Aquatic ecosystems – estuaries MCQs With Answer

Understanding aquatic ecosystems, especially estuaries, is essential for B. Pharm students studying pharmacology, ecotoxicology, and environmental safety. Estuaries are dynamic coastal zones where freshwater meets seawater, creating unique salinity gradients, high productivity, and complex biogeochemical cycles. Knowledge of estuarine processes — nutrient cycling, sediment dynamics, pollutant fate (including pharmaceutical contamination), bioaccumulation, and monitoring techniques — informs drug safety assessments and environmental risk analysis. This introduction focuses on key concepts relevant to pharmaceutical impacts, natural product sources, and environmental monitoring in estuarine systems. Clear grasp of these topics helps future pharmacists evaluate ecological risks and contribute to sustainable drug development. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What defines an estuary?

• A coastal water body where freshwater mixes with seawater
• A freshwater lake near the coast
• Open ocean shelf areas with constant salinity
• A deep-sea trench

Correct Answer: A coastal water body where freshwater mixes with seawater

Q2. Which term best describes the typical salinity of estuarine waters?

• Freshwater
• Brackish
• Hypersaline
• Saltwater identical to the open ocean

Correct Answer: Brackish

Q3. An estuary is often called an ecological ecotone because it is a zone between which two systems?

• Forest and desert
• Freshwater rivers and marine oceans
• Mountains and plains
• Coral reefs and pelagic zones

Correct Answer: Freshwater rivers and marine oceans

Q4. Primary producers that contribute most to estuarine productivity include:

• Terrestrial conifers and kelp
• Phytoplankton, seagrasses, and mangroves
• Deep-sea chemoautotrophs
• Alpine mosses and lichens

Correct Answer: Phytoplankton, seagrasses, and mangroves

Q5. Why are estuaries among the most productive ecosystems?

• Because they have low nutrient inputs and little mixing
• Due to high nutrient input, tidal mixing, and shallow sunlight penetration
• Because they are isolated from terrestrial influence
• Due to constant low temperatures year-round

Correct Answer: Due to high nutrient input, tidal mixing, and shallow sunlight penetration

Q6. Euryhaline species in estuaries are important because they:

• Can tolerate wide salinity ranges through osmotic regulation
• Are restricted to freshwater only
• Require stable, unchanging salinity
• Cannot osmoregulate and die quickly

Correct Answer: Can tolerate wide salinity ranges through osmotic regulation

Q7. Which organisms are commonly used as bioindicators for estuarine pollution monitoring?

• Pelagic sharks and tuna
• Bivalves (oysters, mussels) and benthic macroinvertebrates
• Deep-sea organisms
• Terrestrial insects

Correct Answer: Bivalves (oysters, mussels) and benthic macroinvertebrates

Q8. Typical contaminants of concern in estuaries that affect pharmaceutical fate include:

• Only organic matter with no chemical pollutants
• Nutrients, heavy metals, and pharmaceutical residues
• Pure freshwater with no contaminants
• Only dissolved oxygen and salinity

Correct Answer: Nutrients, heavy metals, and pharmaceutical residues

Q9. Chemical Oxygen Demand (COD) differs from Biological Oxygen Demand (BOD) because COD:

• Measures biological oxygen required over 5 days
• Measures the oxygen equivalent of chemically oxidizable substances
• Is lower than BOD for all waters
• Is unrelated to organic pollution

Correct Answer: Measures the oxygen equivalent of chemically oxidizable substances

Q10. Estuarine hypoxia (low dissolved oxygen) is most commonly caused by:

• Increased tidal flushing only
• Eutrophication leading to algal blooms and decomposition
• Excessive wind mixing
• High levels of groundwater recharge

Correct Answer: Eutrophication leading to algal blooms and decomposition

Q11. Mangroves around estuaries play a critical role by:

• Removing all saline water permanently
• Stabilizing shorelines and filtering nutrients and sediments
• Causing increased erosion and pollution
• Preventing any species from entering the estuary

