Water's unique properties play an important role in shaping life in the sea.

 

Seawater is a complex solution with important properties as a solvent, heat sink, and absorber of light.

Water is also unique because its density as a solid is less that as a fluid (ice floats): Why is this important? 

Water motion in the form of currents, waves, and tides also plays important biological roles.

 

Water as a solvent

Water's molecular polarity allows binding with ions

Hydrophobic reactions also play important biological roles (e.g. lipid membranes, protein folding, etc.)

"Salinity" reflects a to the concentration of a collection of many inorganic solids within water.

The long residency of many elements (Na, Cl, Sr, etc.) keeps them in relatively constant ratio.

Major elements (0.1 - 20 ppt): Cl^-; Na⁺; SO₄^-2; Mg⁺²; Ca⁺²; K⁺; HCO₃^-

Minor elements (0.1 - .001 ppt): Br^-; H₂BO₃^- ; Sr⁺²; F^-

Trace elements (e.g., N, P, Fe) may be biologically limiting

Salinity is influenced by evaporation and dilution

Oxygen dissolves at the sea surface. This, plus photosynthesis, minus respiration, determines local O₂ levels

Oxygen dissolves more in cold water, and less in salty water (i.e. lots of O₂ at the poles)

Anoxic conditions may persist in deep, dark places

Water as a heat sink

It takes 1 calorie to raise 1 gram of water 1 °C

Translation: It takes lots of heat to change water's temperature

Water has a high heat of vaporization

Translation: water absorbs lots of heat before evaporating

Bottom line: Water's ability to hold and transfer heat contributes directly to the world's climate

Water as an absorber of light

Sunlight hits the earth from different angles and is either reflected or absorbed

Absorption of different wavelengths occurs at different rates

Infrared and ultraviolet attenuate rapidly. Blue penetrates better than red

Light intensity diminishes exponentially with depth

Important for photosynthesis and for visual predators seeking prey

Intense light levels in shallow water can be bad

Diffusion within seawater allows molecules (nutrients, gasses) to disperse

All else being equal, this should promote uniformity

Various factors may counter the effects of diffusion

Differences in water density (pycnoclines) may inhibit mixing

Temperature plays a role, as warmer, less dense water tends to rise.

Solar heat creates thermoclines that vertically stratify water masses

Salinity also affects density

Cold, saline waters sink

Pressure increases with depth, this changes the solubility of gasses

Water motion may also disrupt or override diffusion

Ways that water moves: currents, tides, waves

Currents are directional flows

Water movements directly influence patterns of temperature and nutrient availability

Winds and the Earth's west to east rotation cause global currents

Coriolis effects within fluids: particle deflection on a rotating sphere

To the right in the north and to the left in the south

Westerlies and trade winds are driven by atmospheric pressure differentials

 

These winds, plus Coriolis effects, create hemispheric gyres of water motion

Clockwise in the north, counterclockwise in the south

Try the internet for additional explanations of how coriolis effects influence water motion

Yet another water page, including a bit of animation about coriolis effects

Wind on water along western shores produces upwelling of deeper water

These cold, nutrient-rich waters increase biological productivity

Counter currents may occur when water piles up

Strong current reversals (El Nino events) shut down upwelling

Large storms can also cause water to pile up (storm surges)

Not all currents are obvious from the surface

Deep water currents originate at the poles.

They reflect differences in temperature and salinity,

These currents deliver oxygen rich waters to the depths

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Waves: wind driven "packets" of energy

Why are waves important?

Waves are a "predictable" source of disturbance along coastlines

Wave energy determines grain size

Most waves originate from wind

Wave size/height (crest to trough) depends on fetch, wind speed, and duration

Other important components of wave energy

Wavelength (L)

Period (T)

Velocity (V)

These are related: V = L/T

Waves break after "feeling bottom"

Shoreline topography influences wave energy

Refraction may focus energy on headlands

Rip currents sweep away from shore

Longshore currents move down the coast

Seasonal differences in wave energy can shift local topography (berms and bars)  

Tsunamis ("tidal waves"): seismic waves have nothing to do with tides

High speed (600 km/hr) = lots of energy!

Unpredictable catastrophic events on some coasts causes intermittent temporal disturbance on a massive scale

This video gives a sense of how tsunamis come ashore

Tides generate locally predictable patterns of depth and current

Thus, tides influence many aspects of marine biology. For example: The intertidal

Reproduction is also often linked to the tides

What causes tides?

Gravity waves driven by the motion of sun and moon

Relative celestial position influences tides

Here are two good sites that talk about tides and their predictions:

http://co-ops.nos.noaa.gov/tp4days_old.html

http://www.opsd.nos.noaa.gov/about2.html#ABOUT

Tide and current predictions

Neap tides and Spring tides

Patterns of ebb and flood are also influenced by local topography

In combination, the various properties of water create or organize many different ecological patterns within the sea.