🎒 Base knowledge and understanding

🌿 Content

🔑 Key terms

📝 What you will do

Phase 1: Where does aquatic food come from?

Before we look at how humans get food out of rivers, lakes and oceans, take a moment to think about where that food ultimately comes from. Every fish on a plate started its life depending on something else for energy, and that something else depended on something else, and so on, all the way back to the producers at the base of the food web.

1. List three things humans eat that come out of the sea. For each one, write down what you think it eats.

2. List two things humans eat that come out of freshwater (lakes or rivers). For each one, write down what you think it eats.

3. Following the food chains back as far as you can, what do you end up at? Where does the energy in your fish supper originally come from?

Phase 2: Phytoplankton and macrophytes

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4.3.1 Phytoplankton and macrophytes provide energy for freshwater and marine food webs.

Phytoplankton are a type of microscopic plankton capable of photosynthesis that are found in oceans, seas and freshwater. Macrophytes are aquatic plants that are large enough to be visible. They can be emergent from water, submerged or floating.

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Two groups of aquatic producers

In every aquatic ecosystem, freshwater or marine, the food web starts with producers. A producer is an organism that makes its own food from sunlight, water and dissolved nutrients using photosynthesis. In aquatic ecosystems there are two main groups of producers: phytoplankton and macrophytes.

Phytoplankton are microscopic, photosynthesising plankton. The word plankton means any organism that drifts in the water rather than swimming against the current. Plankton splits into two groups:

• Phytoplankton are plant-like and photosynthesise. They are producers.
• Zooplankton are animal-like and eat other plankton. They are consumers, not producers.

Only phytoplankton sit at the base of the aquatic food web. The main groups are diatoms, dinoflagellates and cyanobacteria. In Lake Geneva, diatoms and dinoflagellates dominate. If you have ever swum in the lake in late summer and the water looked slightly green or cloudy, you were swimming through a phytoplankton bloom.

Macrophytes are aquatic plants and large algae that are big enough to see without a microscope. They include reeds, water lilies, pondweeds, seagrasses and seaweeds. Macrophytes are classified by where they sit in the water column:

• Emergent macrophytes are rooted in the sediment but stick up out of the water.
• Submerged macrophytes live entirely beneath the surface.
• Floating macrophytes float on the surface.

Together, phytoplankton and macrophytes capture solar energy as biomass (the mass of living material) and pass that energy on to every other organism in the aquatic food web, from zooplankton up to fish, seabirds, seals and humans.

Comprehension

1. Are all plankton photosynthetic? Explain in one sentence.

2. Why are macrophytes considered producers?

3. Name the three main groups of phytoplankton.

4. Name the three positions a macrophyte can occupy in the water column.

Classification task: place each species in the right column

1. Below is a list of nine aquatic plants. Place the correct term: Emergent, Submerged or Floating in the empty column. The italicised name in brackets is the scientific (binomial) name. The description tells you everything you need to decide.

Hint

Every clue you need is in the description. Look for the words rooted, beneath the surface, above the water, drift, on the surface.

Phase 3: Two species humans consume