Thresholds Database > Lake: ecosystem shift with glaciation, Lake Baikal (USSR) and Lake Hovsgol (Mongolia)

Background

Lake Baikal is 1642 metres deep, located at 455 metres elevation, is 20-30 million years old (Logatchev et al. 1974; Goulden et al. 2004) and has more than 1600 endemic aquatic species (Timoshkin 1999). During the Last Glacial Maximum (LGM)-Holocene transition, glacial tongues extended into the lake and the meltwater streams deposited fine detritus into the water (Bezrukova et al. 1991). The LGM sediments show a lack of diatoms and chrysophyte algae and a low abundance of all other phytoplankton.



Lake Hovsgol is 262 metres deep, 2.5-4 million years old and is located at 1645 m elevation (Karabanov et al. 2004). It has approximately 390 species, but only about 20 species are endemic (Kozhova et al. 2000). During the LGM, lake levels were 200 metres lower than at present (Fedotov et al. 2002) and meltwater containing fine detritus was deposited into the lake.

Alternate Regimes

1. High diversity and abundance of lake flora and fauna (pre-glaciation)

2. Very low diversity and abundance of lake flora and fauna (during and just following glaciation)

3* Relatively high diversity and abundance of lake flora and fauna, but species composition is largely different from Regime 1 (pre-glaciation)

In this example, the function (production or abundance of flora and fauna) was reversible, but the structure (species composition) changed and as such was irreversible.

Mechanism

Lake Baikal

The disappearance of plankton in Lake Baikal during the LGM probably occurred due to a nutrient-depleted and turbid surface layer resulting from glacial meltwater (Karabanov et al. 2004). Meltwater flow came from the base of glaciers each summer and carried a high concentration of fine ground siliclastic grains ('rock-flour'). The small grains (1-0.1 mm) remained suspended in the water for up to a year. The high turbidity made the transparency of the water and the seasonal ice very low. Transparency of seasonal ice is important because in Lake Baikal, planktonic algae have a maximum productivity under ice during late winter and spring (Popovskaya, 1987). A combination of low surface-water temperature, low nutrient loading and very low ice transparency suppressed primary production of algae in the lake (Karabanov et al. 2004). The longevity of the disturbance probably led to a total collapse of the pelagic and benthic food webs including zooplankton, invertebrates and fish, leaving the lake largely abiotic. Recovery started around 17,000 years ago at the beginning of deglaciation, but there have been other glacial periods that have also impacted on lake productivity. Climate cycling every few thousand years has probably been an important mechanism of diatom evolution in Lake Baikal. Other species probably survived the glacial period by occupying refugia within Lake Baikal (e.g. relatively shallow areas such as lagoons and deltas).



Lake Hovsgol

LGM clay in Lake Hovsgol is almost barren of diatoms (Karabanov et al. 2004). It is likely this resulted from a combination of low surface-water temperature, the formation of a nutrient-depleted turbid surface layer and a change in water chemistry. Lake levels were 200 metres lower than present levels, leaving the lake with a water depth of only 50-60 metres (Fedotov et al., 2002). This decrease in water volume drastically altered the water chemistry, leading to an accumulation of carbonates (calcite, dolomite and magnesium calcite). Many glaciers appear to have terminated in the lake and icebergs filled the lake, probably remaining year-round. The effects in Lake Hovsgol were probably more intense than in Lake Baikal as the lake is smaller, shallower and had stronger glacial conditions due to its higher elevation. Lake level rise occurred only around 12,000 years ago with present lake levels occurring around 8,500 years ago. The smaller, shallower depth of the lake and the stronger glacial condition probably did not provide refugia for most groups of plants and animals and therefore the ecosystem is considered to be very young (perhaps 10,000 years old). When the lake levels rose, species from surrounding areas invaded the lake, possibly complemented by the evolution of new species.

Management Decisions in Each Regime