Aquatic Food Webs (HL IB ESS OLD COURSE - IGNORE)

Revision Note

Aquatic Food Webs

  • Aquatic food webs show how energy and nutrients move through freshwater and marine ecosystems

Phytoplankton

  • Phytoplankton are microscopic organisms found in marine and fresh water bodies that can perform photosynthesis

    • Phytoplankton are not plants

    • They include a variety of autotrophic microorganisms, such as:

      • Algae (e.g. diatoms)

      • Cyanobacteria (prokaryotic organisms that are also known as blue-green algae)

  • Role in food webs:

    • They form the base of most aquatic food webs

    • They capture solar energy and convert it into biomass through photosynthesis

    • They are consumed by primary consumers (zooplankton and small fish)

    • They contribute to oxygen production and nutrient cycling

Macrophytes

  • Macrophytes are aquatic plants that are visible to the naked eye

  • They can be:

    • Emergent: plants that grow above the water surface (e.g. cattails or bulrushes)

    • Submerged: plants that grow completely underwater (e.g. seagrass)

    • Floating: plants that float on the water surface (e.g. water lilies or duckweed)

  • Role in food webs:

    • They provide habitat and food for various aquatic organisms

    • They capture solar energy and convert it into biomass through photosynthesis

    • They contribute to oxygen production and nutrient cycling

Energy flow in aquatic food webs

  • Producers: phytoplankton and macrophytes capture energy from sunlight through photosynthesis

  • Primary consumers: zooplankton, small fish and some invertebrates and birds feed on primary producers

  • Secondary consumers: larger fish and birds consume primary consumers

  • Tertiary consumers: top predators like sharks and birds of prey eat secondary consumers

  • Decomposers: aquatic bacteria and fungi break down dead organisms, recycling nutrients back into the ecosystem

Examiner Tips and Tricks

Be clear on the differences between phytoplankton and macrophytes—although they play similar roles in aquatic food webs, they are very different groups of organisms.

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