On land, the most important environmental conditions1 affecting organisms are rainfall and temperature2. In our oceans, there are a number of environmental conditions that affect the growth, survival and productivity of marine organisms. These include light availability, oxygen3 levels, water movement, salinity4, density5 and pH6. These conditions often vary from habitat7 to habitat and will either support or limit the life processes of the marine organisms living there.
Light
On land, most photosynthesis8 occurs at or just above ground level, and there is no easily visible change in light as altitude9 increases. Things are quite different in the sea! Photosynthesis can only occur when enough light penetrates down the water column10. This area is called the photic zone and varies from a few metres11 to as deep as 150 metres in some places.
The amount of light that can penetrate the surface is limited by a number of factors. For example, rays of light that approach at a low angle (at sunrise and sunset) are immediately reflected off the surface. Particles of organic matter12 and sediments13 from land also affect how much light is absorbed. All primary productivity occurs in the photic zone, and as a result, about 90% of all marine life lives in this upper zone.
Oxygen
Just as it is on land, oxygen is a very important gas in the ocean because of its role in the life processes of marine organisms. Dissolved oxygen in seawater comes from mixing (where the surface meets the atmosphere14) and as a byproduct15 of photosynthesis. As a result, the most oxygen-rich water is found near the surface, so this is where the majority of marine organisms live.
Nature of science
Scientists need to understand how a range of factors can interact within a complex ecosystem16 to determine how changes in one factor could impact on organisms within that ecosystem.
Oxygen consistently makes up about 21% of the air that we breathe on land. However, the amount of dissolved oxygen in seawater varies considerably with temperature and salinity. (As temperature and salinity increase, the amount of dissolved oxygen decreases.)
Oxygen availability in the sea can also be affected by human activities on land. For example, nutrient run-off17 from farmland can cause rapid growth of phytoplankton18 resulting in phytoplankton ‘blooms’. When these large numbers of organisms die, the sharp increase in decomposition depletes oxygen levels. In some cases, this can result in the death of large numbers of other organisms such as fish.
Movement of water
Movement of water, in the form of waves, tides and currents, plays an import role in our oceans and impacts on all marine organisms.
- Waves are caused by wind acting on the surface of the sea. However, they impact on life beneath the surface, too. For example, a wave that is 2 metres high above the surface may be felt up to 6 metres below the surface. Waves can be very destructive; however, they also cleanse organisms and help to increase oxygen availability.
- Tides are caused by the interaction of the forces of the sun and moon; in most places, tides occur twice daily. Tides have the biggest impact on marine organisms that live on coastlines. For example, these organisms need to adapt in order19 to resist drying out and to survive daily changes in temperature and salinity, and exposure to land-based predators at low tide.
- Currents are caused by wind, tides and the global circulation of water. Currents play an important role in moving water between the poles and the tropics. Currents also move food and nutrients20 from the coast further out into the sea. Many organisms also rely on currents to transport their eggs and larvae21.
Salinity
The salinity (or saltiness) of seawater varies considerably throughout the world’s oceans. Although most seawater has a salinity of approximately 3–4%, it can reach up to almost 40% in some areas. In estuaries22, salinity is typically low, as there is regular mixing with a freshwater source, such as a river. Salinity is typically highest in isolated bodies of seawater where the evaporation23 rates are high and precipitation24 rates are low.
Marine animals are adapted25 to keep their body salts at a constant level, so that they don’t interfere with the metabolism26 within cells, but significant changes in salinity can cause problems for some. For example, some marine animals are described as stenohaline, which means they can’t cope with large salinity fluctuations; others are euryhaline and can tolerate a wide range of salinities.
Density
Seawater is more than 800 times denser than air. This high density is related to salinity and temperature and means that objects that might sink in freshwater are able to float in seawater. This has a big effect on life in the sea. For example, the marine ecosystem has communities of plankton27 – vast numbers of floating organisms that are kept perpetually afloat due to the density of seawater. It also helps explain why marine animals can grow to be so much bigger than the largest animals on land.
pH
pH is a measure of the acidity or alkalinity of a solution. Pure water is said to be neutral. The pH of seawater is about 8, but this varies slightly throughout the world. There is scientific evidence28 that suggests that the pH in our oceans is decreasing and therefore becoming more acidic. This could have significant impacts on life in the sea. For example, many marine species29 rely on calcium carbonate to build a shell or skeleton.
