Plankton on the reef: it is there, but it is eaten at once

The ecology of coral reefs is one of the most fascinating and complex fields of study in marine biology. It is marked by what has historically been called Darwin's paradox: systems with very high biomass and biodiversity that thrive in oligotrophic tropical waters, that is, waters extremely poor in dissolved inorganic nutrients.

For decades the scientific community and, in turn, the hobbyist community argued about whether phytoplankton and zooplankton were really present in these habitats and of any use. The extreme clarity of ocean water fuelled the myth of an environment without plankton; the pioneering research of Yuri I. Sorokin and modern oceanographic studies have instead revealed the opposite: plankton is not only present, it is the energy pillar that supports the whole coral holobiont.

Understanding these mechanisms is essential for the modern reefkeeper, who finds themselves between conflicting management philosophies. On one side are those who argue that adding plankton is superfluous or harmful because of the organic load; on the other, approaches such as the Reef Moonshiners method (Andre Mueller and Anand Kaimal) stress chemical stability and heterotrophic feeding as the keys to coral resilience and colour. Let's start from the basic ecological data and work our way to practical use in the tank.

1. Phytoplankton in reef ecosystems

One of the most common arguments against dosing phytoplankton is the supposed absence of microalgae in the waters where stony corals grow. Sorokin's systematic analysis shows instead that phytoplankton is a ubiquitous, dynamic part of the reef: its density can seem low if measured with crude methods, but its productivity is extremely high.

Biomass and primary production

The waters around coral reefs are not sterile. Even though chlorophyll a is often below 0.5 µg/l, the total biomass of phytoplankton, made up mostly of pico- and nanoplankton (cells under 20 µm), is significant. Sorokin reports a wet biomass of between 30 and 270 mg/m³, with seasonal peaks that can lead to real blooms in atoll lagoons.

Phytoplankton acts as a "biological pump": it takes up the few inorganic nutrients available (nitrates and phosphates) and turns them into complex organic matter, which then becomes available to zooplankton and, indirectly, to corals. In the aquarium, live phytoplankton imitates this process and helps compete with nuisance algae and cyanobacteria for nutrients.

The island mass effect (IME)

The Island Mass Effect is a key oceanographic phenomenon: it explains why coral reefs are surrounded by waters much richer than their position in the open ocean would suggest. When currents meet the physical structure of an island or atoll, upwellings are triggered that bring deep water rich in iron and other micronutrients to the surface.

This enrichment can increase phytoplankton biomass by more than 80% compared with the surrounding ocean waters, and it does not stay out in the open sea: it reaches the colonies directly, with a constant flow of particulate food.

The idea that corals live in water without phytoplankton is therefore an optical illusion caused by very rapid turnover: phytoplankton is produced quickly, but it is eaten just as quickly by the reef's filter-feeders, and the water stays clear.

1. 1Currents strike the island and push up deep water, rich in iron and nutrients (Island Mass Effect)
2. 2Phytoplankton booms around atolls and reefs, up to 80% more than in the open sea
3. 3The reef eats it at once corals, sponges and filter-feeders: a "wall of mouths" that eats it as soon as it forms
4. 4The water stays clear not because it is empty, but because turnover is very fast

The water is clear not because it is empty, but because the plankton is eaten at once.

Sources: [1]

Why reef water is clear even though plankton is there.

2. Zooplankton: the protein source and the engine of growth

If phytoplankton is the base of the food chain, zooplankton is the direct supplier of organic nitrogen and phosphorus for corals. It is widely documented that Acropora get energy (carbohydrates) from their zooxanthellae, but depend on catching animal prey for the proteins needed to build tissue and DNA.

The demersal cycle and night-time feeding

A crucial and often overlooked feature of reef zooplankton is that it is demersal. Much of the reef's zooplankton (mysids, copepods, amphipods) does not live in the water column all the time: by day it hides in the sediment or in the gaps in the coral to escape the fish.

At night there is a massive vertical migration: zooplankton biomass over the reef can increase 10 to 100 times compared with the day. This is why most Acropora extend their polyps and expose their nematocysts mainly at night.

In the aquarium this migration is greatly limited, partly because skimmers and mechanical filters continually remove these organisms. Dosing live or high-quality zooplankton (Artemia nauplii, copepods) at night is therefore a scientifically sound practice to stimulate tissue growth.

Effect on calcification and growth

Experiments on Stylophora pistillata have shown that eating zooplankton not only increases polyp biomass (2 to 8 times) but also speeds up calcification by about 30%. The energy obtained from prey lets the coral support the metabolic processes needed to precipitate calcium carbonate, reducing its dependence on photosynthesis alone.

