Fig. 4

Shear stress. The shear stress of 11 aggregations of different densities was measured. Measurements were done inside the aggregation (filled diamond) as well as immediately beside each aggregation (open diamond). Shear stress is significantly higher inside the aggregation compared with shear stress outside the aggregation. The shear stress of high density aggregations is significantly higher than the shear stress of low density aggregations. Visualized error bars are standard errors, calculated on five replicate measurements per aggregation