Correct Answer: Stabilizing shorelines and filtering nutrients and sediments

Q12. The turbidity maximum zone in many estuaries is formed by:

• Convergence and trapping of fine suspended sediments at salinity fronts
• Complete absence of sediments
• High concentrations of dissolved oxygen only
• Deep ocean currents pushing sediments in

Correct Answer: Convergence and trapping of fine suspended sediments at salinity fronts

Q13. Denitrification in estuarine sediments converts nitrate primarily into:

• Ammonia in all cases
• Nitrogen gas (N2) under anoxic conditions
• Sulfate under oxidizing conditions
• Phosphate compounds

Correct Answer: Nitrogen gas (N2) under anoxic conditions

Q14. Which estuarine organisms are well-known sources of marine natural products relevant to drug discovery?

• Sponges, tunicates, and associated bacteria
• Terrestrial mammals
• Deep-sea hydrothermal vent worms only
• Freshwater algae exclusively

Correct Answer: Sponges, tunicates, and associated bacteria

Q15. The presence of antibiotics in estuaries can promote:

• Decreased microbial diversity without resistance
• Selection for antibiotic-resistant bacteria and resistance genes
• Complete removal of all bacteria
• Immediate degradation into harmless water

Correct Answer: Selection for antibiotic-resistant bacteria and resistance genes

Q16. Biomagnification refers to:

• Concentration decrease of a contaminant at higher trophic levels
• Increase in contaminant concentration up the food chain
• Sediment deposition in estuaries
• Photosynthetic rate increase with depth

Correct Answer: Increase in contaminant concentration up the food chain

Q17. Estuarine pH can vary diurnally primarily due to:

• Photosynthesis and respiration cycles affecting CO2 levels
• Constant buffering that prevents any change
• Only tidal height with no biological influence
• Salinity alone without biological activity

Correct Answer: Photosynthesis and respiration cycles affecting CO2 levels

Q18. High suspended particulate matter (SPM) in estuaries affects pharmaceuticals by:

• Preventing any chemical reactions indefinitely
• Enhancing sorption of hydrophobic drugs to particles, altering transport
• Reducing all contaminant concentrations to zero
• Only increasing water temperature

Correct Answer: Enhancing sorption of hydrophobic drugs to particles, altering transport

Q19. Mesohaline estuarine regions typically have salinity ranges of approximately:

• 0–0.5 PSU
• 5–18 PSU
• 30–40 PSU
• Above 50 PSU

Correct Answer: 5–18 PSU

Q20. How does residence time of water in an estuary influence contaminant behavior?

• Short residence time always increases contaminant retention
• Longer residence time increases retention and potential transformation of contaminants
• Residence time has no effect on contaminants
• Only salinity controls contaminant fate

Correct Answer: Longer residence time increases retention and potential transformation of contaminants

Q21. The tidal prism concept describes:

• The volume of water exchanged during a tidal cycle
• The number of fish in an estuary
• The amount of sunlight reaching the seabed
• Salinity at high tide only

Correct Answer: The volume of water exchanged during a tidal cycle

Q22. Which estuarine circulation type is characterized by a strong salt wedge and pronounced stratification?

• Well-mixed estuary
• Partially mixed estuary
• Salt-wedge estuary
• Tidal flat without stratification

Correct Answer: Salt-wedge estuary

Q23. Estuarine sediments are often dominated by which particle type that influences contaminant binding?

• Coarse sand with very little adsorption
• Fine-grained muddy sediments with high organic content
• Pure crystalline rock fragments
• Volcanic ash only

Correct Answer: Fine-grained muddy sediments with high organic content

Q24. Estuarine wetlands primarily remove excess nitrogen through which microbial process?

• Nitrification only in oxic water with no removal
• Denitrification in anoxic zones converting nitrate to N2 gas
• Sulfate formation increasing nitrogen levels
• Phosphorus mineralization

Correct Answer: Denitrification in anoxic zones converting nitrate to N2 gas

Q25. Which water quality parameter is most routinely used to indicate organic pollution and oxygen demand?

