Econstudentlog

Lakes (I)

“The aim of this book is to provide a condensed overview of scientific knowledge about lakes, their functioning as ecosystems that we are part of and depend upon, and their responses to environmental change. […] Each chapter briefly introduces concepts about the physical, chemical, and biological nature of lakes, with emphasis on how these aspects are connected, the relationships with human needs and impacts, and the implications of our changing global environment.”

I’m currently reading this book and I really like it so far. I have added some observations from the first half of the book and some coverage-related links below.

“High resolution satellites can readily detect lakes above 0.002 kilometres square (km2) in area; that’s equivalent to a circular waterbody some 50m across. Using this criterion, researchers estimate from satellite images that the world contains 117 million lakes, with a total surface area amounting to 5 million km2. […] continuous accumulation of materials on the lake floor, both from inflows and from the production of organic matter within the lake, means that lakes are ephemeral features of the landscape, and from the moment of their creation onwards, they begin to fill in and gradually disappear. The world’s deepest and most ancient freshwater ecosystem, Lake Baikal in Russia (Siberia), is a compelling example: it has a maximum depth of 1,642m, but its waters overlie a much deeper basin that over the twenty-five million years of its geological history has become filled with some 7,000m of sediments. Lakes are created in a great variety of ways: tectonic basins formed by movements in the Earth’s crust, the scouring and residual ice effects of glaciers, as well as fluvial, volcanic, riverine, meteorite impacts, and many other processes, including human construction of ponds and reservoirs. Tectonic basins may result from a single fault […] or from a series of intersecting fault lines. […] The oldest and deepest lakes in the world are generally of tectonic origin, and their persistence through time has allowed the evolution of endemic plants and animals; that is, species that are found only at those sites.”

“In terms of total numbers, most of the world’s lakes […] owe their origins to glaciers that during the last ice age gouged out basins in the rock and deepened river valleys. […] As the glaciers retreated, their terminal moraines (accumulations of gravel and sediments) created dams in the landscape, raising water levels or producing new lakes. […] During glacial retreat in many areas of the world, large blocks of glacial ice broke off and were left behind in the moraines. These subsequently melted out to produce basins that filled with water, called ‘kettle’ or ‘pothole’ lakes. Such waterbodies are well known across the plains of North America and Eurasia. […] The most violent of lake births are the result of volcanoes. The craters left behind after a volcanic eruption can fill with water to form small, often circular-shaped and acidic lakes. […] Much larger lakes are formed by the collapse of a magma chamber after eruption to produce caldera lakes. […] Craters formed by meteorite impacts also provide basins for lakes, and have proved to be of great scientific as well as human interest. […] There was a time when limnologists paid little attention to small lakes and ponds, but, this has changed with the realization that although such waterbodies are modest in size, they are extremely abundant throughout the world and make up a large total surface area. Furthermore, these smaller waterbodies often have high rates of chemical activity such as greenhouse gas production and nutrient cycling, and they are major habitats for diverse plants and animals”.

“For Forel, the science of lakes could be subdivided into different disciplines and subjects, all of which continue to occupy the attention of freshwater scientists today […]. First, the physical environment of a lake includes its geological origins and setting, the water balance and exchange of heat with the atmosphere, as well as the penetration of light, the changes in temperature with depth, and the waves, currents, and mixing processes that collectively determine the movement of water. Second, the chemical environment is important because lake waters contain a great variety of dissolved materials (‘solutes’) and particles that play essential roles in the functioning of the ecosystem. Third, the biological features of a lake include not only the individual species of plants, microbes, and animals, but also their organization into food webs, and the distribution and functioning of these communities across the bottom of the lake and in the overlying water.”

“In the simplest hydrological terms, lakes can be thought of as tanks of water in the landscape that are continuously topped up by their inflowing rivers, while spilling excess water via their outflow […]. Based on this model, we can pose the interesting question: how long does the average water molecule stay in the lake before leaving at the outflow? This value is referred to as the water residence time, and it can be simply calculated as the total volume of the lake divided by the water discharge at the outlet. This lake parameter is also referred to as the ‘flushing time’ (or ‘flushing rate’, if expressed as a proportion of the lake volume discharged per unit of time) because it provides an estimate of how fast mineral salts and pollutants can be flushed out of the lake basin. In general, lakes with a short flushing time are more resilient to the impacts of human activities in their catchments […] Each lake has its own particular combination of catchment size, volume, and climate, and this translates into a water residence time that varies enormously among lakes [from perhaps a month to more than a thousand years, US] […] A more accurate approach towards calculating the water residence time is to consider the question: if the lake were to be pumped dry, how long would it take to fill it up again? For most lakes, this will give a similar value to the outflow calculation, but for lakes where evaporation is a major part of the water balance, the residence time will be much shorter.”

“Each year, mineral and organic particles are deposited by wind on the lake surface and are washed in from the catchment, while organic matter is produced within the lake by aquatic plants and plankton. There is a continuous rain of this material downwards, ultimately accumulating as an annual layer of sediment on the lake floor. These lake sediments are storehouses of information about past changes in the surrounding catchment, and they provide a long-term memory of how the limnology of a lake has responded to those changes. The analysis of these natural archives is called ‘palaeolimnology’ (or ‘palaeoceanography’ for marine studies), and this branch of the aquatic sciences has yielded enormous insights into how lakes change through time, including the onset, effects, and abatement of pollution; changes in vegetation both within and outside the lake; and alterations in regional and global climate.”