1. 1Day ×1 zooplankton hides in the sand and among the branches
2. 2Night ×10-100 it rises over the reef
3. 3Polyps open at night the polyps open to hunt

At night the food rises from the bottom: that is why Acropora open their polyps in the dark.

At night the reef opens the pantry.

3. Bacterioplankton and the role of coral mucus

Sorokin's research introduced a key concept: bacterioplankton is one of the most important and constant food sources for corals. In many cases the bacterial biomass available on the reef exceeds that of phytoplankton and zooplankton put together.

Mucus as a capture net

The coral is not just a passive predator. It continually secretes mucus, a polysaccharide-rich substance that serves as a substrate for bacteria and as a trap for particles. Once expelled or built up on the surface, the mucus is colonised by bacteria that break down its complex parts; from time to time the coral ingests this biofilm loaded with bacteria, detritus and bacterioplankton.

In this way the coral captures particles too small for its nematocysts, such as single bacterial cells or microbial aggregates of 3-10 µm. Bacteriotrophy supplies organic phosphorus, essential for ATP, and B vitamins (such as B12) that the zooxanthellae do not produce.

Protein hydrolysis and external digestion

The coral's feeding is not limited to ingestion. On the surface of the mucus real protein hydrolysis takes place: the proteins in the particles are split into peptides and free amino acids by the enzymes of the microbiome's bacteria. These amino acids can be absorbed directly by the tissue through pinocytosis, without the polyp having to open.

It is a discovery that changes the approach to feeding: the coral eats even with its polyps closed, as long as there is a constant flow of organic matter and bacteria in its mucus.

1. 1Mucus rich in polysaccharides: a substrate for bacteria and a trap for particles
2. 2Bacterioplankton colonises the mucus; the coral ingests the biofilm
3. 3Particles of 3-10 µm too small for the nematocysts, they stay in the mucus
4. 4Amino acids absorbed by pinocytosis, even with polyps closed

The coral eats even with its polyps closed, through its mucus.

4. Dosing phytoplankton in the aquarium: pros and cons

Discussions about phytoplankton keep coming back in reefkeeping groups. Let's look at the pros and cons in the light of this evidence.

Arguments for

It feeds the microfauna: live phytoplankton is the food of choice for copepods, rotifers and the small organisms that make up the tank's benthos and zooplankton. A thriving microfauna is the best guarantee of cleanliness and of constant live food for the corals.

It brings fatty acids and vitamins: Acropora are not pure filter-feeders, but they consume phytoplankton indirectly or through their muco-ciliary system. Phytoplankton is rich in polyunsaturated fatty acids (PUFA) such as EPA and DHA and in vitamin C, useful for colour and stress response.

Competition for food: live phytoplankton takes up nitrates and phosphates for photosynthesis, "locking away" nutrients that would otherwise feed filamentous algae or cyanobacteria.

It helps pH: during the photoperiod it uses up dissolved CO₂, countering the drop in pH caused by the animals' respiration and by organic acids.

Arguments against (and answers)

"It increases pollution": this is only true with dead phytoplankton or powdered products that are not eaten. Live phytoplankton stays in suspension until it is eaten or removed by the skimmer, and in the meantime it works as a biological filter, not as a polluting load.

"It does not exist in nature": as Sorokin's data and the IME show, this is false. Phytoplankton is abundant, but its turnover is so fast that it does not build up visibly.

For

• Feeds the microfauna
• Brings fatty acids (EPA, DHA) and vitamins
• Takes nutrients away from nuisance algae
• Uses up CO₂ and helps pH

Against (and answers)

• "It increases pollution": only if dead, or uneaten powder
• "It does not exist in nature": it does, but turnover is very fast

The problem is not phytoplankton: it is dead or uneaten phytoplankton.

5. The Reef Moonshiners method and fine chemistry

An important contribution to modern management comes from the Reef Moonshiners method, which moves away from management based on frequent water changes and relies on precise ICP-OES tests and targeted replacement of individual trace elements.

Trace elements as catalysts of biology

Mueller and Kaimal argue that the health of the biology (bacteria and microfauna) depends closely on the availability of metals and trace elements. Many bacterial enzymes involved in the nitrogen cycle and in breaking down organic matter need metal cofactors such as copper, manganese, vanadium and zinc.

If these elements are missing, the chain from plankton to coral breaks. The method stresses, for example, the importance of rubidium for zoanthids and soft corals and of iodine for moulting in zooplankton crustaceans. Without these elements even generous plankton dosing risks being useless, because the tank's biology cannot process it.

