A) From this record of observations construct a food web:

Here are the solutions to your questions:

a) From this record of observations construct a food web:
The food web for the lake ecosystem can be constructed as follows:

• Algae/Phytoplankton $\rightarrow$ Planktonic crustaceans
• Algae/Phytoplankton $\rightarrow$ Worms
• Planktonic crustaceans $\rightarrow$ Small fish
• Worms $\rightarrow$ Small fish
• Planktonic crustaceans $\rightarrow$ Bird species
• Worms $\rightarrow$ Bird species
• Small fish $\rightarrow$ Insect larvae
• Small fish $\rightarrow$ Bird species
• Small fish $\rightarrow$ Large fish

b) From the food web you have constructed in (a) above isolate and write down a food chain that ends with:

i) Bird species as a secondary consumer:
Algae/Phytoplankton $\rightarrow$ Planktonic crustaceans $\rightarrow$ Bird species
(Alternatively: Algae/Phytoplankton $\rightarrow$ Worms $\rightarrow$ Bird species)

ii) Large fish as tertiary consumer:
Algae/Phytoplankton $\rightarrow$ Planktonic crustaceans $\rightarrow$ Small fish $\rightarrow$ Large fish
(Alternatively: Algae/Phytoplankton $\rightarrow$ Worms $\rightarrow$ Small fish $\rightarrow$ Large fish)

c) The biomass of the producers in the lake was found to be greater than that of primary consumers. Give an explanation for this observation?
The biomass of producers is greater than that of primary consumers because energy is lost at each successive trophic level in an ecosystem. Only about 10% of the energy from one trophic level is transferred to the next, with the rest being lost as heat during metabolic processes or remaining unconsumed. Therefore, a larger base of producers is required to support the energy needs of the primary consumers.

d) Using either the observations recorded by the students or the food web you have constructed name:

i) Two organisms that compete for food in the lake.
Bird species and Small fish

ii) The source of food the organisms in d (i) above compete for.
Planktonic crustaceans and/or Worms

e) i) State three ways by which man may interfere with this lake ecosystem: 1. Discharge of industrial waste/sewage (Pollution) 2. Overfishing 3. Introduction of alien/invasive species

ii) Explain how each of the ways you have states may affect life in the lake? 1. Discharge of industrial waste/sewage: Industrial waste often contains toxic chemicals that can directly poison aquatic organisms, leading to death or impaired health. Sewage introduces excessive organic matter and nutrients, causing eutrophication. This leads to rapid growth of algae (algal blooms), which then die and decompose, consuming large amounts of dissolved oxygen in the water. The depletion of dissolved oxygen creates anoxic conditions, making it impossible for most fish and other aquatic animals to survive, leading to mass mortality and a significant reduction in biodiversity. 2. Overfishing: Removing too many fish from the lake, especially specific species, can drastically reduce their populations, potentially leading to their collapse. This disrupts the food web balance. If a predator fish species is overfished, its prey species might overpopulate, leading to overgrazing of producers or primary consumers. Conversely, if a prey species is overfished, its predators may starve, leading to their decline. This can lead to the collapse of fish stocks and affect the entire ecosystem structure. 3. Introduction of alien/invasive species: Non-native species, when introduced, can outcompete native species for resources (food, habitat), prey on native species, or introduce new diseases. They often lack natural predators in the new environment, allowing their populations to grow unchecked. This can lead to a decline or even extinction of native species due to increased competition or predation, altering the existing food web, disrupting ecological relationships, and reducing the overall biodiversity and stability of the lake ecosystem.