In-situ estimations of the density and porosity of flocs of varying sizes in a submarine canyon

This paper introduces a new method to estimate in-situ floc density and porosity of three particle-size classes: 0.45–10, 10–63 and, 63–250 μm. LISST-100 and CatNet (a novel on-board filtration system of nested filters) were used to measure suspended particle volume and mass concentrations of 32 and 3 size-classes, respectively. Two field experiments were conducted in 2004 and 2006 in the head region of Gaoping Submarine Canyon (GPSC) to test this method. In each experiment CTD and LISST-100 profiling and simultaneous water sampling were carried out on board R/V Ocean Research III. In 2004, the water samples were filtered in the laboratory after the cruise using traditional methods. CatNet was used in 2006 and the seawater filtration was done on board. Assuming the observed suspended particles were all flocs composed of primary particles (with assumed density of 2.65 g/cm3) and interstitial water (with assumed density of 1.025 g/cm3) we estimated the volume of primary particles. The volume difference between flocs (measured from LISST-100) and primary particles is the interstitial pore space in flocs. Subsequently, the porosity of a floc particle is the ratio of the volume of the interstitial pore space to that of the floc particle. Results for 2004 show erroneous values of estimated floc porosities, which do not appear in 2006. The major difference is that the seawater was filtered immediately on board using CatNet in 2006. After removing erroneous data related to after-cruise filtering, the averaged floc porosity and density in the three grain-size classes were 47.0, 64.0, 85.9% and 1.89, 1.61, 1.25 g/cm3, respectively. There is a trend that larger particles have lower density, which is supported by literature. The temporal variations of floc density and porosity of all three particle-size groups were larger in the surface than near the bottom of the submarine canyon. This vertical variability resulted from flocculation and biogenic processes in the surface of the coastal ocean, and the terrestrial source of the Gaoping River. Floc density is an important physical parameter to evaluate the settling flux of suspended sediments in estuarine and marine environments. This approach provides direct in-situ floc bulk densities of different grain-size classes, which has advantages over conventional methods.