Marine Communities

The study of the interaction of living things with each other and with their ocean environment is called marine ecology. The study of marine ecology requires and understanding of the biotic and abiotic factors found within the ocean. Biotic and abiotic factors interact within an ecosystem. Biotic factors include all of the living things in a particular community, such as plants, animals, and bacteria. The abiotic factors include all of the non-living things such as the physical surroundings, density, dissolved oxygen, and pH.

There are other factors called limiting factors that determine how successfully organisms can live within an environment. Some of biotic and abiotic factors are considered limiting factors. Specifically, limiting factors are physical or biological aspects of an environment that regulate an organism’s distribution.

Physical limiting factors include temperature, salinity, pressure, nutrients, and light availability.

  • Temperature – Ocean temperatures determine metabolic rate. Most marine organisms are ectothermic some are endothermic. Endotherms require more energy, but can handle more extreme temperatures. Although the average temperature of the ocean is about 17oC, the temperature of the ocean can vary based on latitude.
  • Salinity – Normal salinity in ocean environments is 35 parts per thousand. Too much salinity increases evaporation and decreases biodiversity. Organisms maintain salt balance through diffusion, osmosis, and active transport.
  • Pressure –The average ocean depth is about 3800 meters. The mass of this water creates hydrostatic pressure. An organism that lives near the surface feels little effect from hydrostatic pressure. However, an animal living at a depth of 1000 feet is influenced by 500 pounds of hydrostatic pressure. Thankfully, these marine organisms have adaptations that allow them to adjust to the pressure.
  • Nutrients – Nutrients such as nitrates, phosphates, iron, and silicon are essential to marine organisms. These nutrients cycle with carbon, oxygen, and sulfur to provide the energy needed for ocean organisms. Nutrient levels are highest in estuaries and marshes.
  • Light – Organisms must adapt to the light available in their habitat. Light penetrates deeper in the open ocean than in the coastal regions. Light availability defines photosynthesis and productivity which provides the foundation for all living things in the ocean.
  • Density – Water density is determined by the mass of the water divided by the space it occupies. The density of sea water is between 1.020 and 1.030g/cm3 compared with pure water which has a density of 1.00g/cm3. Seawater density increases with increasing salinity, increasing pressure, and decreasing temperature. Much of the ocean is divided into three density zones: the surface zone, the pyconocline, and the deep zone.
  • Dissolved Oxygen – Dissolved oxygen (DO) is the amount of oxygen dissolved in the water. DO is measured in units of parts per million (ppm) or milligrams per liter (mg/L). DO levels in ocean water range from about 1 to 12 ppm. Just for comparison, the amount of O2 in the air is 200 ppm! The minimum DO level needed by fish and other aquatic animals to breathe is 4 ppm; below this level, a condition called hypoxia occurs. Hypoxia occurs when oxygen levels in the water decrease causing fish and other aquatic animals to suffocate. As depth increases, the DO decreases until the measured DO is zero. The depth at which there is the lowest amount of oxygen is called the O2 minimum zone. This is typically found at a depth of 1000 meters.
  • pH – pH is known as the degree of acidity or alkalinity of a liquid based on hydrogen ion concentration. The pH of a substance is measured on a scale from 0 to 14. This scale is called the pH scale. When water disassociates it forms positive ions and negative ions. The negative ions are the OH- ions, which cause a solution to become alkaline and the positive ions are the H+ ions, which cause a solution to become acidic. Pure water has a pH of 7, but most ocean water is, on average, about 8, which means it is slightly alkaline.

Biological limiting factors include competitionpredation, and herbivory.

  • Competition is a naturally occurring process that takes place when organisms must share an environment. Organisms typically compete for food, shelter, and mates. Competition tends to increase when resources are limited.
  • Predation is the interaction between organisms in which one organism captures and feeds upon another. Therefore, marine organisms must find ways to avoid predation and ensure reproduction.
  • Herbivory is described as the act of organisms eating producers. Producers are the foundation for any environment. Autotrophic availability determines how an ecosystem will flourish. If there are limited amounts of primary food sources, the consumers will perish.

All organisms depend on other organisms for survival within a community and it is therefore important to understand the relationships between the interacting organisms. A single individual of a species is referred to as an organism. Several organisms of the same species make up a population. Several populations of varying species make up a community and various communities make up an ecosystem.

Although all of the organisms in an ecosystem live together, they each live within a particular place in the community. A habitat is the place where an organism lives out its life. Several species may share the same habitat, but each organism has its own unique niche. A niche is the role and position that a particular species has in an environment. In the marine environment, there are particular habitat categories based on an organism’s lifestyle.

Life consists of relationships in which people depend on one another for their everyday needs, such as food and shelter. In the marine world, there are relationships among living things in which organisms depend on one another for survival. A relationship in which organisms interact to ensure survival is called interdependence. Some species enhance their chances of survival by forming relationships with other species. These relationships are referred to as symbiotic relationships. The three types of symbiotic relationships are mutualismcommensalism, and parasitism.


Mutualism is a relationship in which both species benefit. A common example found in the ocean is the relationship between the clownfish and the sea anemone. The clownfish benefits from this relationship because it is protected from predators by the sea anemone’s stinging tentacles, which do not harm the clownfish. The sea anemone benefits from the relationship by receiving scraps of food left over by the clownfish and the bright colors of the clownfish attract other fish to the anemone tentacles.

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Commensalism is a relationship in where one species benefits, while the other is not affected by the association. A marine example includes the remora and sharks. The remora often swims below or attached to the underside of a shark. The remora benefits by eating leftovers from the shark’s meals and the shark appears to be neither helped nor harmed by the remora.


Parasitism is a relationship in which one species benefits at the expense of the other. In the case of fin rot, the parasite is the beneficiary, while the fish, also referred to as the host is harmed. From this relationship, the fish suffers and eventually dies from disease while the parasite lives by causing the infection in host fish.


Biodiversity is most easily defined as the variety of life in an area. The easiest way to determine biodiversity is to account for the number of species that live in a particular area. The coral reefs would be a good example of a marine environment high in biodiversity. There are several different species present in the reefs and their presence provides stability to the ecosystem. The kelp forest off the coast of California is a good example of what happens when biodiversity is threatened. Many sea otters were hunted and killed within the last century and the loss of their presence resulted in an increase in their prey, the sea urchin. The increase in the sea urchin population further resulted in the depletion of the kelp forest, the primary food source for sea urchins. The protection of sea otters has resulted in their reintroduction to the kelp forest and a decline in the sea urchin population which in turn has allowed the kelp forest to flourish again.

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