Biological effects on flocculation of fine-grained suspended sediment in natural seawater

The flocculation process is ever occurring in the marine and estuarine environment, and better knowledge of the biological influence as a governing factor is central to improve the understanding and modelling of the horizontal distribution of sediments. In this study we performed a series of laboratory experiments with fine-grained sediments from Sermilik Fjord and Young Sound, situated in Southeast and Northeast Greenland, respectively. The sediment was suspended in natural low-turbidity seawater, filtered seawater and NaCl-supplemented tap water to quantify the isolated effect of particulate organic material (POM) and dissolved substances on flocculation of fine-grained cohesive sediment. A high-resolution camera system (PCam) was applied to observe the effects, and the images and videos were processed in MatLab. The results showed a significantly enhanced flocculation from the presence of POM with larger floc sizes and higher settling velocities compared to flocs formed in seawater filtered with a retention diameter of 0.7 μm. The influences of the remaining dissolved substances, however, led to an enhancement of the flocculation and mass settling of particles compared to those of the control experiments (NaCl-supplemented tap water). The comparison of flocculation potential of sediments from the two different geographic locations in Greenland showed large differences in size and effective density of the flocculated particles, likely caused by the observed differences in texture and organic content of the sediments. •Quantification of flocculation efficiency through laboratory experiments.•Significantly increased flocculation in unfiltered natural seawater.•Enhanced flocculation from dissolved substances in filtered natural seawater.