50 Aquatic Ecology MCQs with Answers

Introduction

Aquatic Ecology is the study of life and ecological processes in water environments like freshwater, marine, estuarine, and wetlands.
It examines how physical, chemical, and biological components interact in aquatic systems, influencing biodiversity, productivity, and ecosystem balance.
This Aquatic Ecology MCQ set is designed for students of Environmental Science, Marine Biology, and Ecology, and is useful for exams such as UGC NET Environmental Science, GATE Ecology & Evolution, CSIR-JRF, and university entrance tests.

Aquatic Ecology MCQs (Questions Only)

The study of inland water ecosystems is known as
a) Oceanography
b) Limnology
c) Hydrology
d) Ichthyology
Marine ecology focuses on
a) Forest ecosystems
b) Oceanic and coastal systems
c) Mountain ecosystems
d) Grasslands
The vertical layering of aquatic ecosystems is called
a) Stratification
b) Turbidity
c) Mixing
d) Thermal inversion
The euphotic zone refers to
a) Dark deep layer
b) Zone with sufficient light for photosynthesis
c) Zone with no oxygen
d) Sediment zone
Thermocline is
a) A layer of rapid temperature change in a water body
b) The topmost warm layer
c) Deep cold zone
d) Mixing zone
The benthic zone includes
a) Surface waters
b) Bottom-dwelling organisms and sediments
c) Floating plankton
d) Shore vegetation
Plankton are defined as
a) Bottom dwellers
b) Free-floating organisms
c) Strong swimmers
d) Attached plants
Phytoplankton are primarily
a) Bacteria
b) Microscopic plants and algae
c) Small fish
d) Zooplankton
The primary productivity of oceans is highest
a) In open ocean
b) In upwelling zones
c) In abyssal plains
d) In polar regions
The main limiting nutrient in freshwater ecosystems is
a) Nitrogen
b) Phosphorus
c) Iron
d) Sodium
The main limiting nutrient in marine ecosystems is
a) Phosphorus
b) Nitrogen
c) Calcium
d) Potassium
Eutrophication is caused by
a) Excess nutrient input
b) Lack of light
c) Sediment removal
d) Overfishing
Oligotrophic lakes are characterized by
a) High nutrients and turbidity
b) Low nutrients and clear water
c) High organic load
d) No oxygen
Hypolimnion refers to
a) Upper warm layer
b) Middle transition zone
c) Deep cold layer of a stratified lake
d) Shoreline vegetation
The major source of oxygen in water bodies is
a) Wind aeration
b) Photosynthesis by aquatic plants
c) Chemical oxidation
d) Diffusion from soil
Zooplankton feed mainly on
a) Fish larvae
b) Phytoplankton
c) Benthos
d) Detritus
Detritus is
a) Dead organic matter
b) Living phytoplankton
c) Minerals
d) Dissolved salts
Estuaries are
a) Freshwater lakes
b) Transitional zones between rivers and seas
c) Underground aquifers
d) Glacial streams
Estuaries have
a) Uniform salinity
b) Variable salinity (brackish water)
c) No nutrients
d) Cold water only
Mangroves are best described as
a) Marine algae
b) Salt-tolerant trees growing in coastal intertidal zones
c) Deep-sea plants
d) Freshwater shrubs
The benthos community consists of
a) Floating algae
b) Organisms living on or in sediments
c) Air-breathing fish
d) None
Coral reefs are formed by
a) Mollusks
b) Polyps secreting calcium carbonate
c) Sponges
d) Algae alone
The largest coral reef system in the world is
a) Great Barrier Reef
b) Red Sea Reef
c) Andaman Reef
d) Caribbean Reef
Coral bleaching occurs due to
a) Cold temperature
b) Loss of symbiotic algae (zooxanthellae)
c) Excess nutrients
d) Sedimentation
Primary producers in the ocean include
a) Bacteria and phytoplankton
b) Zooplankton
c) Fish
d) Mollusks
The intertidal zone is periodically
a) Covered and uncovered by tides
b) Permanently submerged
c) Dry
d) None
Salinity of seawater is approximately
a) 1 ppt
b) 15 ppt
c) 35 ppt
d) 100 ppt
Ocean acidification results from increased
a) CO₂ absorption
b) O₂ concentration
c) Nitrogen fixation
d) Evaporation
The oxygen minimum zone (OMZ) is found
a) Near the surface
b) At mid-depths (200–1000 m)
c) On sea floor
d) In polar regions only
Wetlands are ecologically important because they
a) Support biodiversity and purify water
b) Are barren areas
c) Produce methane only
d) Lack vegetation
Ramsar Convention (1971) is related to
a) Forest conservation
b) Wetland conservation
c) Desert ecology
d) Fisheries
The Ganga River Dolphin is an indicator of
a) Desert ecosystem
b) Freshwater ecosystem health
c) Marine fisheries
d) Tundra biodiversity
Lake turnover occurs due to
a) Density-driven mixing during seasonal changes
b) Wind erosion
c) Ice melting
d) Sedimentation
Turbidity in aquatic systems affects
a) Light penetration and photosynthesis
b) Oxygen diffusion
c) Temperature
d) All of the above
Bioindicator species in aquatic ecosystems include
a) Diatoms and macroinvertebrates
b) Elephants
c) Eagles
d) Camels
Anoxic conditions indicate
a) High oxygen
b) No oxygen
c) Low carbon dioxide
d) High temperature
Upwelling increases
a) Nutrient concentration and productivity
b) Salinity only
c) Temperature
d) Sediment load
The El Niño phenomenon leads to
a) Warmer Pacific waters and reduced upwelling
b) Colder Pacific
c) Higher fish yield
d) Drier conditions globally
Periphyton are
a) Microbes attached to submerged surfaces
b) Floating plankton
c) Sediment organisms
d) None
The Redfield ratio (C:N:P = 106:16:1) represents
a) Average elemental composition of marine phytoplankton
b) Fish nutrient ratio
c) Soil nutrient ratio
d) None
Dissolved Oxygen (DO) decreases when
a) Temperature increases
b) Eutrophication occurs
c) Organic load is high
d) All of the above
Hydrophytes are
a) Air plants
b) Water-loving plants
c) Desert plants
d) None
Lotic ecosystems refer to
a) Standing waters (lakes)
b) Flowing waters (rivers, streams)
c) Glaciers
d) None
Lentic ecosystems refer to
a) Flowing water
b) Standing water
c) Groundwater
d) Ice ecosystems
Estuarine organisms are often
a) Euryhaline (tolerant to wide salinity)
b) Stenohaline
c) Halophobic
d) None
Mangrove roots are adapted for
a) Gas exchange in anaerobic mud
b) Photosynthesis
c) Buoyancy
d) Salt excretion only
Microbial decomposition in water consumes
a) Dissolved oxygen
b) CO₂
c) Nitrogen
d) None
Biochemical Oxygen Demand (BOD) is an indicator of
a) Organic pollution level
b) Temperature
c) Nutrient ratio
d) Turbidity
Seagrasses differ from algae because they
a) Are true flowering plants
b) Are microscopic
c) Lack roots
d) Float on surface
The largest freshwater lake by volume is
a) Lake Victoria
b) Lake Tanganyika
c) Lake Baikal
d) Lake Superior