Phytoplankton: Producers of the Aquatic World
Phytoplankton are microscopic, photosynthetic organisms suspended in the sunlit surface waters of oceans, seas, and freshwater bodies.
Phytoplankton occupy the euphotic zone, the upper layer of water where sunlight is sufficient for photosynthesis. Like plants on land, they use sunlight, carbon dioxide, and dissolved nutrients to build organic matter. What makes them remarkable is their scale of influence: despite accounting for only around 1% of global plant biomass, they carry out close to 50% of all photosynthesis on Earth and generate roughly half of the planet's oxygen.
The group is taxonomically diverse:
- Diatoms: single-celled algae enclosed in intricate silica shells
- Dinoflagellates: some bioluminescent, some symbiotic, some producing toxins
- Cyanobacteria: photosynthetic bacteria, not true algae, but functionally similar as primary producers
Phytoplankton form the foundation of aquatic food webs. Zooplankton graze on them as primary consumers; small fish and crustaceans feed on zooplankton; larger fish, seabirds, and marine mammals occupy higher trophic levels. Decomposing bacteria break down dead organic matter and return nutrients to the water, completing the cycle.
In upwelling zones off Peru and Namibia, nutrient-rich deep water rises to the surface, fuelling dense phytoplankton populations that support some of the world's largest sardine and anchovy fisheries.
Not all aquatic producers are plants. Phytoplankton include bacteria and single-celled organisms that are not true plants, even though they photosynthesize. Do not conflate phytoplankton with macrophytes.
Macrophytes: Aquatic Plants in Freshwater and Coastal Systems
Macrophytes are aquatic plants large enough to be visible to the naked eye, found in both freshwater and marine environments.
Unlike microscopic phytoplankton, macrophytes have recognisable plant structures: roots, stems, and leaves. They can be categorised by their relationship to the water surface:
- Emergent plants: rooted underwater but with leaves and stems extending above the surface (e.g., reeds, cattails)
- Submerged plants: grow entirely below the surface (e.g., eelgrass, pondweed)
- Floating plants: rest on the surface, often with roots hanging freely (e.g., water lilies, duckweed)
Macrophytes are less dominant in open marine systems than phytoplankton, but they play important roles in freshwater ecosystems and in coastal habitats such as seagrass beds and mangrove margins.
Their ecological functions are wide-ranging:
- Primary production: converting solar energy into organic biomass that herbivores consume
- Habitat structure: providing shelter and feeding grounds for invertebrates, fish, and microbial communities
- Nutrient uptake: absorbing nitrates and phosphates, which reduces the risk of eutrophication
- Sediment stabilisation: slowing water flow and reducing erosion
- Water quality: filtering suspended particles and taking up heavy metals and pollutants
Macrophytes are used in constructed wetlands specifically because of their ability to strip nitrogen, phosphorus, and suspended solids from wastewater — a practical application of their natural filtration role.