Aquatic Food Webs (HL IB ESS OLD COURSE - IGNORE)
Revision Note
Written by: Alistair Marjot
Reviewed by: Bridgette Barrett
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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