Water stratification
Introduction
Water stratification refers to the phenomenon where water masses with different properties - salinity (read more), oxygenation (read more), density (read more), temperature (read more) - form layers that act as barriers to water mixing. These layers are normally arranged according to their density, with the least dense water masses sitting above the denser ones. Stratification can significantly impact the physical and biological dynamics of the water body.
Causes of Water Stratification
Water stratification is primarily driven by two forces: heat and freshwater input. The sun heats the surface water, causing it to expand and become less dense than the cooler, unheated water below. This creates a thermal stratification with a warm, less dense layer of water (the epilimnion) over a cooler, denser layer (the hypolimnion).
Freshwater input, such as from rain or melting ice, can also cause stratification. Freshwater is less dense than saltwater, so when a large amount of freshwater enters a saltwater body, it can form a layer on top of the denser saltwater. This is known as haline stratification.
Types of Water Stratification
There are three main types of water stratification: thermal, haline, and chemocline.
Thermal Stratification
Thermal stratification occurs when water masses with different temperatures form layers. The water's temperature affects its density, with warmer water being less dense than cooler water. This results in a layer of warmer water floating on top of cooler water. Lakes are commonly subject to thermal stratification, which varies seasonally and daily.
Haline Stratification
Haline stratification occurs when fresh water from rivers, precipitation, melting ice, or groundwater inflow forms a layer on top of denser salt water. This type of stratification is common in estuaries and the ocean, especially in areas with significant freshwater inflow.
Chemocline
A chemocline is a type of stratification caused by a sharp gradient in chemical composition. For example, in some bodies of water, a layer of water with a high concentration of nutrients or oxygen can sit on top of a layer with lower concentrations.
Effects of Water Stratification
Water stratification can have significant effects on the physical and biological characteristics of a water body.
Physical Effects
Stratification can affect the circulation of a water body, limiting the vertical mixing of water. This can lead to the formation of distinct layers with different temperatures, salinities, or oxygen concentrations. Stratification can also impact the transport of nutrients, pollutants, and sediments within a water body.
Biological Effects
Stratification can also have significant biological effects. It can create distinct habitats within a water body, each with its own set of species. Stratification can also affect the distribution of nutrients, which can impact the productivity of a water body and influence the types of organisms that can live there.
Dealing with Water Stratification
Stratification can pose challenges for water management, particularly in reservoirs and lakes used for drinking water supply. Various methods can be used to mitigate the effects of stratification, such as aeration, destratification, and hypolimnetic oxygenation.
Aeration
Aeration involves introducing air bubbles into a water body to promote mixing and increase oxygen levels. This can help to break down stratification and improve water quality.
Destratification
Destratification involves mechanically mixing a water body to break down stratification. This can be achieved using devices such as mixers or water circulators.
Hypolimnetic Oxygenation
Hypolimnetic oxygenation involves injecting oxygen into the hypolimnion to increase oxygen levels and prevent anoxia. This can help to maintain a healthy ecosystem and improve water quality.