Heat, Water, and Suspended Solids: A Complex Relationship

Hot weather can significantly influence the concentration and behaviour of suspended solids in natural water bodies, though the exact effect is rarely straightforward. Suspended solids—comprising fine particles such as silt, clay, organic detritus, and plankton—respond to elevated temperatures through a combination of physical, chemical, and biological mechanisms. These interactions often produce conflicting outcomes, making the net impact highly dependent on local conditions.

Evaporation and Hydrological Concentration

The most direct physical effect of prolonged hot weather is increased evaporation. As water volume decreases, the same mass of suspended particles becomes concentrated in a smaller volume, leading to a measurable rise in turbidity and suspended solids concentration. This effect is particularly pronounced in shallow lakes, ponds, and slow-flowing rivers with limited inflow. 

At the same time, reduced rainfall—common during hot spells—curtails runoff, thereby diminishing the transport of soil particles from surrounding land into the water. In some systems, these two forces may offset each other, resulting in little net change.

Thermal Stratification and Internal Cycling

In deeper lakes and reservoirs, intense heating of the surface layer creates thermal stratification—a warm, less dense upper layer (epilimnion) overlying a cooler, denser bottom layer (hypolimnion). This density gradient inhibits vertical mixing, effectively trapping particles in the upper layer. Under these conditions, larger suspended particles tend to settle out, often decreasing near-surface turbidity, while fine colloidal particles may remain suspended for extended periods. 

The stratification also reduces oxygen exchange with the lower layer, which can trigger the release of dissolved iron and manganese from bottom sediments. Upon re-oxidation near the surface during occasional mixing events, these elements can precipitate as fine particles, temporarily increasing suspended solids concentrations.

Algal Blooms and Organic Suspended Matter

Perhaps the most significant biological response to hot weather is the rapid proliferation of phytoplankton—commonly known as algal blooms. Many blue-green algae thrive at elevated water temperatures and in nutrient-rich conditions. 

As their population surges, these microscopic organisms become a major component of suspended solids. Unlike inorganic sediment particles, algal suspensions have low specific gravity and settle very slowly, thus maintaining high turbidity over large areas. Furthermore, when these blooms die and decompose, they contribute additional fine particulate organic matter to the water column, further sustaining elevated suspended solids levels for extended periods.