A freshwater marsh has a Gross Primary Production (GPP) of $5000{\text{kJm}}^{-2}{\text{d}}^{-1}$ and ecosystem respiration of $2000{\text{kJm}}^{-2}{\text{d}}^{-1}$. The energy transfer efficiencies are 12% from producers to primary consumers, 8% from primary to secondary consumers and 5% from secondary to tertiary consumers. Calculate the total energy available to tertiary consumers per square metre per day, accounting for respiration losses before each transfer.

Step-by-step solution

1. Find Net Primary Production (NPP) = GPP – respiration = $5000-2000=3000{\text{kJm}}^{-2}{\text{d}}^{-1}$.
2. Energy transferred to primary consumers = NPP × 12% = $3000\times 0.12=360{\text{kJm}}^{-2}{\text{d}}^{-1}$.
3. Energy transferred to secondary consumers = $360\times 0.08=28.8{\text{kJm}}^{-2}{\text{d}}^{-1}$.
4. Energy transferred to tertiary consumers = $28.8\times 0.05=1.44{\text{kJm}}^{-2}{\text{d}}^{-1}$.
5. Thus the energy that reaches tertiary consumers is $1.44{\text{kJm}}^{-2}{\text{d}}^{-1}$.

Concept tested: Energy flow calculation

Published 4 October 2026 · How this solution was prepared and checked