We have considered the physico-chemical characteristics of water bodies and the processing of organic matter by biological communities. This has led us to consider many areas where water bodies are used by humans, but we are terrestrial animals and don’t easily identify with water as a medium. We have already considered the food and feeding of aquatic organisms and in the next chapters I will describe further what water is like to live, and move, in.

Unlike air, water provides a medium in which many organisms live and it is important that all water bodies are viewed as 3-dimensional habitats. Indeed, it can be argued that the waters of lakes and oceans are the most important sources of primary and secondary production in the World because of the numbers of organisms supported within their huge volume. To understand more about living in water we must first examine some principles of hydrodynamics and three important equations are presented in Equations 8a – 8c (formulae used in hydrodynamics - based on those in Steven Vogel's book "Life in Moving Fluids"). At the outset we know that water is much denser than air and organisms that are buoyant, or maintain their position by swimming, are supported within the water column.

Equation 8a. Dynamic viscosity refers to the "stickiness" of imaginary sheets (laminae) of water relative to each other and is given by:

µ = FL/US

where µ = dynamic viscosity (kg m^-1 s^-1)
F = force to maintain uniform velocity of one sheet of water relative to another (kg m s^-2)
L = distance between sheets (m)
U = uniform velocity between sheets of water (m s^-1)
S = area of sheets (m²)

From this equation, it is clear that a greater force is required to maintain uniform movement of an object as a fluid becomes more viscous. We use the concept of dynamic viscosity in two further equations that help our understanding of movement of objects in water: sinking rate, which relates to buoyancy; and Reynolds number, which relates to the pattern of flow around an object.

Equation 8b. Sinking rate of an object is given by:

R = (Sg (D₁-D₂))/µ

where R = sinking rate (m s^-1)
S = cross-sectional area of the object (m²)
g = gravitational acceleration (m s^-2)
D₁ = density of the object (kg m^-3)
D₂ = density of fluid in the environment (kg m^-3)
µ = dynamic viscosity (kg m^-1 s^-1)

Equation 8c. Reynolds number is given by:

Re = DLU/µ

where Re = Reynolds number (dimensionless number)
D = density of fluid (kg m^-3)
L = length of the moving object (m)
U = velocity of the moving object (m s^-1)
µ = dynamic viscosity (kg m^-1 s^-1)

Buoyancy

As seen from equation 8b, sinking rate is decreased when dynamic viscosity increases and this, in turn, is related to the density of the object and the density of water in which the object is moving. We are aware of this as it is much easier for a human to float in the Dead Sea than in an oligotrophic lake, as the water in the Dead Sea is made very dense by high levels of dissolved salts [8.1.]. Organisms living in the oceans similarly have a higher buoyancy than those in fresh waters but it is still insufficient to prevent them sinking, unless they have a mechanism that promotes buoyancy.

For an organism of a given size, the only effective way to increase buoyancy is to alter its density relative to that of the surrounding water. One way in which this is achieved is for cells to contain oils and other fluids of low density. This is a mechanism used by many planktonic algae and protists.

Jellyfish have a gelatinous skeleton that keeps the body shape [8.2.] and the jelly is also of lower density than sea water and thus acts as a buoyancy aid [8.3.]. Comb jellies [8.4.] also have a skeleton of jelly and some replace heavier sulphates with lighter chlorides within their bodies, another method of reducing the total density of body fluids. In both jellyfish and comb jellies, the animals are not dependent on buoyancy alone as they have means of swimming to maintain position, jellyfish using pulses of the umbrella and comb jellies the beating of fused cilia over their body surface.

Perhaps the commonest means of reducing body density in aquatic organisms is by the use of gases. When humans first learn to float on the surface of water a deep breath of air is always the precursor, the lungs becoming inflated as a buoyancy aid. First swimming lessons in the developed world are often also accompanied by some strap-on inflatable device. Some examples of aquatic animals that use gas inclusions show the variety of mechanisms that have evolved to use gas for buoyancy. Larvae of the fly Chaoborus [8.5.] live in ponds where they feed on other planktonic animals. These larvae have a transparent body which gives them the colloquial name "phantom midges" and their eerie appearance is magnified by the presence of silvery paired gas bags at the anterior and posterior of the body. These structures are filled with gas from the insects' respiratory system and provide floats that maintain buoyancy, with the body held level in the water. Other animals that use gas floats to maintain buoyancy within the water column are the many fish that have swim bladders (the means by which they remain apparently suspended in the water column without swimming) [8.6., 8.7.], nautiloids (relatives of the octopus that have a coiled shell containing chambers filled with gas [8.8.]), and cuttlefish (that have an internal shell that is porous and which has the amount of fluid within it varied to maintain position in the water column). Some surface-dwelling animals gulp air into their guts to act as a float and yet others use rafts of bubbles which are blown and coated with mucus to make them semi-permanent (e.g., Janthina [8.9.]). A permanent aerial gas bag is used by primitive colonial organisms like the "Portuguese Man o' War" (Physalia, [8.10.]), a relative of jellyfish with several polyps attached to a single polyp modified into a large gas-filled bag. Intriguingly, the bag can be dipped into the water and is also used as a sail to enhance movement in water currents by harnessing wind energy.

The other way in which organisms reduce their sinking rate is to increase the drag over their bodies so that there is an increased resistance to sinking, just as occurs when sedimenting faecal pellets become more diffuse and porous. Several very small planktonic organisms bear numerous spines or setae, and some trail feathery extensions. All these modifications serve to increase drag.

