Suspended macro-aggregates of a tropical estuary in Northeast Brazil: composition and settling velocities

Abstract

The settling velocity and composition of large estuarine suspended aggregates (macro-aggregates) were investigated in the well-mixed, tropical estuary of Todos os Santos Bay (NE Brazil). These aggregates, reaching up to several decimeters in length, are uncommon in estuaries where flow shear promotes the disintegration of larger particles, and might expedite the transit of suspended particulate matter towards the bottom. Sets of sediment traps, sieves, and a still chamber were used to collect and measure aggregate’s properties in 14 field campaigns between 2015 and 2017. The results showed that suspended particulate matter concentrations in the order of 4 mg l−1 are associated with vertical fluxes of 51.5 ± 7.0 g m−2 day−1, of which 23 % are combustible material. The measured aggregate’s settling speed, ranging in size from 14 to 103 mm (longest axis), varied from 12 to 34 mm s−1. The particulate organic carbon content of the aggregates is lower than the SPM’s, varying from 10 to 31 %, and increase towards the bottom. Diatoms, inorganic, and organic detritus are common particles carried by the aggregates, whose speedy fall velocities make them an efficient medium to remove suspended particles from the water column and potentially increase sedimentation rates.