Field Observations on Flocculation of Suspended Sediment in Coastal Algal Reef Environments

Sediment flocculation in subaqueous environments is vital for morphodynamics, biogeochemical cycles, and ecological processes; however, the effects of biophysical cohesion on flocs are not well understood or quantified. This study provides quantitative field evidence that suspended sediments on a coastal algal reef primarily flocculate due to bio-cohesion. Measurements of mass and volume concentrations of suspended sediment and turbulent Reynolds stresses were performed at various heights above the seabed using Optical Backscatter Sensors (OBSs), Laser In-Situ Scattering and Transmissometry (LISSTs), and Acoustic Doppler Velocimeters (ADVs). Observed mass concentration profiles were compared with Rouse's law. Results indicate that while mass concentration decreases as expected with height, volume concentration increases away from the bed. Notably, mass concentration profiles align with the Rouse formula when assuming a settling velocity for flocculated sediment rather than non-cohesive sediment. Microscope images confirmed sediment flocculation, likely due to bio-cohesion. Direct measurements showed that particle effective density depends on mean particle diameter. Regression analysis determines a three-dimensional fractal dimension of 2.18. The reduced effective density and low fractal dimension are characteristic of flocs comprising lower-density saltwater and organic materials. The organic content was determined using the weight loss on ignition method.  We found that organic content negatively correlates with effective density and positively correlates with the mean particle diameter, reinforcing the role of bio-cohesion in flocculation. Further information on the findings is published in "Field evidence of flocculated sediments on a coastal algal reef," Volume 6, Article 8, Communications Earth & Environment, 2025.