A-Level Marine Science: Hard Biochemical & Ecological Principles

Front

Explain the chemical mechanism by which CO2 lowers seawater pH (ocean acidification) differently from carbonic acid addition in pure water.

Back

In seawater, CO2 reacts to form H2CO3, which dissociates into H+ and HCO3-. However, the H+ ions rapidly react with carbonate ions (CO3 2-) to form bicarbonate (HCO3-), buffering the pH drop initially but reducing the saturation state of calcium carbonate (CaCO3), hindering calcification. This 'buffering' effect distinguishes seawater chemistry from pure freshwater systems where alkalinity is lower.

Front

Analyze the energetic trade-off between active transport and osmoregulation in stenohaline marine teleosts.

Back

Stenohaline marine fish drink seawater to offset osmotic water loss. They actively excrete excess Na+ and Cl- via chloride cells in the gills using Na+/K+-ATPase pumps, which requires significant ATP (basal metabolic cost). In contrast, euryhaline species or elasmobranchs retain urea/TMAO to reduce osmotic gradients, minimizing this energy expenditure but requiring nitrogen recycling adaptations.

Front

Contrast chemosynthesis in hydrothermal vents vs. cold seeps regarding electron donors and carbon sources.

Back

Hydrothermal vent symbionts typically oxidize hydrogen sulfide (H2S) using oxygen as the terminal electron acceptor (aerobic respiration), often in environments with high temperature gradients. Cold seep communities rely on methane (CH4) or sulfide oxidation but face different temperature and flow regimes, often using anaerobic methanotrophy (sulfate-driven AOM) involving archaeal-bacterial consortia, reflecting different biochemical pathways.

Front

Derive the likely impact of a halocline on primary productivity in an estuarine environment.

Back

A strong halocline creates density stratification (pycnocline), preventing vertical mixing. This traps nutrients in the bottom layer and restricts their resupply to the euphotic zone. Consequently, light is abundant in the surface layer but nutrients are limited (light-limited bottom, nutrient-limited top), potentially reducing total primary productivity unless physical mixing (wind/tides) breaks down the stratification.

Front

Evaluate the trade-off between r-selection and K-selection in the context of commercial fisheries management.

Back

r-strategists (e.g., cod, squid) produce many small eggs with high natural mortality; populations are variable and sensitive to overfishing of adults but can recover quickly if spawning stock is preserved. K-strategists (e.g., sharks, rockfish) produce few live young with low mortality; they recover slowly from depletion. Management must set lower quotas for K-selected species due to their low intrinsic rate of increase (r).