Phytoplankton and macrophytes support aquatic food webs by acting as primary producers. They convert light energy into chemical energy through photosynthesis, producing biomass that supplies energy to consumers in freshwater and marine ecosystems.
How They Support Food Webs
This is core SL and HL content in Subtopic 4.3, Aquatic food production systems. Phytoplankton are microscopic photosynthetic organisms suspended in water, while macrophytes are larger aquatic plants or algae that may be submerged, floating or rooted.
Both occupy the first trophic level. Through photosynthesis, they use light energy to produce organic carbon compounds. When herbivores consume this biomass, its chemical energy passes to higher trophic levels such as predatory fish, seabirds and humans.
| Producer | Main contribution to aquatic food webs |
|---|---|
| Phytoplankton | Are consumed by zooplankton and filter feeders, supporting fish and higher-level predators; contribute dead organic matter to decomposer food chains. |
| Macrophytes | Are eaten by herbivores; provide shelter, nursery areas and surfaces for other organisms; contribute leaf litter and other material to detrital food chains. |
Macrophytes also increase habitat complexity, reducing predation risk for juvenile organisms. Both producer groups release oxygen during photosynthesis, although their respiration and decomposition also consume oxygen.
For example, nutrient-rich upwelling off Peru supports high phytoplankton productivity, which sustains zooplankton and the commercially important Peruvian anchoveta fishery. In Florida Bay, seagrass beds provide food and nursery habitat for fish and invertebrates, supporting biodiversity and fisheries.
A common misconception is that phytoplankton and macrophytes only support food webs when eaten directly. Their dead biomass also enters detrital food chains, where decomposers and detritivores transfer energy and matter onward. However, excessive nutrient inputs can cause algal blooms and eutrophication, showing that greater producer growth is not always beneficial to ecosystems or fishing communities.
Exam Technique
For an explain question, link photosynthesis to biomass, feeding and energy transfer through trophic levels. Include both direct grazing and detrital pathways, then support the explanation with a named aquatic example.
