Q1. Which causes harmful algal blooms (HABs) leading to red tides in marine ecosystems?
a) Diatoms
b) Dinoflagellates
c) Cyanobacteria
d) Euglenoids
Answer: (b) Dinoflagellates
Explanation :
• Red tides are typically caused by dinoflagellates, especially Karenia brevis, Alexandrium, Gymnodinium.
• They release brevetoxins, saxitoxins → neurotoxicity in fish, marine mammals, and humans (shellfish poisoning).
• Dinoflagellates proliferate under warm SSTs, stratified water, excess nitrogen–phosphorus, and low grazing pressure.
• The 2025 Gulf of Mexico bloom (NOAA) was linked to early-summer warming + Mississippi nutrient discharge, intensifying HAB frequency under climate change.
Q2. Primary driver of 2025 mass coral bleaching (Great Barrier Reef)?
a) Ocean acidification
b) Marine heatwaves (MHW)
c) Oil spill
d) Invasive crown-of-thorns starfish
Answer: (b) Marine heatwaves (MHW)
Explanation :
• Bleaching ≠ mortality, but high MHW intensity increases mortality risk.
• GBR’s 2025 bleaching coincided with a ~+2°C SST anomaly (Jul–Sep 2025) (GBRMPA).
• Heat stress disrupts the coral–zooxanthellae symbiosis → expulsion → whitening → reduced photosynthesis.
• Acidification is a chronic stressor, but acute thermal anomalies are the primary trigger for mass bleaching events globally.
Q3. Estuaries rank highest NPP among aquatic ecosystems due to:
a) High salinity
b) Nutrient trapping + tidal mixing
c) Deep aphotic zone
d) Low biodiversity
Answer: (b) Nutrient trapping + tidal mixing
Explanation :
• Estuaries are ecotones where rivers meet oceans → nutrient-rich inputs.
• Tidal mixing prevents stratification, ensuring high oxygen + nutrient availability.
• NPP values: ~2500–3000 gC/m²/yr, among the highest in aquatic systems (NCERT + global synthesis).
• Example: Sundarbans estuarine complex, where both mangroves + phytoplankton drive productivity and fisheries.
Q4.Dimictic lakes undergo turnover:
a) Once a year
b) Twice a year
c) Never
d) Continuously
Answer: (b) Twice a year
Explanation :
• Dimictic = two mixing cycles annually: spring and autumn.
• Occur mainly in temperate climates where surface water reaches 4°C (maximum density) twice yearly → entire water column mixes.
• Result: nutrient redistribution from hypolimnion → epilimnion, improving productivity.
• Indian example: high-altitude lakes like parts of Dal Lake showing dimictic tendencies due to seasonal temperature gradients.
Q5. Pneumatophores in mangroves are for:
a) Salt excretion
b) Aerial respiration in anaerobic soil
c) Vivipary
d) Prop roots
Answer: (b) Aerial respiration
Explanation :
• Mangrove soils are waterlogged, saline, anaerobic.
• Species like Avicennia develop pencil-like pneumatophores with lenticels.
• Function: O₂ diffusion to underground roots via aerenchyma tissues.
• Pneumatophore density increases after storms/cyclones; noted in 2025 Sundarbans restoration where replantation favored Avicennia for rapid soil oxygenation.
