Ecosystem Set-4

Q1. A grassland ecosystem has Gross Primary Productivity (GPP) = $800\ \mathrm{g\ C\ m^{-2}\,yr^{-1}}$ and total autotrophic respiration $R_a = 200\ \mathrm{g\ C\ m^{-2}\,yr^{-1}}$. Using $NPP = GPP - R_a$, the Net Primary Productivity (NPP) is:

Q2. In a lake, dissolved inorganic nitrogen (reported as elemental N) is $112\ \mu\mathrm{g\ L^{-1}}$ and phosphate (reported as elemental P) is $3.1\ \mu\mathrm{g\ L^{-1}}$. Using atomic masses $N=14$, $P=31$ and the Redfield molar ratio $N:P = 16:1$, which nutrient is most likely limiting phytoplankton growth?

Q3. Producers fix $GPP = 10\,000\ \mathrm{kJ\ m^{-2}\,yr^{-1}}$. Autotrophic respiration is $30\%$ of GPP. Herbivores ingest $20\%$ of NPP, assimilate $40\%$ of ingested energy and allocate $25\%$ of assimilated energy to production. Carnivores consume $50\%$ of herbivore production, assimilate $70\%$ of ingested energy and allocate $15\%$ of assimilated energy to production. Calculate annual production of carnivores (in $\mathrm{kJ\ m^{-2}\,yr^{-1}}$). (Use $NPP = (1-0.30)\times GPP$ and propagate efficiencies.)

Q4. A lake's phosphate pool is modelled as a single box with steady external input $I=1000\ \mathrm{kg\ yr^{-1}}$ and a first‑order loss rate constant $k=0.2\ \mathrm{yr^{-1}}$. With initial pool $P_0=2000\ \mathrm{kg}$, the steady‑state pool is $P_{ss}=\dfrac{I}{k}$ and the time‑course is $P(t)=P_{ss}-(P_{ss}-P_0)e^{-kt}$. Approximately how long will it take for the pool to reach $90\%$ of $P_{ss}$?

Q5. A coastal region receives an annual phosphate input of $18\,600\ \mathrm{mol P\ yr}^{-1}$ which is entirely available and is limiting for primary production. Using Redfield stoichiometry $C:P = 106:1$ and atomic mass of carbon $12\ \mathrm{g\ mol^{-1}}$, estimate the maximum potential carbon fixation per year (in metric tonnes; $1\ \text{tonne}=10^3\ \text{kg}$). (Use $C_{\text{mol}}=106\times P_{\text{mol}}$, then $C_{\text{g}}=12\times C_{\text{mol}}$.)

Q6. In a grassland ecosystem the gross primary productivity (GPP) is $1200\ \mathrm{g\,C\,m^{-2}\,yr^{-1}}$. Autotrophic respiration is $450\ \mathrm{g\,C\,m^{-2}\,yr^{-1}}$ and heterotrophic respiration (consumers + decomposers) is $600\ \mathrm{g\,C\,m^{-2}\,yr^{-1}}$. Using $\mathrm{NEP} = \mathrm{GPP} - (R_{\text{auto}} + R_{\text{hetero}})$, what is the net ecosystem productivity (NEP)?

Q7. A forest has an annual net primary productivity (NPP) of $5000\ \mathrm{kJ\,m^{-2}\,yr^{-1}}$. Herbivores ingest $15\%$ of NPP; of the ingested energy assimilation efficiency is $60\%$ and production efficiency (fraction of assimilated energy converted to herbivore biomass) is $20\%$. What is the annual herbivore production (biomass gain) per m$^2$?

Q8. Assertion (A): A climax community always shows the highest net primary productivity ($\mathrm{NPP}$) compared to earlier successional stages.
Reason (R): During succession biomass generally increases, but $\mathrm{NPP}$ often peaks at mid-successional stages because early and mid-successional species have higher relative growth rates while later communities allocate more energy to maintenance.

Q9. In an oceanic ecosystem phytoplankton biomass is $B_p = 200\ \mathrm{g\,C\,m^{-2}}$ and primary production is $P_p = 1000\ \mathrm{g\,C\,m^{-2}\,yr^{-1}}$. If trophic transfer efficiency from phytoplankton to zooplankton is $10\%$ and zooplankton production-to-biomass ratio is $(P/B)_{z} = 4\ \mathrm{yr^{-1}}$, estimate zooplankton biomass $B_z$ (in $\mathrm{g\,C\,m^{-2}}$).

Q10. A coastal water sample has dissolved inorganic nitrogen (DIN) = $8\ \mu\mathrm{M}$ and dissolved inorganic phosphorus (DIP) = $1\ \mu\mathrm{M}$. Using the Redfield atomic ratio C:N:P = $106:16:1$, which nutrient is most likely to limit phytoplankton growth in this water?

Q11. In a grassland ecosystem annual gross primary productivity (GPP) is $5000\ \text{kcal m}^{-2}\ \text{yr}^{-1}$ and plant respiration is $1800\ \text{kcal m}^{-2}\ \text{yr}^{-1}$. The net primary productivity (NPP) is:

Q12. In a temperate forest $1000\ \text{g}$ of leaf litter decreases to $400\ \text{g}$ in $2$ years. Assuming first‑order decomposition $M_t = M_0 e^{-kt}$, the time (in years) required for the litter mass to fall to $100\ \text{g}$ is approximately:

Q13. Net primary production (NPP) of an ecosystem is $10000\ \text{kJ yr}^{-1}$. Herbivores consume $20\%$ of NPP. Of the consumed energy $70\%$ is lost as egesta (unassimilated), and $40\%$ of assimilated energy is converted into herbivore biomass (production efficiency). The annual secondary production by herbivores (in $\text{kJ yr}^{-1}$) is approximately:

Q14. Consider a lake where phytoplankton have biomass $B_p=4\ \text{g m}^{-2}$ and annual production $P_p=1500\ \text{g m}^{-2}\ \text{yr}^{-1}$, while zooplankton have biomass $B_z=120\ \text{g m}^{-2}$ and annual production $P_z=300\ \text{g m}^{-2}\ \text{yr}^{-1}$. Which of the following best describes pyramids constructed using producers (phytoplankton) and primary consumers (zooplankton)?

Q15. A forest receives annual litter input $L=500\ \text{g C m}^{-2}\ \text{yr}^{-1}$. Decomposition follows first‑order kinetics and at steady state soil carbon $C_{ss}=\dfrac{L}{k}$. If the decomposition rate constant increases from $k=0.05\ \text{yr}^{-1}$ to $k=0.08\ \text{yr}^{-1}$ due to warming, the steady‑state soil carbon will change by approximately: