Aquatic organisms are living organisms that are close-knit and reliant on one another in the aquatic environment. They live in the oceans, rivers, and other bodies of water. Aquatic organisms live their entire lives in the water. They use particular organs to take oxygen from the water. Some ingest it through their skin, while others ingest it through their gills. Neuston, Periphyton, Plankton, Nekton, and Benthos are the categories of aquatic organisms. This article will explain to you about Aquatic Organisms which will be helpful in preparing the Environment syllabus for the UPSC Civil service exam.

Aquatic Organisms - Categories

• Aquatic organisms are categorized according to their occurrence zones and their capacity to cross them.
• The species (both flora and fauna) in the aquatic ecosystem are scattered but can be divided into five groups based on their life form or location.

Neuston

• Neuston, also called pleuston, is a group of organisms that live on the surface of the ocean, an estuary, a lake, river, or pond.
• The term neuston was coined by Naumann in 1917 to characterize organisms found in freshwater ecosystems' surface layers.
• Neuston can live on top of the water's surface or on the underside of it.
• They could also be found in the surface microlayer that arises between the top and bottom sides of the surface.
• They serve as a food source for zooplankton traveling from deeper layers to the surface, as well as seabirds flying above the oceans.

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Periphyton

• In most aquatic ecosystems, periphyton is a complex mixture of algae, cyanobacteria, heterotrophic microorganisms, and detritus clinging to submerged surfaces.
• Periphyton has evolved to live on hard surfaces.
• The temperature range required for periphyton growth may be connected to the absolute temperature range and temperature stability in its environment.
• Periphyton communities are also utilized to remove solids and dissolved contaminants in aquaculture food production systems.
• The cyanobacterium Mastigocladium laminosces is the most common periphyton taxon, mainly because it can withstand freezing and desiccation.

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Plankton

• Plankton is a diverse group of organisms that live in water (or air) and are unable to swim against the stream (or wind).
• Plankters are the individual creatures that make up plankton.
• Planktons range in size from 0.2 meters to more than 20 meters.
• Their distribution varies depending on the availability of light and nutrients, from microscopic bacteria to giant species like jellyfish.
• Planktons are a source of food for large aquatic species.

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Nekton

• Nekton (swimmers) are living organisms that can swim and move without being affected by currents.
• German biologist Ernst Haeckel first used the term nekton to distinguish between active swimmers in a body of water and plankton, which are passive organisms carried along by the current.
• Nektons can be found in either the photic or aphotic zones.
• In all marine food webs, they serve as a vital link between primary consumers and the higher trophic levels.

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Benthos

• The group of organisms that live on, in, or near the bottom of a sea, river, lake, or stream is known as benthos.
• The benthic zone, which includes the sediment surface and some sub-surface layers, is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream.
• The macrobenthos which is greater than 1 mm, is by far the most well-studied benthos to date.
• Benthic creatures can be grouped into two categories depending on whether they live on the ocean floor or a few centimeters below it.

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Factors Limiting the Productivity of Aquatic Habitats

Sunlight

• As sunlight moves through the water column, its penetration decreases fast.
• The extent of plant dispersal is determined by the depth to which light penetrates a lake.
• They are divided into photic and aphotic zones based on light penetration and plant distribution.

Photic zone

• It is the upper layer of aquatic ecosystems, where light can enter but photosynthetic activity is restricted; the depth of this zone is determined by the transparency of the water.
• Photosynthesis and respiration occur simultaneously.
• The photic (or "euphotic") zone is the sunlit, typically well-mixed area that extends from the lake's surface to a light level that is 1% of that at the surface.

Aphotic Zone

• The aphotic zone is formed by the bottom layers of aquatic environments, where light penetration and plant development are restricted.
• The aphotic zone is located below the littoral and photic zones to the lake's bottom, where light levels are too low for photosynthesis.
• Because breathing happens at all depths, the aphotic zone is an area where oxygen is consumed.
• The profundal zone is the name given to this dark, unlit area.

Dissolved Oxygen

• Oxygen is dissolved in water in aquatic ecosystems, where its concentration varies constantly depending on factors that influence oxygen input and output.
• The average concentration of dissolved oxygen in freshwater is 0.0010 percent by weight (also known as 10 parts per million or 10 ppm), which is 150 times lower than the oxygen concentration in an equivalent volume of air.
• Oxygen enters the aquatic ecosystem via the air-water contact and aquatic plants' photosynthetic activity.
• As a result, the amount of dissolved oxygen in an environment is determined by the pace at which the aforementioned two processes occur.
• Dissolved oxygen leaves the water body via the air-water interface via organism respiration (fish, decomposers, zooplanktons, etc).
• Temperature has an effect on the amount of dissolved oxygen retained in water.
• Warm water makes oxygen less soluble. Decomposer activity is also boosted by warm water.
• As a result, raising the temperature of a water body accelerates the depletion of oxygen in the water.
• Many aquatic organisms will die if the dissolved oxygen content goes below 3-5 ppm.

Transparency

• Transparency is another limiting factor that has an impact on aquatic productivity.
• Light penetration is affected by transparency.
• Suspended particulate matter such as clay, silt, phytoplankton, and other particles make the water turbid, limiting light penetration and photosynthetic activity significantly.

