Factors Affecting Phytoplankton Biodiversity and Their Importance
Environment & Ecology › Terrestrial and Aquatic Ecosystems · Topic 4.6
Phytoplankton diversity and abundance in the sea depend on a set of physical, chemical and biological factors. Light is the first: it limits photosynthesis to the euphotic zone, and its quantity and colour vary with depth, turbidity and season. Temperature affects growth rate and species composition. Nutrients, mainly nitrate, phosphate, silicate and iron, often limit growth, and their supply depends on upwelling, river inflow, mixing and dust. Water column stability, shaped by stratification and the thermocline, decides whether nutrients reach the surface. Salinity, ocean currents, grazing by zooplankton, competition and viral attack also matter. Phytoplankton are important as the base of the food web, producers of about half of Earth's oxygen, carbon sinks through the biological pump, indicators of ocean health, and a source of fisheries food.
5 min read · UPSC Prelims · Environment & Ecology · Terrestrial and Aquatic Ecosystems
At a Glance
- Light Euphotic zone only
- Key nutrients N, P, Si, Fe
- Nutrient supply Upwelling, rivers, mixing
- Barrier Stratification
- Importance Oxygen, carbon, food
- Exam link UPSC Prelims, Environment & Ecology: Terrestrial and Aquatic Ecosystems
Where Does It Fit?
This topic follows the account of plankton. It explains what controls the growth and variety of phytoplankton, and why they matter for climate and food. Previous topic: Plankton: Phytoplankton and Zooplankton. Next topic: Sea-Grass and Seaweeds.
Which Factors Affect Phytoplankton?
| Factor | Effect |
|---|---|
| Light | Needed for photosynthesis. It falls with depth, cloud and turbidity. Different species prefer different light levels |
| Temperature | Controls metabolic rate. Cold waters favour diatoms, and warm stratified waters favour small cells and cyanobacteria |
| Nutrients | Nitrate, phosphate, silicate (for diatoms) and iron limit growth. The Redfield ratio of about 106 carbon to 16 nitrogen to 1 phosphorus describes needs |
| Upwelling and mixing | Bring nutrients from deep water to the surface, causing blooms |
| Stratification | A warm layer on top blocks mixing, so surface nutrients run out; warming strengthens stratification |
| River input and dust | Rivers carry nutrients to coasts; wind-blown dust brings iron to remote oceans |
| Salinity and pH | Species have preferred ranges; acidification can harm calcifiers |
What Biological Factors and Importance Matter?
- Grazing: zooplankton eat phytoplankton and control their numbers
- Competition: species compete for nutrients and light
- Viruses and parasites: infection can end blooms
- Iron limitation: in High Nutrient Low Chlorophyll regions such as the Southern Ocean and equatorial Pacific, iron is scarce
- Base of the food web: all marine animals depend on phytoplankton directly or indirectly
- Oxygen production: about half of global oxygen
- Biological pump: carbon fixed in the surface sinks as dead cells and fecal pellets
- Indicators: chlorophyll levels show productivity and climate effects
What Else Should You Remember?
- Spring bloom: increase in light and mixing in temperate seas
- Monsoon blooms: in the Arabian Sea upwelling raises chlorophyll
- Oligotrophic gyres: central ocean areas have little nutrient and low phytoplankton
- Eutrophication: excess nutrients cause harmful algal blooms
- Climate change effect: warming reduces mixing and may cut productivity in tropical seas
- Acidification: affects coccolithophores
- Ocean fertilisation: proposal to add iron to increase carbon uptake; effects uncertain and controversial
- Chlorophyll-a: green pigment measured by satellites
- Diversity: thousands of species, with high turnover
- Diatoms: need silica, dominant in cool waters
- Cyanobacteria: small cells dominate warm nutrient-poor seas
- Coccolithophores: bloom in stable water
- Dinoflagellates: swim, and may cause red tides
- Fishery link: sardine catch depends on plankton
- India’s data: INCOIS gives potential fishing zone advice using chlorophyll and temperature, as in Project Brainwave and Sagar Vani System
- Biodiversity measures: species richness and Shannon index in water samples
- Phosphorus: often limiting in freshwater
- Link to productivity: see Aquatic Ecosystems and Factors Affecting Their Productivity
- Link to plankton: see Plankton: Phytoplankton and Zooplankton
- Caution: a nutrient limiting in one sea may not limit in another
- Nutrient ratios: a shift in nitrogen to phosphorus ratio can change which species dominate
- Light quality: blue light penetrates deepest in clear seas, so deep phytoplankton are adapted to it
- Seasonal succession: diatoms bloom first, followed by dinoflagellates and smaller cells as nutrients are used up
- Human effects: sewage, fertilisers, dredging and climate change alter phytoplankton communities
- Monitoring: buoys, ships and satellites track chlorophyll to manage fisheries and bloom risks
- Species diversity: high diversity makes the food web more stable
- Plankton and monsoon: the southwest monsoon drives upwelling along the Indian west coast and raises chlorophyll
- Dust and iron: dust from the Thar and Arabia may supply iron to the Arabian Sea
- Exam hint: remember that limiting nutrients vary: iron in HNLC areas, nitrogen in many tropical seas, silica for diatoms
- Importance for fisheries: good plankton years bring good fish catches, so phytoplankton data guide fishing advice
- Satellite sensors: instruments such as MODIS and the Indian Oceansat missions measure ocean colour
Why Does It Matter for the Exam?
