Exercises: Matter cycles

Explain the difference between a gaseous and a sedimentary biogeochemical cycle, and classify the carbon, nitrogen, and phosphorus cycles into one of the two types, justifying your choice.

Describe the role of nitrogen-fixing bacteria and nitrifying bacteria, indicating which chemical transformation each group performs and why they are essential for plant nutrition.

Write the overall chemical equations for photosynthesis and aerobic cellular respiration, and explain the role both processes play, in opposite directions, within the carbon cycle.

Order the following stages of the nitrogen cycle as they occur in an ecosystem and briefly describe what each consists of: nitrification, fixation, denitrification, and ammonification.

Phosphorus is often the limiting nutrient in terrestrial ecosystems. Explain what it means for a nutrient to be 'limiting', why the phosphorus cycle is so slow, and what consequence its lack of a gaseous phase has.

The diagram represents the main reservoirs and fluxes of the carbon cycle. Name the processes marked with the letters a, b, and c, indicate which process corresponds to the arrow marked with '?', and explain how this flux alters the balance of the cycle.

An environmental monitoring team studies a coastal lagoon after an agricultural runoff discharge and records the following data:

• Day 5: Surface reactive phosphorus concentration of $0.8\text{ mg/L}$ and chlorophyll $a$ of $65\ \mu\text{g/L}$; dissolved $\text{O}_2$ saturation of $145\,\%$ at noon in the top $40\text{ cm}$ of depth.

• Day 20: Surface $\text{O}_2$ saturation of $35\,\%$ at dawn; extreme turbidity ($40\text{ NTU}$); dissolved $\text{O}_2$ at the bottom ($3\text{ m}$) at $0.3\text{ mg/L}$, with the release of hydrogen sulfide ($\text{H}_2\text{S}$) and benthic fauna die-off.

A municipal technician states: "It is impossible for fish to die of asphyxiation if algae produce plenty of oxygen through photosynthesis; the cause must be direct chemical toxicity from nitrate."

1. a) Refute the technician's claim using biogeochemical arguments: justify why a massive autotrophic bloom inevitably leads to hypolimnetic and nocturnal anoxia.
2. b) Explain which trophic group proliferates from day 15 onwards and which anaerobic metabolic pathways account for the presence of $\text{H}_2\text{S}$ at the bottom.
3. c) Predict what immediate ecological consequences treating the lagoon with a chemical algaecide to eradicate phytoplankton would have on the matter cycle.

Two high-mountain lakes, located at equal distances from an industrial park with coal-fired power plants, receive continuous precipitation with an average pH of $4.1$. Biological and chemical monitoring reveals very contrasting data:

• Lake A (situated on a granitic basin): Has lost its salmonid and amphibian populations; a dissolved $\text{Al}^{3+}$ concentration of $0.35\text{ mg/L}$ is detected in its waters.

• Lake B (situated on a limestone basin): Keeps its fish richness intact and shows no detectable dissolved aluminum.

1. a) Justify on a lithological and chemical scale why the same flux of acid rain causes a biological collapse in lake A but proves harmless in lake B.
2. b) Explain the biogeochemical mechanism by which acidification of the surrounding soil mobilises toxic ions such as $\text{Al}^{3+}$ and leaches essential nutrients ($\text{Ca}^{2+}$, $\text{Mg}^{2+}$), and predict the impact of this phenomenon on the forest in basin A.
3. c) To restore lake A, an association proposes periodically adding large volumes of slaked lime or calcium carbonate directly to the water. Argue the limitations and potential harmful effects of this measure compared to reducing $\text{SO}_2$ emissions at the source.

A science sceptic posts the following message: "The atmosphere contains about $850\text{ Gt}$ of carbon and the oceans more than $38\,000\text{ Gt}$. Our annual emissions from burning fossil fuels are barely around $10\text{ Gt}$. Since the planet's total carbon is constant and anthropogenic emissions are less than $0.03\,\%$ of the marine reservoir, it is absurd to claim that we can alter the global carbon cycle."

1. a) Refute their argument by distinguishing between the dynamics of the biological (fast) cycle and the geological (slow) cycle: use the concepts of flux, residence time, and dynamic equilibrium to justify why $10\text{ Gt/year}$ destabilises the global climate.
2. b) If natural sinks (biosphere and ocean surface) absorb on average $45\,\%$ of the anthropogenic carbon emitted each year, calculate how many ppm of $\text{CO}_2$ accumulate annually in the atmosphere, knowing that $1\text{ ppm} \approx 2.13\text{ Gt}$ of atmospheric carbon.
3. c) Predict what will happen to the ocean's capacity to continue acting as a chemical sink for $\text{CO}_2$ as surface water temperature increases and its pH decreases, justifying through the chemical equilibrium of carbonic acid and carbonates.