When an object moves through water, or when water flows over an object, it is subjected to drag. Two forms of drag occur: skin friction drag created by water moving over the surface of the object and pressure drag that results from turbulence. All organisms moving in water create skin friction drag but it is only significant in the smallest organisms moving at the slowest speeds

The extent of pressure drag is dependent on the degree of turbulence in the wake and the pattern of turbulence relates to Reynolds number (equation 8c). If we imagine a cylindrical object over which water is flowing, we can identify four patterns of wake [8.11.]. Where Re < 10 there is creeping flow around the object, with sheets of water displaced and then rejoining behind the object without a wake being formed. The re-joining of the sheets serves to push the object from behind (rather in the manner of a zip-fastener) so that the only drag experienced is from water passing over the surface. This pattern of flow is characteristic of unicellular organisms. When Re < 40 but > 10 attached vortices begin to appear in the wake so that the laminar nature of the sheets is maintained, although energy is lost from the zip-fastener effect. This range of Re values is typical of small and slow-moving organisms and pressure drag is now becoming significant. Larger, faster animals with Re of < 200,000 but > 40 show von Karman trails, where vortices are detached alternately and trail behind the swimming animals in a broad wake. Above Re of 200,000 we reach the realm of large and fast-swimming animals where the wake is narrower but fully turbulent and considerable energy is required to push the animal through the water against very high pressure drag.

Given the pattern of flow around objects it is not surprising that the shapes of large, rapidly moving organisms show adaptations to reduce drag. Organisms creating turbulent wakes are often streamlined. For all but the largest and fastest-moving organisms the ideal streamlined shape has maximum width about a third of the way along the body and a width : length ratio of about 0.25. We are familiar with this shape as it is shown by some fast-swimming fish that we eat (e.g. mackerel [8.12.] or herring [8.13.]). A streamlined shape does not prevent drag but narrows the cross-sectional area of the turbulent wake so that pressure drag from the body is reduced. The shape causes the water laminae to follow along the animal's body for a greater distance before they break away to form the wake. Streamlining is also expected where stationary organisms create turbulent wakes in flowing water and may explain the body shape of some organisms found on rocky shores (e.g., limpets [8.14.]) or in rivers (e.g. some mayfly larvae [8.15.]).

Very large, fast-swimming animals such as whales sailfish (with Re well in excess of 200,000) have a variant of this streamlined shape with their bodies being roughly parallel-sided for much of their length before tapering towards the posterior [8.16.]. Water flows relatively smoothly along the body before breaking up into the turbulent wake. The retention of water laminae close to the body is probably caused by roughening of the body. This increases skin friction drag but this is a minute fraction of total drag in these large animals, so power needed to overcome skin friction drag is a similarly tiny fraction of the overall power requirement. Roughness of the body surface creates local microturbulence that acts as a lubricant over which water laminae can pass with minimal disruption. This mechanism is probably commonplace among aquatic organisms that move rapidly. It is also a characteristic that is used in the design of fabrics use in making costumes for competitive swimmers, the latest styles extending from neck to knee and using fabrics with alternating smooth and rough bands that generate microturbulence close to the body.

Drag is disadvantageous for most organisms. Power is needed to overcome the effects of drag and to overcome the pull on the body of sessile animals that can cause them to be dislodged. However, there are some ways in which animals exploit drag to their advantage. For example, skin friction drag of very small organisms may decrease their sinking rate and thus reduce the amount of energy needed to maintain position within the water column [8.17.]. Larger pelagic animals can also use drag to reduce sinking rate and extensions from the body probably serve this function. Jellyfish, with buoyancy provided by their jelly, also use the umbrella as a form of underwater parachute.

8.1. http://www.jewishvirtuallibrary.org/images/deadsea.jpg
8.2. http://www.ucmp.berkeley.edu/cnidaria/scyphozoa.html
8.3. http://jellieszone.com/gelatinousanimal.htm
8.4. http://www.whoi.edu/cms/images/DSC_0246_C_97125.jpg
8.5. http://www.microscopy-uk.org.uk/mag/imagsmall/chaoborus2.jpg
8.6. http://www.ucmp.berkeley.edu/vertebrates/actinopterygii/actinomm.html
8.7. http://australianmuseum.net.au/image/Fish-Dissection-Swim-bladder-exposed/
8.8. http://woodwose.files.wordpress.com/2009/10/800px-nautilus_profile.jpg
8.9. http://www.roboastra.com/hastmoll1/images/hppr8411.jpg
8.10. http://australianmuseum.net.au/Uploads/Images/7652/j032_big.jpg
8.11. http://www.princeton.edu/~asmits/Bicycle_web/blunt.html
8.12. http://www.glaucus.org.uk/Mackerel.jpg
8.13. http://i01.i.aliimg.com/photo/v1/109204783/Atlantic_Herring_Clupea_harengus_.jpg
8.14. http://www.univ-lehavre.fr/cybernat/pages/patevulg.htm
8.15. http://www.royal-flyfishing.com/cms/upload/bilder/Berichte/Entomologie/Eintagsfliegen/Eintagsfliege_Heptagenia_Bild4.jpg
8.16. http://animals.nationalgeographic.com/staticfiles/NGS/Shared/StaticFiles/animals/images/1024/sailfish.jpg
8.17. http://www.microscopy-uk.org.uk/mag/imgsep01/dinmon.jpg