**5.1 Classification of lakes**

Lakes are often classified according to their trophy or degree of enrichment with nutrients and organic matter. They are classified by their trophic state with the main classes of oligotrophic, mesotrophic, eutrophic, and dystrophic (**Table 1**).

Several natural water bodies referred as oligotrophic have clearwater ecosystems with limited primary and secondary productivities due to a shortage of major nutrients [60]. These water bodies under natural succession will require thousands of years to transform into eutrophic. The oligotrophic lake is deep and receiving effluents that are nutrient-poor from its drainage basin. Organic matter production is less in the well-illuminated epilimnion. Therefore, the material sinking into the hypolimnion is the small quantity and little oxygen is consumed there during the summer. In contrast, a eutrophic lake is often, but not necessarily, shallower, the drainage basin is richer, and rivers and groundwater discharge into its epilimnion a substantial amount of nutrients. Primary productivity is higher as compared to that of oligotrophic lakes, and therefore, more organic material settles into the hypolimnion resulting in oxygen depletion. As a result, the deeper layers of water of a eutrophic lake become anoxic during summer. Oligotrophic water bodies have <5–10 μg l−1 of phosphorus and < 250–600 μg l−1 nitrogen. Oligotrophic water bodies have mean primary productivity ranging between 50 and 300 mg carbon m−2 day−1. In eutrophic water bodies, the phosphorus concentration is 10–30 μg l−1, while nitrogen concentration content is 500–100 μg l−1. Primary productivity in eutrophic water bodies is >1 g carbon m−2/day−1. If excessive quantities of phosphorus and nitrogen are added to the water, excessive growth of aquatic plants and


**Table 1.**

*Lake classification on the basis of trophy or degree of enrichment with nutrients and in relation to P and N [59].*

algae takes place. As these algae die, they are decomposed by bacteria and in this process, dissolved oxygen is utilized. The decomposers use up the dissolved oxygen of the water body. Due to this dissolved oxygen, concentrations often fall considerably for fish to breathe resulting in fish kills [61].
