On land, the most important environmental conditions affecting organisms are rainfall and temperature. In our oceans, there are a number of environmental conditions that affect the growth, survival and productivity of marine organisms. These include light availability, oxygen levels, water movement, salinity, density and pH. These conditions often vary from habitat to habitat and will either support or limit the life processes of the marine organisms living there.
On land, most photosynthesis occurs at or just above ground level, and there is no easily visible change in light as altitude increases. Things are quite different in the sea! Photosynthesis can only occur when enough light penetrates down the water column. This area is called the photic zone and varies from a few metres 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 matter and sediments 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.
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 atmosphere) and as a byproduct 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 ecosystem 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-off from farmland can cause rapid growth of phytoplankton 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 order 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 nutrients from the coast further out into the sea. Many organisms also rely on currents to transport their eggs and larvae.
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 estuaries, 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 evaporation rates are high and precipitation rates are low.
Marine animals are adapted to keep their body salts at a constant level, so that they don’t interfere with the metabolism 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.
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 plankton – 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 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 evidence 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 species 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 acidification has the potential to decrease marine biodiversity on a very large scale.