The total suspended solids constitute the amount of solid material within a given volume of water. However, the specific particles will have to be bigger than 2 microns in order to qualify within this measurement. This is partly a function of the fact that it is just too difficult to measure particles that are much smaller than that.
This is also a function of the fact that particles that are smaller than that are not going to affect the turbidity of the water and are not going to be visible. Those particles are typically bacteria, viruses, or something of that nature, and they become more relevant in the field of microbiology.
The suspended solids in given bodies of water will always vary. In many cases, they are basically inorganic material of some kind or another. Some of the suspended solids can include sand, algae, silt, plankton, or clay. Of course, the suspended solids do not have to include materials that are naturally occurring. Chemical precipitates will certainly count towards the total suspended solids. Testing for chemical precipitates in the total suspended solids can be an important part of water quality monitoring in the first place.
In most bodies of water, even if they are heavily polluted, the total suspended solids will largely consist of inorganic and organic material that is naturally occurring within the ecosystem. There will be exceptions, but that is usually what people will find when measuring the total suspended solids in a body of water in nature.