower densities [22].…”

“…In all experiments, aggregate excess density was measured for aggregates in each growth phase by quantifying the size and settling velocity of individual aggregates following the method described in References [21,22]. In Experiments 3 and 4, the aggregates were very fragile for some of the growth phases resulting in lower sample sizes (Table 1); it is possible that in these experiments, aggregate density measurements may have been biased because aggregate density could not be quantified for the more fragile aggregates which may have been less dense on average.…”

“…In Experiment 2, delayed settling time through a sharp density gradient was measured for 5-7 aggregates per growth phase using the method described in References [21,22]. Briefly, a two-layer water column was set up with a sharp density transition in the middle of a tank of the same size as used for the aggregate excess density measurements.…”

“…Given that TEP can impact aggregate density, with higher TEP concentrations resulting in lower sinking velocities [12,20], carbon export through aggregate settling may vary significantly for different growth phases, and thus at different stages of phytoplankton blooms. In addition, less dense aggregates are more likely to have reduced sinking velocities as they pass through sharp density gradients, a phenomenon referred to as delayed settling [21,22], allowing thin layers of aggregates to form [23,24], which will further affect carbon remineralization and export.…”