|The LISST-STX (and before that, the LISST-ST) obtains the settling velocity by looking at the time-history of the concentration variability in 8 size classes. When the STX takes in a sample, and closes the doors, the sediment in the settling column starts to settle out. If it is assumed that all the sediment in the column is well mixed when the sediment starts to settle out, then one can measure the settling velocity of a particular size class simply by detecting when the concentration in that size class drops to 0. This general principle is described in much more detail in the paper by Agrawal & Pottsmith (2000).
So, if the concentration in a size class does not drop to 0, then it is (generally) not possible to estimate the settling velocity of that size class. Since density is estimated from Stokes’ law, describing the relation between particle settling velocity, size and density, it follows that if we can’t get a measurement of the settling velocity then we can’t get the density. And if we can’t get the density, then we can’t get the total suspended sediment concentration (by multiplying the volume concentration with the density).
Why does the concentration in a size class not drop to 0? The standard time for a full settling experiment is 22 hours; this should normally ensure that all particles in the column have settled out. However, if the duration of an experiment is shortened to less than 22 hours, then only some size classes will have time to settle out. Hence no estimate of settling velocity (thus density and TSS) of the remaining size classes.
It can also happen that even if the settling experiment is 22 hours long, some particles are still in suspension. This is particular so during the summer, and in waters with a high biological productivity, where small pieces of algal detritus and living and/or dead biological matter may be in suspension. Such material has a density much lower than then 2650 kg/m3 that is commonly used for mineral grains, often just a little bit higher than that of the water itself (~1050 kg/m3). If the particle density is so low, then the particles simply will never settle out.
For example, as a rule of thumb a particle with a diameter of 33 micrometer settles roughly at 1 mm/s if it is a mineral grain. Such a particle has an excess density of 1600 kg/m3 (2650 kg/m3-1050 kg/m3). A biological particle with an excess density of, e.g. 50 kg/m3 (1100 kg/m3-1050 kg/m3) settles a factor of 1600/50 = 32 times slower = 0.03 mm/s.
Consequently, if the smallest size classes in the LISST-STX have just a small fraction of biological material in them, then the particles will probably not settle out until much after the 22 hours have passed, and no good estimate of the settling velocity (hence density and TSS) can be obtained.
So which size classes are this? It completely depends on the deployment and the characteristics of the suspended matter composition during the deployment – there is unfortunately no good, firm answer to this question.
The good news in this respect is that:
- these small particles seldom make up a significant fraction of the suspended sediment
- even if they do, the fact that they settle so slowly that they don’t settle the length of the settling column (30 cm) in 22 hours also means that they wouldn’t settle out in natural waters under normal circumstances, given the impact of turbulence produced by waves, tides, boat traffic etc.