One of the effects of increasing acidity is a reduction in the availability of carbonate. This means that any animal that produces a calcium carbonate skeleton will find it much more difficult to do so. Organisms could grow more slowly, their shells could become thinner or they might dispense with shells altogether. It is difficult to predict the overall impact on the marine ecosystem, but many scientists fear that ocean acidification30 has the potential to decrease marine biodiversity31 on a very large scale.
- condition: An existing state or situation; a mode or state of being.
- temperature: A measure of the degree of hotness or coldness of an object or substance. Temperature is measured with a thermometer calibrated in one or more temperature scales. Kelvin scale temperature is a measure of the average energy of the molecules of a body.
- oxygen: A non-metal – symbol O, atomic number 8. Oxygen is a gas found in the air. It is needed for aerobic cellular respiration in cells.
- salinity: The amount of chemicals dissolved in water. In seawater, the main chemical is sodium chloride (salt), but there are many others in smaller quantities.
- density: How tightly a certain amount of matter (atoms or molecules) of a substance is compacted in a given volume. Density is commonly measured in grams per millilitre (g/ml) or cubic centimetre (g/cm3).
- pH: A measure of the acidity or alkalinity of a substance, based on a scale of 0 to 14. Acidic solutions have pH values less than 7, whereas alkaline solutions have pH values greater than 7.
- habitat: The natural environment in which an organism lives.
- photosynthesis: A process that uses the energy from sunlight to convert carbon dioxide and water into carbohydrates, releasing oxygen as a byproduct. Photosynthesis occurs in the green parts of plants, in algae and in some microorganisms.
- altitude: 1. The height of something, usually height above sea level. 2. In astronomy, the angular distance of a natural or artificial satellite above the horizon.
- water column: The vertical section of water between the freshwater or ocean floor and the surface.
- metre: The base unit of length in the International System of Units (SI).
- organic matter: The decomposed remains of living organisms and their waste products.
- sediments: Material that settles to the bottom of a liquid. In geology, it describes the solid fragments of inorganic or organic material that come from the weathering of rock and are carried and deposited by wind, water or ice.
- atmosphere: 1. The layer of gas around the Earth. 2. (atm) A non-SI unit of pressure equivalent to 101.325 kPa.
- byproduct: A secondary product produced during manufacturing, mining or refining. Something unexpected or unintended caused as a result of something else.
- ecosystem: An interacting system including the biological, physical, and chemical relationships between a community of organisms and the environment they live in.
- nutrient run-off: Nutrients, especially excess nutrients from fertiliser applications on pastures and from livestock dung, are carried by surface or ground water and flushed into waterways. This can cause eutrophication.
- phytoplankton: Very small plant organisms that drift with water currents and, like land plants, use carbon dioxide, release oxygen and convert minerals to a form animals can use.
- order: A classification grouping that ranks above family and below class (kingdom > phylum > class > order > family > genus > species).
- nutrient: A substance that provides nourishment for growth or metabolism.
- larva: An immature form that some animals (such as insects, crustaceans and amphibians) pass through before metamorphosing into an adult form.
- estuary: A partially enclosed body of water where freshwater mixes with saltwater from the sea.
- evaporation: The process by which a liquid is converted into a gas, without necessarily reaching the boiling point.
- precipitation: 1. The formation of an insoluble solid (precipitate) from a given solution by altering either its temperature, concentration or chemical composition. 2. In meteorology, this term describes the formation of rain, hail, snow or ice in the atmosphere.
- adaptation: A change in the structure or function of something. In biology, a change in a species, as a result of natural selection. Individuals with a particular feature (adaptation) are more likely to survive and reproduce than individuals without this feature.
- metabolism: Chemical processes that occur in living organisms, for example, producing energy and building the compounds that cells need to survive and divide.
- plankton: A group of marine organisms including single-celled and multi-celled organisms.
- evidence: Data, or information, used to prove or disprove something.
- species: (Abbreviation sp. or spp.) A division used in the Linnean system of classification or taxonomy. A group of living organisms that can interbreed to produce viable offspring.
- ocean acidification: Decrease in ocean pH due to higher levels of dissolved carbon dioxide.
- biodiversity: The range of species found in a particular region. The more species that exist (the higher the biodiversity), the more likely it is that an ecosystem will survive episodes of change.