- Factors: light, nutrients, temperature
- Upwelling: nutrient supply
- Importance: oxygen, carbon, food
- Current affairs link: ocean fertilisation and potential fishing zones
- Common errors: saying stratification boosts nutrient supply or phytoplankton live in the dark
As an analytical point, phytoplankton are sensitive to warming and nutrient changes, and their response will shape fish catches and the ocean’s role in absorbing carbon. Monitoring them is therefore essential for climate and food security planning.
Why Is This Topic Important?
- Factors Light, nutrients
- Supply Upwelling
- Value Food, oxygen
What Came Next?
The next topic covers sea-grass and seaweeds. Next topic: Sea-Grass and Seaweeds.
Key People and Terms
- Euphotic zone Sunlit layer
- Upwelling Rise of deep water
- Redfield ratio C:N:P ratio
- HNLC High nutrient low chlorophyll region
- Chlorophyll-a Pigment index
- Biological pump Carbon export
Exam Corner
Points to Remember
- Phytoplankton need light, N, P, Si, Fe.
- Diatoms need silica.
- Stratification blocks nutrient supply.
- HNLC areas lack iron.
- They give about 50 percent of oxygen.
Do Not Confuse With
- Upwelling and stratification Brings nutrients versus blocks mixing
- Diatoms and cyanobacteria Silica-using large cells versus small warm-water cells
- Eutrophication and upwelling Human nutrient load versus natural supply
Memory Hook
Light above, nutrients from below; stratification shuts the door, upwelling opens it.
Mains Angle
Questions ask for factors and importance. These points help.
- Factors Physical, chemical
- Importance Oxygen, carbon
- Climate Warming effect
A Question You May Face
An original practice question, not a past paper question.
“Discuss the factors that control phytoplankton growth and diversity in the ocean and their importance for climate and fisheries.”
How to Answer
- Factors Light, nutrients.
- Supply Upwelling.
- Importance Oxygen, carbon.
- Climate Warming.
GS Relevance
Frequently Asked Questions
What factors affect the growth of phytoplankton?
Phytoplankton growth depends on light, temperature, nutrients such as nitrate, phosphate, silicate and iron, water column stability, salinity and ocean currents. Biological factors such as grazing by zooplankton, competition and viral infection also control numbers. Upwelling and mixing supply nutrients, while stratification limits them.
Why is iron important for phytoplankton?
Iron is a trace nutrient needed for photosynthesis and nitrogen use. In large parts of the ocean such as the Southern Ocean and equatorial Pacific, nitrate is plentiful but iron is scarce, so phytoplankton remain few. Wind-blown dust and upwelling supply iron, and these areas are called high nutrient, low chlorophyll regions.
How does stratification affect phytoplankton?
Stratification is the formation of a warm, light surface layer over cold, dense deep water. This stops mixing, so the nutrients in the surface layer are used up and not replaced from below, and phytoplankton growth falls. Global warming strengthens stratification, so it may reduce ocean productivity in many tropical regions.
Why are phytoplankton important?
Phytoplankton are the base of marine food webs, produce about half of the oxygen in the atmosphere, absorb carbon dioxide and export carbon to the deep sea through the biological pump, support the world's fisheries, and serve as indicators of ocean health and climate change.
What is the biological pump?
The biological pump is the process by which carbon dioxide fixed by phytoplankton in the surface ocean is transferred to the deep sea, as dead cells, faecal pellets and animal remains sink. It stores carbon away from the atmosphere for long periods and is a key part of the ocean's role in regulating climate.
PYQ Practice — Statement Analysis
1 Upwelling brings nutrients to the surface for phytoplankton.
Raising productivity.
2 Stratification increases nutrient supply to surface waters.
It reduces it.
3 Diatoms need silica for their shells.
A limiting nutrient.
4 Phytoplankton produce about half of Earth's oxygen.
By photosynthesis.
Past Mains Questions on This Topic
This topic comes under Terrestrial and Aquatic Ecosystems. UPSC Mains questions asked on the matching syllabus topic, year-wise with marks:
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