• Conductivity
• Biological Oxygen Demand (BOD)
• Chloride concentration only
• Water color alone

Correct Answer: Biological Oxygen Demand (BOD)

Q26. Which estuarine organism is commonly sampled for heavy metal biomonitoring due to its filter-feeding and accumulation?

• Detritivorous worms only
• Oysters and mussels
• Pelagic fish that never enter estuaries
• Sea grasses exclusively

Correct Answer: Oysters and mussels

Q27. Major processes determining the environmental fate of pharmaceuticals in estuaries include:

• Only dilution with no transformation
• Sorption to particles, microbial biodegradation, and photolysis
• Immediate precipitation as solids in all cases
• Evaporation to the atmosphere entirely

Correct Answer: Sorption to particles, microbial biodegradation, and photolysis

Q28. Photodegradation of pharmaceuticals in estuarine systems is most effective in:

• Deep anoxic sediments
• Surface waters with light penetration
• Buried sediment layers
• Complete darkness

Correct Answer: Surface waters with light penetration

Q29. Riparian buffer zones and constructed wetlands adjacent to estuaries primarily reduce contaminant flux by:

• Increasing direct industrial discharge
• Filtering sediments, promoting nutrient uptake and microbial degradation
• Adding more pharmaceuticals to the water
• Preventing tidal exchange completely

Correct Answer: Filtering sediments, promoting nutrient uptake and microbial degradation

Q30. Sediment porewater redox potential is commonly measured as:

• pH only
• Eh (oxidation-reduction potential)
• Salinity units
• Conductivity at 25°C only

Correct Answer: Eh (oxidation-reduction potential)

Q31. Which nutrients most commonly drive harmful phytoplankton blooms in estuaries?

• Iron and silicon only
• Nitrogen and phosphorus
• Gold and silver ions
• Potassium exclusively

Correct Answer: Nitrogen and phosphorus

Q32. The most effective long-term strategy to mitigate estuarine eutrophication is:

• Increasing fertilizer application upstream
• Reducing external nutrient inputs from agriculture and wastewater
• Removing all vegetation near estuaries
• Adding more fish to consume algae

Correct Answer: Reducing external nutrient inputs from agriculture and wastewater

Q33. Microbial communities in estuaries contribute to pharmaceutical removal primarily through:

• Photosynthesis of antibiotics
• Biodegradation and transformation of organic compounds
• Preventing any sorption to sediments
• Increasing persistence of all compounds

Correct Answer: Biodegradation and transformation of organic compounds

Q34. Passive sampling devices (PSDs) like semipermeable membrane devices (SPMDs) are used to:

• Directly count fish populations
• Accumulate hydrophobic contaminants for time-integrated monitoring
• Measure salinity fluctuations instantaneously only
• Replace the need for chemical analysis

Correct Answer: Accumulate hydrophobic contaminants for time-integrated monitoring

Q35. LC50 reported for an estuarine species refers to:

• The lethal concentration at which 50% of test organisms die under specified conditions
• The concentration that causes growth in 50% of organisms
• The concentration that is safe for 100% of organisms
• The light coefficient at 50 meters depth

Correct Answer: The lethal concentration at which 50% of test organisms die under specified conditions

Q36. Environmental monitoring for antibiotic resistance in estuaries commonly detects:

• Only chemical oxygen demand values
• Antibiotic resistance genes (ARGs) in microbial communities
• Photosynthetic pigments exclusively
• Salinity changes as the sole indicator

Correct Answer: Antibiotic resistance genes (ARGs) in microbial communities

Q37. When sampling estuarine water for contaminants, why is depth-profile sampling often necessary?

