Marine Organisms – Marine Biology – Environment

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Marine Organisms

Marine organisms are a diverse group of living organisms that inhabit the world’s oceans and play crucial roles in marine ecosystems. From the smallest plankton to the largest marine mammals, these organisms contribute to the balance and functioning of aquatic environments. By studying the different categories of marine organisms, such as plankton, phytoplankton, zooplankton, seagrass, and seaweeds, we can gain a comprehensive understanding of their ecological significance, the factors influencing their biodiversity, and the threats they face.


  • Plankton refers to a group of organisms that float in the surface waters of rivers, lakes, and oceans.
  • It includes microscopic plants (phytoplankton) like algae, as well as animals (zooplankton) such as crustaceans and protozoans. They can be found in all aquatic ecosystems except for fast-moving waters.
  • Plankton’s movement is limited, so their distribution is mostly influenced by currents in aquatic ecosystems.
  • Plankton in tropical waters, especially in mangrove areas, exhibit high growth rates, productivity, and species diversity.


  • Phytoplankton are microscopic plant organisms that reside in both saltwater and freshwater environments. The name “phytoplankton” is derived from the Greek words for plant (phyto) and wander or drift (plankton).
  • They encompass various types, including bacteria, protists, and single-celled plants. Common examples include cyanobacteria, diatoms with silica shells, dinoflagellates, green algae, and coccolithophores with chalk-coated bodies.
  • Phytoplankton contribute over 60% of the oxygen produced by all plants.
  • Similar to terrestrial plants, phytoplankton contain chlorophyll to harness sunlight and utilize photosynthesis to convert it into chemical energy. They consume carbon dioxide and release oxygen. While all phytoplankton photosynthesize, some also obtain energy by consuming other organisms.
  • These micro-algae inhabit illuminated areas in seas and oceans, including the Polar Regions.
  • The total biomass of phytoplankton exceeds that of all land plants combined, and they serve as vital food sources within aquatic environments.

Factors Influencing Phytoplankton Biodiversity:

  • Phytoplankton are found in the upper layers of the ocean where there is enough light for photosynthesis to occur.
  • The rate of photosynthesis varies depending on the intensity of light.
  • Phytoplankton require nitrogen and phosphorus as major inorganic nutrients for growth and reproduction.
  • Diatoms and silicoflagellates also need significant amounts of silicate (SiO2).
  • Some phytoplankton can fix nitrogen and grow in areas with low nitrate concentrations.
  • They also require trace amounts of iron, which is often scarce in large ocean areas, limiting phytoplankton growth.
  • Temperature, along with other factors, influences the variation in photosynthetic production.
  • Generally, photosynthesis increases with rising temperature but declines after a certain point is reached.
  • Temperature, combined with light, affects the seasonal variation of phytoplankton production in temperate regions.
  • Salinity, in addition to light and temperature, influences primary production.
Grazing by Zooplankton:
  • The grazing activity of zooplankton is a significant factor in determining the size of the phytoplankton population and the overall production rate.
  • Marine phytoplankton are not evenly distributed across the world’s oceans. The highest concentrations are found at high latitudes, except for upwelling areas on continental shelves, while tropical and subtropical regions have much lower concentrations (10 to 100 times lower).
  • Apart from nutrients, temperature, salinity, and light availability, high levels of solar UV-B radiation in tropical and subtropical regions may also impact phytoplankton distribution.
  • Phytoplankton productivity is limited to the euphotic zone, the upper layer of the water column where there is sufficient sunlight for net productivity.
  • The positioning of organisms within the euphotic zone is influenced by wind and wave actions.

Importance of Phytoplankton:

The Food Web:

  • Phytoplankton serve as the foundation of the aquatic food web as primary producers. They provide sustenance for various organisms, ranging from microscopic zooplankton to large whales. Small fish and invertebrates feed on phytoplankton, and these smaller animals become prey for larger ones.

