Suspended particulate matter <2.5 μm (SPM) in shallow lakes: Sedimentation resistance and bioavailable phosphorus enrichment after sediment resuspension.

Resuspended particulate matter in shallow lakes contributes remarkable phosphorus (P) concentrations to the water column that potentially support algal/cyanobacterial growth. However, only fine particulate matter can be retained in the water column for a long time after sediment resuspension events. The size at which fine particulate matter has ecological implications remains undefined. This research defined suspended particulate matter with a median grain size <2.5 μm (SPM) in shallow lakes, which resists sedimentation and enriches bioavailable P. The relationship between the size of suspended particulate matter (SPM) and water disturbance was characterized by conducting a lab-scale jar test with sediments in a shallow lake. The sedimentation of completely resuspended particulate matter occurred under a series of turbulence shear rates (G) ranging from 0 to 50 s. When G was larger than 20 s, the SPM had a median grain size (D) ranging from 9 μm to 11 μm for the three samples. When G was <10 s, only SPM <2.5 μm remained in suspension. The SPM larger than 2.5 μm settled when G was between 10 s and 20 s, and the SPM remained in complete suspension when G was larger than 20 s. Furthermore, P fractionation was conducted on different size-grouped particles that were sorted using gravity sedimentation. The concentration of iron/aluminium bound-P (Fe/Al-P) decreased exponentially as the particle size increased. The concentration of Fe/Al-P in SPM ranged from 902.8 mg/kg to 1212.1 mg/kg, accounting for over 80 % of extractable total phosphorus. SPM contributed a remarkable amount of bioavailable P to the algal/cyanobacterial biomass in the shallow lake with frequent sediment resuspension.