Definitions and Limnology

The definition of lakes or ponds, and even more so wetlands, may vary according to the historical time of the definition, countries and scientists. What a “lake” is is still a matter of debate. By some definitions, a lake would be made up of fresh water, unlike seas and oceans that are salty. This definition is incorrect because the Baltic Sea has less than 4 g/l of salt and the Great Salt Lake about 250 g/l.

It is also not possible to classify a water body as a lake or pond according to its local name in toponymy: the same water body is sometimes popularly called a pond or lake or lake and sea. The boundary between the terms lake and pond is blurred. We also know underground lakes (Križna jama in Slovenia) and also underwater lakes, which represent the accumulation of hypersaline water in the deep sea bottom.

In 1892, the Swiss François-Alphonse Forel, a pioneer of limnology (a science engaged in inland water research), clearly defined the lake. He described the 'lake' as a mass of standing water with no direct connection to the sea, located in a hollow in the ground, closed on all sides. A lake is an inland body of water that represents the accumulation of depressed water in land soils and, unlike the inland sea, has no direct connection to the world’s oceans, so it has no inflow or outflow due to sea currents. The inflow and outflow are usually small compared to the total amount of water in the lake.

In contrast to the river, the lake has no slope. The term inland lake is used to distinguish inland lakes from coastal lakes (coastal lakes, coastal brackish lakes or lakes formed by embankments along the coast), as well as to denote lakes in general. A lake in terms of limnological definition is usually much deeper than a pond, pool, or pond, so stable temperature stratification can develop in days to months. The frequency of their mixing is used to classify lakes, as it also has far-reaching ecological implications.

Global Abundance and Scale

In this respect, shallow steppe lakes, such as Lake Neusiedl or Lake Balaton, are not considered "real" lakes (limnologically defined as "shallow lakes"). By more commonly used definition, lakes are just standing water with a depth of more than two meters. Lakes in the true sense are natural bodies of water, but in a broader sense they also include artificial bodies of water, such as reservoirs and lakes left over from mining (eg Lake Velenje). However, the exact demarcation between lakes and pools/ponds, etc., is vague and always subjective.

That’s why some limnologists call any depression filled with water a lake. For their categorization, it would then be irrelevant whether the lake is constantly or occasionally filled with water. Conversationally, allocation often depends on salinity, but this is not a criterion. Although the lake usually contains fresh water, there are also large salt lakes such as the Caspian Sea, the Aral Sea and the Dead Sea. In these cases, the term sea is merely historically/culturally descriptive and refers to lakes.

About 100 million surface lakes, with an area of more than one hectare, have been identified on earth through satellite observation. In total, they cover an area of more than 300 million hectares or about 2% of the land area. The share of the remaining small lakes has long been underestimated: in 2006, the global model made on the basis of Pareto’s law doubled the previous estimate to a total of 4.2 million square kilometers of land area. Taking into account ponds and small reservoirs, 4.6 million square kilometers of inland land area (3%) would be covered by water.

Volume and Climate Impact

Loch Ness in Scotland has an average depth of 132 meters, more than four times deeper than Lake Erie in North America, which is 4500 times larger in area. Approximate models infer that the total volume of the lakes is between 160,000 and 280,000 square kilometers. Cael, in collaboration with Adam Heathcote and David Seekell, showed that the number of lakes in a given area follows a mathematical power law: the smaller the lakes, the more they are arranged in a cluster.

This reassessment suggests that if these lakes are actually shallower than previously thought, they are likely emitting more methane than expected. Their role as carbon sinks would be less important than expected and their contribution to global warming greater than expected. This contribution could be further increased by warming and increasing intake of organic matter due to eutrophication and erosion caused by human activities. The latest estimate gives a total lake volume of 199,000 km³ and an average depth of only 42 meters.

Classification by Origin

In 1957, British ecologist George Evelyn Hutchinson proposed a classification identifies 11 main types of lakes:

Note: Based on Hutchinson's classification, other types include Aeolian, Coastal, Organic/Peat, Anthropogenic, and Meteorite lakes.