Managing pH and CO₂

Another pillar is managing CO₂ and pH. Many aquariums have low pH (below 7.8) in the morning because of the carbon dioxide built up in the house: carbonic acid inhibits calcification and stresses the holobiont. Phytoplankton, together with scrubbing CO₂ from the skimmer's air or night-time micro/nano-bubbling (the Cruz Arias method), helps keep pH above 8.2, optimising metabolic efficiency.

6. Feeding Acropora: the holobiont and the feeding mash

The concept of the holobiont is essential: we are not feeding just an animal, but a consortium of animal, algae (zooxanthellae) and microbiome (bacteria and fungi).

Amino acids and pinocytosis: Acropora absorb free amino acids (DFAA) directly from the water through the epidermis. It is "passive" feeding but very efficient. The dose must be balanced, though: Mueller warns that too many amino acids can feed pathogenic bacteria and darken the tissue, if the water chemistry is not perfect.

Organic phosphorus: in the aquarium we fear inorganic phosphates (PO₄), but the organic phosphorus in food (plankton and detritus) is essential for ATP, the universal energy molecule. Without organic phosphorus from heterotrophy the coral does not have enough energy to repair tissue or grow quickly, even with low inorganic nutrients.

7. Deep-sea corals and shallow-water corals

Martin Hovland's work on deep-sea corals (Lophelia pertusa) shows the extreme potential of heterotrophic feeding. These corals live in total darkness, without zooxanthellae: they survive by catching zooplankton and bacterioplankton, helped also by hydrocarbon-rich fluids seeping from the sea floor (the "hydraulic theory").

If a deep-sea coral can build whole reefs on external feeding alone, it is logical to think that our photosynthetic corals, even with the support of their algae, gain enormous benefit from plankton, reaching growth and colours impossible with light alone.

8. Technical considerations for dosing

Skimming and mechanical filtration: in a system pushed with feeding, the skimmer is a "necessary evil". It removes pollutants, but also the bacteria-enriched mucus and much of the plankton dosed. Many reefkeepers switch it off for 1-4 hours after dosing, to give the corals time to catch the particles.

Activated carbon: the Moonshiners method recommends specific (non-dusty) carbon in fluidised-bed reactors, to remove organic toxins (mainly from soft corals and zoanthids) that can hold Acropora back. Carbon also keeps the water crystal clear, but it can remove elements such as iodine, which must be checked and replaced.

9. Absence or usefulness of plankton?

The absence argument rests on the clarity of the water, which however is the result of ultra-efficient filtering (the reef is a gigantic biological sieve), not of a lack of production.

The usefulness argument: science confirms that corals are predators that get proteins, minerals and essential vitamins from plankton. Phytoplankton, even when it is not eaten directly by Acropora in large amounts, is vital for the health of the whole tank ecosystem.

10. Heavy metals and pollutants

A well-done ICP-OES analysis is not only for checking useful elements, but also for spotting toxins that can block the metabolism of plankton and corals. Aluminium, tin or antimony above natural levels can stress the microbiome and hold back the zooxanthellae's photosynthesis. Chemical stability is the condition needed for plankton feeding to give the hoped-for results.

Summary and conclusions

Plankton should not be considered an optional "extra", but the means by which the coral ecosystem manages energy and nutrients. In nature the coral lives immersed in a continuous flow of bacterioplankton, micro-zooplankton and photosynthetic nanoplankton, sustained by powerful oceanographic dynamics such as the IME.

In the aquarium, live phytoplankton supports the food chain, stabilises pH and competes with unwanted algae; zooplankton is the fuel for tissue growth and calcification.

Success, however, is inseparable from chemical stability. Without proper management of trace elements (iodine, vanadium, manganese, rubidium) and physical parameters (pH, CO₂), plankton may give no results or create imbalances.

In conclusion: plankton is useful and naturally present. The secret is not to eliminate it for fear of pollution, but to dose living organisms wisely, with adequate filtration and carefully controlled water chemistry.

Who wrote this article

Sources

1. Gove J.M., McManus M.A., Neuheimer A.B. et al. (2016). Near-island biological hotspots in barren ocean basins. Nature Communications 7: 10581.
2. Houlbrèque F., Tambutté E., Allemand D., Ferrier-Pagès C. (2004). Interactions between zooplankton feeding, photosynthesis and skeletal growth in the scleractinian coral Stylophora pistillata. Journal of Experimental Biology 207(9): 1461-1469.
3. Hoeksema B.W., Cairns S., Samimi-Namin K., World List of Scleractinia (2026). Desmophyllum pertusum (Linnaeus, 1758). World Register of Marine Species (WoRMS).