• Because estuarine properties like salinity and contaminant concentrations vary with depth due to stratification
• Because only the surface matters and depth profiles are irrelevant
• Because estuaries have identical conditions from surface to bottom
• Because deeper samples are always pollutant-free

Correct Answer: Because estuarine properties like salinity and contaminant concentrations vary with depth due to stratification

Q38. Estuaries are important for drug discovery because they:

• Contain no biological diversity useful to pharmacology
• Host diverse organisms that produce bioactive natural products
• Only have terrestrial plants of pharmaceutical interest
• Prevent any microbial metabolism useful for drugs

Correct Answer: Host diverse organisms that produce bioactive natural products

Q39. Halophyte plants in estuarine salt marshes can be useful in pollution control by:

• Acting as biofilters to uptake and sequester contaminants
• Releasing large amounts of pollutants
• Preventing nutrient uptake entirely
• Only increasing salinity of groundwater

Correct Answer: Acting as biofilters to uptake and sequester contaminants

Q40. Stable isotope analysis (e.g., δ13C, δ15N) in estuaries is used to:

• Measure pH directly
• Trace food web structure and sources of organic matter
• Directly quantify antibiotic concentrations
• Replace all chemical analyses

Correct Answer: Trace food web structure and sources of organic matter

Q41. Which constituent behaves largely conservatively (mixes without significant biological or chemical removal) in estuaries?

• Chloride
• Ammonium subject to rapid transformation
• Reactive phosphorus
• Bioavailable nitrate undergoing denitrification

Correct Answer: Chloride

Q42. The end product of complete denitrification in estuarine sediments is typically:

• Nitrous oxide only
• Nitrogen gas (N2)
• Phosphate
• Sulfate

Correct Answer: Nitrogen gas (N2)

Q43. Under anoxic sediment conditions, sulfate-reducing bacteria produce which gas that can be toxic to benthic life?

• Methane exclusively
• Hydrogen sulfide (H2S)
• Ozone
• Carbon monoxide only

Correct Answer: Hydrogen sulfide (H2S)

Q44. Mangrove root adaptations like pneumatophores help plants by:

• Increasing salt excretion only
• Facilitating gas exchange in waterlogged, anoxic soils
• Preventing seed dispersal
• Raising water temperatures locally

Correct Answer: Facilitating gas exchange in waterlogged, anoxic soils

Q45. Why are estuaries often important nursery grounds for many fish species?

• They provide shallow, productive habitats with food and shelter for larvae and juveniles
• They have high predation with no shelter
• They are too saline for juvenile fish survival
• They contain no nutrients for growth

Correct Answer: They provide shallow, productive habitats with food and shelter for larvae and juveniles

Q46. How can salinity fluctuations alter the toxicity of contaminants, including drugs, in estuarine organisms?

• Salinity has no impact on contaminant bioavailability or toxicity
• Salinity changes can alter ionic speciation and bioavailability, affecting toxicity
• Higher salinity always neutralizes all contaminants
• Only temperature controls drug toxicity

Correct Answer: Salinity changes can alter ionic speciation and bioavailability, affecting toxicity

Q47. Estuarine buffering capacity against pH changes is mainly controlled by:

• Alkalinity and carbonate systems
• Only dissolved oxygen
• Salinity with no carbonate influence
• Chloride concentration exclusively

Correct Answer: Alkalinity and carbonate systems

Q48. Mercury methylation, producing toxic methylmercury, is typically promoted under which conditions in estuarine sediments?

• Oxic, sandy sediments with high oxygen
• Anoxic, organic-rich sediments with active microbial communities
• Completely dry and exposed sediments
• Only in the water column with no sediments

Correct Answer: Anoxic, organic-rich sediments with active microbial communities

Q49. Effective bioremediation strategies for estuarine contamination may include:

• Phytoremediation, microbial biodegradation, and constructed wetlands
• Dumping more untreated waste to dilute pollutants
• Removing all vegetation and leaving bare mudflats
• Adding heavy metals intentionally to precipitate contaminants

Correct Answer: Phytoremediation, microbial biodegradation, and constructed wetlands

Q50. Which regulatory framework or guideline is commonly referenced for protecting estuarine water quality from chemical contaminants?

• Local folklore with no scientific basis
• National and international water quality criteria such as EPA or WHO guidelines
• Only geological surveys unrelated to water quality
• Random daily weather reports

Correct Answer: National and international water quality criteria such as EPA or WHO guidelines

G S Sachin

I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.

Mail- Sachin@pharmacyfreak.com