In situ observations of turbulence-induced disruption of marine particle aggregates and associated backscatter in the upper ocean and implications for remote sensing of particle size

Steven G Ackleson, Naval Research Laboratory, Washington, DC, United States and Matthew Rau, Pennsylvania State University Main Campus, University Park, United States
Ocean turbulence is largely responsible for bringing marine particles in contact with each other and enhancing aggregate (floc) formation. Floc formation and size dependent sinking is well documented and is believed to be a primary mechanism for enhancing the removal of particulate carbon from the upper ocean. High turbulent energy, on the other hand, e.g., due to current shear, wave breaking, and ship wakes can potentially disrupt flocs resulting in a shift towards smaller particle size and slower particle sinking rate. While marine floc formation, also referred to as marine snow, has been investigated extensively, the significance of floc disruption is less understood. It has also been suggested that the particle size distribution, including flocs, may result in changes in spectral backscatter, thus raising the possibility of using ocean color and LIDAR to quantify marine particle size and estimate vertical carbon flux. In situ measurements of turbulence-induced changes in particle size and associated observations of particulate backscatter collected in near-surface coastal and pelagic waters were used to test two hypotheses; 1) turbulence typical of the upper ocean is large enough to disrupt naturally occurring marine aggregates and 2) aggregate disruption can be detected by optical backscatter. Measurements were made using a commercial in situ particle size meter (LISST-100X, Sequoia Scientific) with an attachment designed to subject ambient marine flocs to controlled levels of turbulence typical of the upper ocean. We show that upper ocean turbulence can influence the size distribution of near-surface marine flocs. However, backscatter appears to be relatively insensitive to floc formation or disruption. Thus, the retrieval of information directly related to marine particle aggregation from ocean color remote sensing seems unlikely.