Discuss the general features of the global water balance
- More water evaporates from oceans than is returned via precipitation, opposite on land
- Until recently, global water balance fluctuated very little but we have introduced regional fluctuations by extensive environmental modifications - irrigation, industrial and domestic use e.g. exploitation of groundwater reserves
Discuss the water balance of a lake
- Change in storage per unit time = rate of inflow - rate of loss
- Inflows: precipitation directly on lake surface, surface influences (surface runoff and rivers), groundwater seepage, groundwater entering lakes as discrete springs
- Water losses: outlet of lake, seepage into groundwater, evaporation, evapotranspiration due to emergent and floating vegetation
Discuss how human activity can interfere with the water balance of lakes
Reduction in volume of lake due to extraction for irrigation, damming leads to water being diverted so little water reaches the estuary (e.g Colorado River). Because most large lakes are shallow - small changes in elevation result in large changes in water surface e.g. Lake Chad has continuous change in water level largely because of climatic changes and damming
What happened to Aral Sea?
1960: Aral sea was 4th largest lake in the world, massive diversion of water from its inflowing rivers used for irrigation AND building of Karakum Canal drew water from one of its two major inflowing rivers for irrigation thousands of kilometers away (inefficient - evaporation and lost through seepage) 1960-1990: water level dropped 13m, volume shrunk by 60%. City Aralsk used to be a harbor but the shoreline is now 60km further south. Left behind a desert of salts - winds transport and deposit this dust and salt to cities causing health problems (tuberculosis, kidney deisease) and destroying soils, income declined due to loss of harbor and fisheries
Why are most lakes on earth relatively young?
- Lakes of glacial origin were formed just after the ice age, they are at most 10,000 years old - erosional and depositional effects of glacial ice movements
- Many lakes are artificial reservoirs created by man
What is a graben lake + give an example
Tectonic/formed by faulting - deep lakes e.g. Lake Tahoe
What is an oxbow lake?
- Originate as dead arms of meandering rivers. Are generally productive as they are shallow + lake basin is enriched with sediments from the river
- River meanders with irregularities of topography - greater erosion occurs on the outer, concave side while deposit of sediment happens on the inner side. Increase in erosion until main course of river cuts a channel through and leaves oxbow lake
Give an example of how lake origin can determine productivity
- Oxbow lake - originate as dead arms of meandering rivers. Are generally productive as they are shallow + lake basin is enriched with sediments from the river
- Lake basins that originated by volcanic activity are unproductive - low conc of nutrients
Explain why shallow lakes tend to be more productive than deep ones
- Shallow lake = lakes that do not stratify due to mixing effect of wind. Mean depth of less than 3-5m. Shallow lakes are more productive
- Light can reach the bottom - photosynthetic activity is possible to a depth near the sediments . Because decomposition yields the nutrients necessary for photosynthetic production of organic molecules and photosynthesis now takes place near the sediment = high productivity
- Shallow lakes do not stratify so wind can resuspend sediments = increases availability of nutrients, nutrients do not sink to the bottom. So anthropogenic activity (eutrophication) is worse/ long term in shallow lakes as in deep lakes, nutrients just sink to bottom
- Shallow lakes have extensive littoral zone = area available for growth of rooted aquatic plants. Littoral communities have high productivity
- Sediments can be heated by sunlight when then act as a secondary heat source for the lake. Higher water temp = increased biological productivity
Why are ancient lakes special and important?
- Not many lakes are more than 10,000 years old. Harbor a large number of endemic species (species that only occur in a certain area). Sufficient time for speciation to occur. Species flocks: a large number of species are very strongly related,
- Often play a central role in economy and history of surrounding countries e.g. Lake Malawi - 75% of animal protein derived from fish, viable tourist attraction
What are endemic species + examples
Species that only occur in a certain area e.g. Caspian seal. E.g. cichlid fish in Lake Malawi have evolved to be mouth breeders - keep larvae in their mouth to protect them from predation by other fish
Which of the following lakes are meromictic and why (+ list of three lakes) ?
- Only part of water column undergoes circulation, layers of water that do not mix - most of the time arise because lower layer of lake is highly saline + denser. Due to continuous decomposition of organic matter in absence of any circulation, deeper water layer becomes anoxic
> Ectogenic meromixis: events bring salt water into lake either - often in marine coastal regions where catastrophic instructions of salt water caused by storms are common
> Biogenic meromixis: accumulation of salts in deeper layer caused by settling organic matter + decomposition of this matter. Initiated when abnormal meteorological conditions prevent complete circulation of lake that is normally holomictic - once certain conc of salinity has achieved, salinity gradient is too high so meromixis persists. All very deep lakes of equatorial tropics are meromictic in nature. Can also occur in small lakes with low area to depth ratio and are sheltered from wind action - Lake Tanganyika in Central Africa, Lake Malawi, Lake Nagada in Papua New Guinea, Lake Windermere in UK
What are the major threats to the ecological integrity and functioning of Lake Victoria?
- Eutrophication has led to cyanobacteria blooms + resulted in lake becoming permanently anoxic below a depth of 20m- cannot sustain large biomass of fish
- Overfishing: in 1905, caught 50-100 fish per day. In 1957, average was only 1.6
- Invasive species: introduction of piscivorous nike perch has led to extinction of more than 100 endemic species, mainly cichlids
Give some examples illustrating the far-reaching consequences of species introductions.
- Lake Victoria - introduction of piscivorous nike perch has led to extinction of more than 100 endemic species, mainly cichlids
- After introduction of tilapia species in man made lake in Sri Lanka, yields increased from 1.07 kg/ha in 1953 to 400 kg/ha in 1978
