Ecosystem Set-5

Q1. In a grassland study the gross primary productivity (GPP) was measured as $2500\ \text{kJ m}^{-2}\text{yr}^{-1}$ and total respiration by producers was $1800\ \text{kJ m}^{-2}\text{yr}^{-1}$. Using $NPP = GPP - R$, the net primary productivity (NPP) is:

Q2. In a marine patch daily $GPP = 1500\ \text{kJ m}^{-2}\text{day}^{-1}$, producer respiration $R = 900\ \text{kJ m}^{-2}\text{day}^{-1}$ and standing producer biomass = $3000\ \text{kJ m}^{-2}$. Using $NPP = GPP - R$ and turnover time $T = \dfrac{\text{standing biomass}}{NPP}$, and assuming herbivore assimilation efficiency $=30\%$, all NPP is consumed by herbivores, herbivore respiration is negligible and their turnover time equals producer turnover, the maximum steady-state herbivore biomass (in $\text{kJ m}^{-2}$) is:

Q3. Phytoplankton require elements in approximate atomic ratio $C:N:P = 106:16:1$. A lake sample has dissolved inorganic nitrogen (DIN) = $5\ \mu\text{M}$ and dissolved inorganic phosphorus (DIP) = $0.25\ \mu\text{M}$. Using the critical $N:P = 16:1$, identify the limiting nutrient initially and after adding $0.5\ \mu\text{M}$ DIP to the water:

Q4. Assertion (A): Eutrophication in lakes frequently leads to hypoxia and consequent fish kills.
Reason (R): Algal blooms formed during eutrophication directly consume dissolved oxygen ($O_2$) through photosynthesis, causing oxygen depletion.
Choose the correct evaluation of A and R:

Q5. Two ecosystems A and B have equal annual net primary production $NPP = 2000\ \text{kJ m}^{-2}\text{yr}^{-1}$. Their trophic transfer efficiencies are $TE_A = 10\%$ and $TE_B = 20\%$. If the energy reaching trophic level $t$ is $E_t = NPP \times TE^{\,t-1}$ and a minimum of $1\ \text{kJ m}^{-2}\text{yr}^{-1}$ is required to sustain a trophic level, the maximum whole-number of trophic levels (count producers as level 1) that each ecosystem can sustain is:

Q6. In a grassland ecosystem the gross primary productivity (GPP) is $1200\ \mathrm{g\,C\,m^{-2}\,yr^{-1}}$ and total autotrophic respiration is $700\ \mathrm{g\,C\,m^{-2}\,yr^{-1}}$. Using the relation $NPP = GPP - R_a$, what is the annual net primary productivity (NPP) per m$^2$?

Q7. Leaf litter decomposition in a forest follows $L_t = L_0 e^{-kt}$. At $20^\circ\mathrm{C}$ the decomposition constant is $k = 0.10\ \mathrm{month^{-1}}$. If an increase to $30^\circ\mathrm{C}$ raises $k$ by $50\%$ (per $10^\circ\mathrm{C}$), what fraction of the original litter mass remains after $6$ months at $30^\circ\mathrm{C}$?

Q8. In a shallow lake phytoplankton have standing biomass $B_{phyto}=0.25\ \mathrm{g\,C\,m^{-2}}$ and turnover rate $r_{phyto}=150\ \mathrm{yr^{-1}}$, while submerged macrophytes have $B_{mac}=100\ \mathrm{g\,C\,m^{-2}}$ and $r_{mac}=0.2\ \mathrm{yr^{-1}}$. Using $P = B\times r$ (annual primary productivity), which statement correctly compares their annual contributions?

Q9. In a terrestrial ecosystem annual NPP = $1000\ \mathrm{g\,C\,m^{-2}\,yr^{-1}}$. A herbivore consumes $5\%$ of NPP. Plant tissue has $C:N = 40:1$ and herbivore tissue requires $C:N = 8:1$. If the herbivore's production efficiency (fraction of ingested carbon converted to herbivore biomass) is $30\%$, determine whether nitrogen in the consumed plant matter is sufficient to support the herbivore's biomass production. If insufficient, give the nitrogen deficit in g N m$^{-2}$ yr$^{-1}$.

Q10. Assertion (A): "Increasing species richness always increases ecosystem stability (resistance to perturbations)." Reason (R): "Functional redundancy among species can buffer ecosystem processes when some species are lost, thereby enhancing stability."

Q11. In a grassland ecosystem the gross primary productivity (GPP) is $6000\ \mathrm{kJ\ m^{-2}\ yr^{-1}}$ and autotrophic respiration $R_a$ is $2200\ \mathrm{kJ\ m^{-2}\ yr^{-1}}$. Using $\mathrm{NPP}=\mathrm{GPP}-R_a$, the net primary productivity (NPP) is:

Q12. In a grassland, net primary productivity (NPP) = $2000\ \text{kJ m}^{-2}\text{yr}^{-1}$. Herbivores consume $30\%$ of NPP. Of the consumed energy, $60\%$ is assimilated (rest egested) and $15\%$ of assimilated energy is converted into herbivore biomass (secondary production). Using the relation $\text{Secondary production} = \text{NPP}\times f_{\text{consume}}\times f_{\text{assimil}}\times f_{\text{prod}}$, the annual secondary production is:

Q13. In a forest, GPP = $6000\ \text{kJ m}^{-2}\text{yr}^{-1}$ and plant respiration $R_a = 3500\ \text{kJ m}^{-2}\text{yr}^{-1}$. Herbivores consume $25\%$ of NPP; $40\%$ of consumed energy is egested; of the assimilated energy, $20\%$ becomes herbivore biomass. Calculate (i) NPP using $\mathrm{NPP}=\mathrm{GPP}-R_a$ and (ii) the annual herbivore biomass production (in $\mathrm{kJ\ m^{-2}\ yr^{-1}}$).

Q14. In a temperate forest experiment equal dry mass of two leaf litters were added to separate plots: Litter X with C:N = $15:1$ and low lignin content; Litter Y with C:N = $80:1$ and high lignin content. After one year, which outcome is most consistent with decomposition dynamics and soil nitrogen availability?

Q15. A coastal lagoon has dissolved inorganic nitrogen (DIN) = $2\ \mu\mathrm{M}$ and dissolved inorganic phosphate (DIP) = $0.5\ \mu\mathrm{M}$ (atomic ratio DIN:DIP = $4:1$). The phytoplankton community includes abundant diazotrophic cyanobacteria capable of fixing atmospheric $N_2$. Considering the Redfield ratio C:N:P = $106:16:1$, which management action will most likely produce the fastest immediate increase in total phytoplankton primary production?