Phytoplankton – Carbon Cycle and Climate Change:

Carbon Cycle:

  • Phytoplankton play a crucial role in transferring carbon dioxide from the atmosphere to the ocean. During photosynthesis, carbon dioxide is consumed, and the carbon becomes incorporated into phytoplankton. When phytoplankton are consumed or decompose, most of the carbon is returned to near-surface waters, but some sinks into the ocean depths.
  • This biological carbon pump transfers approximately 10 gigatonnes of carbon from the atmosphere to the deep ocean annually. Even slight changes in phytoplankton growth can impact atmospheric carbon dioxide concentrations, which in turn can affect global surface temperatures.


  • Zooplankton play a vital role in the food web, nutrient recycling, and the transfer of organic matter from primary producers to secondary consumers like fish.
  • They are more abundant in mangrove waterways than in nearby coastal waters, and a significant portion of the juvenile fish in mangrove habitats rely on zooplankton as their food source.
  • Zooplankton populations determine the abundance of fish stocks. Therefore, the quality and species diversity of zooplankton communities are used to assess ecosystem productivity, fisheries resources, fertility, and overall ecosystem health.
  • Zooplankton range from tiny flagellates to large jellyfish (>50 μm) in size.


  • Seagrasses are marine flowering plants (angiosperms) that resemble grass in appearance.
  • They produce flowers, have strap-like or oval leaves, and possess a root system.
  • Seagrasses grow in shallow coastal waters with sandy or muddy bottoms and require relatively calm areas.
  • They are the only group of higher plants adapted to saltwater environments.
  • Major seagrass meadows in India are found along the southeast coast of Tamil Nadu and in the lagoons of some Lakshadweep Islands. There are also some grass beds around the Andaman and Nicobar Islands. The abundant growth of seagrasses along the Tamil Nadu coast and Lakshadweep islands can be attributed to factors such as high salinity, water clarity, and sandy substrates.

Functions of Seagrass:

  • Seagrass beds provide physical benefits by:
  1. Reducing wave and current energy.
  2. Filtering suspended sediments from the water.
  3. Stabilizing bottom sediments to prevent erosion.
  • They serve as habitats for marine invertebrates and fishes.
  • Seagrass beds, particularly in lagoon areas, support higher populations of fish and migratory birds due to the availability of food and shelter.
  • Seagrasses on reef flats and near estuaries act as nutrient sinks, buffering and filtering nutrient and chemical inputs to the marine environment.

Threats to Seagrass Beds:

  • Major threats to seagrass beds include eutrophication, siltation, trawling, coastal engineering constructions, and overexploitation for commercial purposes.


To preserve seagrass beds, it is important to:

  • Map major seagrass beds and identify areas for conservation.
  • Carry out dredging activities far away from seagrass beds to avoid siltation and turbidity that can harm the seagrass.


  • Seaweeds are macroscopic algae that lack true tissues like roots, stems, and leaves. They have leaf-like appendages.
  • They are visible marine plants attached to rocks, corals, and other submerged surfaces in the intertidal and shallow subtidal zones.
  • Seaweeds grow in shallow coastal waters where suitable substrates are available.
  • Seaweeds are classified into different classes based on the color of their pigmentation, such as blue-green, green, brown, and red.

Functions of Seaweeds:

  • Seaweeds serve as food for marine organisms.
  • They provide habitats for fish breeding grounds.
  • They contribute to sediment production.

Uses of Seaweeds:

  • Seaweeds have various uses, including:
  • Food for humans and animals.
  • Fertilizer for plants.
  • Medicinal purposes, such as treating goiter and gastrointestinal disorders.
  • Extraction of commercial products like agar-agar, alginates, and iodine.
  • Production of economically important gases through seaweed biodegradation.
  • Displaying antibacterial activity in some species.
  • Acting as potential indicators of pollution, particularly heavy metal pollution, due to their ability to bind and accumulate metals.

Harmful Effects of Seaweeds:

  • Rotting seaweed can release hydrogen sulfide, a highly toxic gas that has been associated with incidents of poisoning, causing symptoms like vomiting and diarrhea.

Threats to Seaweeds:

  • Threats to seaweeds are similar to those faced by seagrass, including factors like pollution, coastal development, and overexploitation.

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