Temperature

• The temperature of the water fluctuates more slowly than the temperature of air because water has a far higher specific heat than air.
• This denotes that more heat energy must be given to or taken away from water to raise or lower its temperature.
• Since water temperatures are less susceptible to change, aquatic creatures have a narrow temperature tolerance limit.
• As a result, even minor changes in water temperature pose a greater threat to aquatic organisms' existence than changes in air temperatures.
• All aquatic ecosystems, including lakes, ponds, rivers, streams, estuaries, oceans, and seas, are classified by the creatures and limiting forces addressed here.

Conclusion

From a strain of bacteria to a blue whale, each aquatic organism is distinct. The genetic composition of plants and animals, not their size, distinguishes all life forms. For our survival, we rely on various aquatic plants and animals, as well as their ecological roles. Our lives would be worse, the availability of medications would be limited, career opportunities would be scarce, and the economy would be less healthy without the rich natural diversity of aquatic organisms.

FAQs 

Q1: What are aquatic organisms?

Q2: How are aquatic organisms classified?

Q3: Why are aquatic organisms important for the ecosystem?

Q4: What factors affect the survival of aquatic organisms?

Q5: How do human activities impact aquatic ecosystems?

MCQs 

1. Which of the following is an example of a nektonic organism?

A) Seaweed

B) Plankton

C) Fish

D) Coral

2. What term is used for organisms living at or near the bottom of water bodies?

A) Nekton

B) Benthos

C) Plankton

D) Algae

3. Which factor can negatively affect aquatic organisms?

A) Optimal oxygen levels

B) Stable pH levels

C) Pollution from industrial discharge

D) Natural sunlight

4. What role do aquatic organisms play in ecosystems?

A) They only serve as food for humans

B) They regulate nutrient cycling and water purification

C) They have no impact on biodiversity

D) They reduce oxygen levels in water

5. Which of the following is a threat to aquatic ecosystems?

A) Deforestation near water bodies

B) Conservation efforts

C) Establishing marine protected areas

D) Controlled water flow

GS Mains Questions and Model Answers

Q1: Discuss the ecological significance of aquatic organisms in freshwater and marine ecosystems.

Answer: Aquatic organisms play a crucial role in maintaining the ecological balance of both freshwater and marine ecosystems. They contribute to nutrient cycling, control water quality, and form the base of the food web, supporting diverse life forms. In freshwater ecosystems, organisms such as fish, algae, and microorganisms help purify water by breaking down pollutants and recycling nutrients. In marine ecosystems, species like phytoplankton sequester carbon dioxide, influencing global carbon cycles and climate regulation. The health of aquatic ecosystems directly impacts biodiversity, food security, and human well-being, underscoring the need for their conservation and sustainable management.

Q2: Analyze the impact of pollution on aquatic organisms and the measures needed to mitigate it.

Answer: Pollution, including chemical runoff, plastic waste, and industrial discharge, poses a severe threat to aquatic organisms. Contaminants degrade water quality, reduce oxygen levels, and introduce toxins that accumulate in the food chain, harming fish, invertebrates, and plant life. Eutrophication, caused by excessive nutrient input, leads to algal blooms that deplete oxygen, creating dead zones. To mitigate these impacts, measures such as enforcing pollution control laws, promoting wastewater treatment, reducing plastic usage, and fostering community awareness are essential. International cooperation and local conservation efforts are necessary to protect aquatic ecosystems from further degradation.

Q3: Explain the challenges faced by aquatic organisms due to climate change.

Answer: Climate change poses significant challenges to aquatic organisms through rising water temperatures, ocean acidification, and altered precipitation patterns. Warmer waters affect the metabolism, reproductive cycles, and distribution of aquatic species. Coral reefs, which are home to diverse marine life, are particularly vulnerable to bleaching due to temperature changes. Ocean acidification, caused by increased carbon dioxide absorption, weakens the shells and skeletons of marine organisms like mollusks and coral. Climate-induced changes in water availability and quality can also impact freshwater ecosystems. Addressing these challenges requires global efforts to reduce greenhouse gas emissions and enhance the resilience of aquatic habitats.

Previous Year Questions on  Aquatic Organisms

1. UPSC CSE 2020

Question: Evaluate the role of aquatic organisms in maintaining water quality and ecosystem health.

Answer: Aquatic organisms are integral to maintaining water quality and overall ecosystem health. Species such as phytoplankton and algae contribute to primary production, supporting the food web and oxygenating water. Microorganisms decompose organic matter and recycle nutrients, purifying water and maintaining its chemical balance. Aquatic plants and filter feeders like mussels absorb pollutants, reducing water contamination. Healthy populations of fish and invertebrates control algae blooms and maintain ecological stability. The decline or imbalance of these organisms can lead to degraded water quality, highlighting their critical role in sustaining aquatic ecosystems and ensuring environmental and human health.

2. UPSC CSE 2019

Question: Discuss the threats to aquatic biodiversity and strategies for its conservation.

Answer: Aquatic biodiversity faces threats from pollution, habitat destruction, overfishing, climate change, and the introduction of invasive species. These factors disrupt ecosystems, reduce species populations, and lead to loss of genetic diversity. Strategies for conservation include establishing marine protected areas, enforcing sustainable fishing practices, reducing pollutants through stricter regulations, and restoring degraded habitats. Community-based conservation, public awareness campaigns, and international cooperation are also vital for protecting aquatic biodiversity. Implementing adaptive management practices can ensure that aquatic ecosystems remain resilient to environmental changes and continue to provide essential ecological services.