“Sampling for palaeolimnological analysis is typically undertaken in the deepest waters to provide a more integrated and complete picture of the lake basin history. This is also usually the part of the lake where sediment accumulation has been greatest, and where the disrupting activities of bottom-dwelling animals (‘bioturbation’ of the sediments) may be reduced or absent. […] Some of the most informative microfossils to be found in lake sediments are diatoms, an algal group that has cell walls (‘frustules’) made of silica glass that resist decomposition. Each lake typically contains dozens to hundreds of different diatom species, each with its own characteristic set of environmental preferences […]. A widely adopted approach is to sample many lakes and establish a statistical relationship or ‘transfer function’ between diatom species composition (often by analysis of surface sediments) and a lake water variable such as temperature, pH, phosphorus, or dissolved organic carbon. This quantitative species–environment relationship can then be applied to the fossilized diatom species assemblage in each stratum of a sediment core from a lake in the same region, and in this way the physical and chemical fluctuations that the lake has experienced in the past can be reconstructed or ‘hindcast’ year-by-year. Other fossil indicators of past environmental change include algal pigments, DNA of algae and bacteria including toxic bloom species, and the remains of aquatic animals such as ostracods, cladocerans, and larval insects.”

“In lake and ocean studies, the penetration of sunlight into the water can be […] precisely measured with an underwater light meter (submersible radiometer), and such measurements always show that the decline with depth follows a sharp curve rather than a straight line […]. This is because the fate of sunlight streaming downwards in water is dictated by the probability of the photons being absorbed or deflected out of the light path; for example, a 50 per cent probability of photons being lost from the light beam by these processes per metre depth in a lake would result in sunlight values dropping from 100 per cent at the surface to 50 per cent at 1m, 25 per cent at 2m, 12.5 per cent at 3m, and so on. The resulting exponential curve means that for all but the clearest of lakes, there is only enough solar energy for plants, including photosynthetic cells in the plankton (phytoplankton), in the upper part of the water column. […] The depth limit for underwater photosynthesis or primary production is known as the ‘compensation depth‘. This is the depth at which carbon fixed by photosynthesis exactly balances the carbon lost by cellular respiration, so the overall production of new biomass (net primary production) is zero. This depth often corresponds to an underwater light level of 1 per cent of the sunlight just beneath the water surface […] The production of biomass by photosynthesis takes place at all depths above this level, and this zone is referred to as the ‘photic’ zone. […] biological processes in [the] ‘aphotic zone’ are mostly limited to feeding and decomposition. A Secchi disk measurement can be used as a rough guide to the extent of the photic zone: in general, the 1 per cent light level is about twice the Secchi depth.”

“[W]ater colour is now used in […] many powerful ways to track changes in water quality and other properties of lakes, rivers, estuaries, and the ocean. […] Lakes have different colours, hues, and brightness levels as a result of the materials that are dissolved and suspended within them. The purest of lakes are deep blue because the water molecules themselves absorb light in the green and, to a greater extent, red end of the spectrum; they scatter the remaining blue photons in all directions, mostly downwards but also back towards our eyes. […] Algae in the water typically cause it to be green and turbid because their suspended cells and colonies contain chlorophyll and other light-capturing molecules that absorb strongly in the blue and red wavebands, but not green. However there are some notable exceptions. Noxious algal blooms dominated by cyanobacteria are blue-green (cyan) in colour caused by their blue-coloured protein phycocyanin, in addition to chlorophyll.”

“[A]t the largest dimension, at the scale of the entire lake, there has to be a net flow from the inflowing rivers to the outflow, and […] from this landscape perspective, lakes might be thought of as enlarged rivers. Of course, this riverine flow is constantly disrupted by wind-induced movements of the water. When the wind blows across the surface, it drags the surface water with it to generate a downwind flow, and this has to be balanced by a return movement of water at depth. […] In large lakes, the rotation of the Earth has plenty of time to exert its weak effect as the water moves from one side of the lake to the other. As a result, the surface water no longer flows in a straight line, but rather is directed into two or more circular patterns or gyres that can move nearshore water masses rapidly into the centre of the lake and vice-versa. Gyres can therefore be of great consequence […] Unrelated to the Coriolis Effect, the interaction between wind-induced currents and the shoreline can also cause water to flow in circular, individual gyres, even in smaller lakes. […] At a much smaller scale, the blowing of wind across a lake can give rise to downward spiral motions in the water, called ‘Langmuir cells‘. […] These circulation features are commonly observed in lakes, where the spirals progressing in the general direction of the wind concentrate foam (on days of white-cap waves) or glossy, oily materials (on less windy days) into regularly spaced lines that are parallel to the direction of the wind. […] Density currents must also be included in this brief discussion of water movement […] Cold river water entering a warm lake will be denser than its surroundings and therefore sinks to the buttom, where it may continue to flow for considerable distances. […] Density currents contribute greatly to inshore-offshore exchanges of water, with potential effects on primary productivity, depp-water oxygenation, and the dispersion of pollutants.”

Links:

Limnology.
Drainage basin.
Lake Geneva. Lake Malawi. Lake Tanganyika. Lake Victoria. Lake Biwa. Lake Titicaca.
English Lake District.
Proglacial lakeLake Agassiz. Lake Ojibway.
Lake Taupo.
Manicouagan Reservoir.
Subglacial lake.
Thermokarst (-lake).
Bathymetry. Bathymetric chart. Hypsographic curve.
Várzea forest.
Lake Chad.
Colored dissolved organic matter.
H2O Temperature-density relationship. Thermocline. Epilimnion. Hypolimnion. Monomictic lake. Dimictic lake. Lake stratification.
Capillary wave. Gravity wave. Seiche. Kelvin wave. Poincaré wave.
Benthic boundary layer.
Kelvin–Helmholtz instability.

January 22, 2018 - Posted by | Biology, Books, Botany, Chemistry, Geology, Paleontology, Physics

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