**4. Physico-chemical characteristics of lentic ecosystems**

Closed systems are lentic systems. Due to the lack of an exit for the water body, persistent substances, such as the byproducts of the decomposition and mineralisation of organic matter and the degraded or partially degraded pollutants that are released into these aquatic bodies, continue to exist in the system. The biotic communities are significantly impacted by this. Due to the lack of water movement, the physico-chemical environment of lentic aquatic systems is exceptional. The following list of key characteristics of lentic water bodies includes:

## **4.1 Water quality**

Physical–chemical study of a water sample provides a picture of the physical and chemical components, however this analysis simply provides a numerical value. To determine the precise quality of water, an indexing method called the "Water Quality Index (WQI)" has been devised. WQI provides information on the whole aquatic system's quality. "A rating of water quality, which shows the composite influence of different water quality criteria on the overall quality of water," is how the term "water quality index" is defined [20]. Physical, chemical, and biological characteristics of water must all be considered when evaluating its quality because they are interconnected [21]. The majority of the water in the lentic system comes from rainfall, surface runoff, or subsurface water sources. The makeup of these sources does not alter all that much over time. As a result, the water quality in lentic systems has essentially not changed over a lengthy period of time.

### **4.2 Seasonal changes**

A lentic body's positive association with seasonal variations in the physicochemical properties of water affect how productive it is. The make-up of the living community is significantly impacted by the seasonal variations in water quality. A significant increase in phytoplankton is seen during periods of intense sunlight, which is also accompanied by a striking decrease in the concentration of numerous plant nutrients.

#### **4.3 Assessment of productivity**

The entire surface area of the water body is more significant than the water volume or the depth of the water body when determining the productivity of a lentic water body. The productivity is based on the combined length of the limnetic and littoral zones. Even if some lentic bodies' profundal zones are nutrient-rich, productivity is scarcely impacted by a lack of sunlight.

#### **4.4 Stratification and water movement**

One of the most significant characteristics of a lentic water body is the stratification phenomena, often known as vertical zonation. The variation in density brought on by the varied heating of lake waters causes stratification to exist. If the temperature stays consistent at more than 4.0 C when there are significant breezes present, the lake water is well mixed. This is seen in lakes and ponds where the static waters are often deeper, measuring more than 6 to 8 metres. Because of stratification, the lentic water's many strata range in temperature, oxygen concentration, and nutrient status [22]. The lake is stratified into the epilimnion, hypolimnion, and thermocline when the temperature is not constant due to density differences. Lakes are categorised into amictic, meromictic, holomictic, oligomictic, monomictic, dimictic, and polymictic lakes based on their circulation patterns. Thus, the temperature and wind pattern have a significant impact on how the water moves. Water in lakes frequently moves in multiple directions.

### **4.5 Effect of light**

The depth to which light can penetrate depends on the turbidity. The temperature varies with depth and the season. The spectral makeup and intensity of the light there govern how deep rooted macrophytes and associated algae can develop on suitable surfaces.

A lake can be separated into tropogenic zone and tropolytic zone based on the amount of light it receives. The primary production of a lake is influenced by light, and phytoplankton in turn affects the depth of light penetration. The oxygen level of the lentic ecosystem is considerably lower than that of the lotic because only a tiny fraction is in direct contact with the atmosphere and because decomposition occurs intensively at the bottom.
