UPSC Prelims 2021 Q22: Primary Producers in Ocean Food Chains
Consider the following kinds of organisms:
1. Copepods
2. Cyanobacteria
3. Diatoms
4. Foraminifera
Which of the above are primary producers in the food chains of oceans?
- 1 and 2
- 2 and 3
- 3 and 4
- 1 and 4
Answer: (b) 2 and 3
Statement-wise explanation
- 1: incorrect — Copepods are tiny crustaceans and among the most common zooplankton. They cannot photosynthesise. They graze on phytoplankton such as diatoms or eat other zooplankton. NOAA Fisheries calls them the 'cows of the sea' because they pass the energy stored in phytoplankton up to fish and whales, which makes them primary consumers, not producers.
- 2: correct — Cyanobacteria (blue-green algae) are bacteria that photosynthesise, using sunlight to turn carbon dioxide and water into food and releasing oxygen. Marine cyanobacteria such as Prochlorococcus and Synechococcus are a major part of ocean phytoplankton. NOAA notes that Prochlorococcus alone produces up to 20% of the oxygen in the biosphere.
- 3: correct — Diatoms are single-celled algae with glass-like silica cell walls. They photosynthesise and are among the most common phytoplankton. A NASA-supported model study estimated that diatoms made about half of global ocean phytoplankton production over 1998–2011.
- 4: incorrect — Foraminifera ('forams') are single-celled amoeba-like protists that build shells (tests), mostly of calcium carbonate. They are heterotrophs: they eat bacteria, diatoms and other small organisms. Some reef species host photosynthetic algae as symbionts, but the foram itself does not photosynthesise, so it is a consumer.
How to eliminate
Ask one question of each organism: can it photosynthesise? Cyanobacteria are the most familiar photosynthetic microbes, and NCERT calls them blue-green algae, so statement 2 is almost certainly right. That leaves only the options with 2 in them: '1 and 2' and '2 and 3'. Now compare copepods and diatoms. Copepods are crustaceans (animals), and animals do not photosynthesise. Diatoms are algae. That gives '2 and 3' without knowing anything about foraminifera. If you remember only one thing, remember the producer–consumer split in plankton: phytoplankton (diatoms, dinoflagellates, cyanobacteria, coccolithophores) are producers, and zooplankton (copepods, krill, foraminifera, radiolarians) are consumers. A shell does not make something a plant. Forams and radiolarians have hard tests but eat other organisms.
Know more
Primary producers are organisms that make organic matter from inorganic carbon, mostly by photosynthesis. On land these are plants. In the open ocean they are mainly microscopic phytoplankton that drift in the sunlit (photic) upper layer. NOAA estimates that about half of Earth's oxygen comes from the ocean, mostly from this plankton.
Plankton are organisms that drift rather than swim against currents. They split by how they get food:
| Group | Examples | Feeding | Food-chain role |
|---|---|---|---|
| Phytoplankton | Diatoms (silica walls), dinoflagellates, coccolithophores (calcite plates), cyanobacteria (Prochlorococcus, Synechococcus, Trichodesmium) | Photosynthesis | Primary producers |
| Zooplankton | Copepods, krill, foraminifera, radiolarians, jellyfish, fish larvae | Eat other organisms | Consumers |
A typical ocean chain runs diatoms → copepods/krill → small fish (sardine, herring) → large fish, seabirds, whales.
Phytoplankton growth is limited by light and nutrients (nitrogen, phosphorus, iron, and silica for diatoms). This is why upwelling zones, such as those off Peru or along India's west coast during the monsoon, are rich fishing grounds. Cold, nutrient-rich water rises, phytoplankton bloom, and the food chain above them grows. Too many nutrients cause harmful algal blooms and low-oxygen 'dead zones'.
A few facts worth keeping in mind:
- Some cyanobacteria, such as Trichodesmium, also fix nitrogen, fertilising nutrient-poor seas.
- Diatoms and coccolithophores sink carbon to the deep sea (the biological carbon pump).
- Foraminifera shells pile up on the sea floor as calcareous ooze. They are used as paleoclimate proxies.
- Satellites such as ISRO's Oceansat measure ocean colour (chlorophyll) to track phytoplankton and map potential fishing zones.
NCERT Concepts to Revise
- Plankton: All four organisms in the question are plankton; separating plant-like phytoplankton from animal-like zooplankton is the key to this question.
- Blue-green algae: Statement 2 tests whether you recognise cyanobacteria as blue-green algae and know how they make their food.