Ancient Time plus Size

While differing geomorphology, as measured by the above parameters, influences lake ecology, there are many other factors involved, one of which is geological time. Most lakes exist for hundreds to thousands of years, perhaps even a few hundred thousand years, but some persist for many millions of years, and cognisance of this, together with their size (volume, perhaps area), allows greater understanding. Scale is indeed important in limnology.

By considering a new coefficient, Touchart divides lakes into four broad groups. He estimates the scale of a lake, by taking the logarithm of the product of the age (expressed in millennia) by its volume (in cubic kilometres). Lakes with values 4-8 are almost exclusively large old structural lakes such as Baikal, Caspian, Tanganyika (values of 8), Victoria (7), Issyl-Kul, Aral, Titicaca (6), Balkhash, Tahoe (5), and Biwa, Toba, Taupo (4), among others. Most of these are largely independent of morphoclimatic hazards and have high biological endemism. For instance in Lake Baikal there are 255 species of gammarid amphipods in 35 genera, 34 of which are endemic. New Zealand's

Lake Taupo is an exception, going back perhaps only a million years, but subject to many catastrophic reincarnations as recent as 1800 years ago). Biodiversity is low as a consequence of this Recent age, and also it small size (623 km2) and being on an isolated island.

The next group usually have values 1-4 and are medium sized, largely of morphoclimatic heritage, i.e., shaped largely by the Wurm glaciation. Examples include, Lakes Superior, Great Bear (4), Ontario, Winnipeg (3), Constance, Geneva (2), Te Anau, Wakatipu (both NZ)(1) among a host of others. They are often large (>10 000 km2) and deep (200-500 m), and formed by glacial erosion. Life expectancy is in the order of only thousands/tens of thousands of years. Biodiversity is lower and morpho-dynmaic processes (delta-building, sedimentation) in their basin are relative important in limnological processes. These are the lakes most common in the Temperate Zone of the Northern Hemisphere where limnology developed, so they are the 'standard' lakes of text books. To a large degree both of these classes of lakes are dominated by their shear size, so that morphometric parameters other than area and depth are generally unimportant in influencing limnological processes.

A third group of lakes are small (Touchart coefficients of 1 or less) and largely influenced by current morphodynamic processes or recently inherited ones. These include fluviatile lakes, karstic lakes, aeolian lakes, and landside lakes in the first subgroup and many small glacial and volcanic lakes in the second. All are subject to changing shape and dimensions and many have a particularly precarious existence (e.g., landslide lakes). It is in these lakes where key abiotic variables (e.g., Secchi depth) are related in varying degrees to lake morphology and catchment area.

Touchart's fourth group of lakes also have coefficients of 1 or less; they are the lakes (reservoirs) of human origin. They are of variable size (in area, some challenge lakes with coefficients of 4), but are recent in origin and have a short life span. Moreover, their morphometry is forever changing and their catchments dominate their limnological processes.

Touchart largely omits consideration of short-term changing morphology, partly seen in reservoirs, but absolutely characteristic of intermittent lakes. Such lakes are uncommon where most limnologists live in Europe and North America, but prevalent in the drier parts of Asia, Africa, South America and especially Australia. Some are of very ancient lineage, e.g., Lake Chad, Lake Eyre, but lack ancient/endemic biodiversity because intermittent lakes are useless evolutionary loci. Others (e.g., seasonal lakes due to flooding; shallow lakes due to wind deflation)

are temporary landscape features and if they lie in a distinct basin, are best depicted with inverse contours (i.e., bottom at 0 m, highest level at x metres/ centimetres to account for their fluctuations in water level (Figure 5). Most abound in geomorphic expressions of their shallowness and variability, so that a consideration of their geomorphic parameters together with their hydrologic variability can explain some ecological features.

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