EOSC 315: Module C

What is the pelagic zone?

Open water of the ocean not including the bottom

What is the neritic zone?

Shallow water from the coast to the edge of the continental shelf

What is the oceanic zone?

Deep water beyond the continental shelf

What is the benthic zone?

The ocean bottom, both shallow and deep

What proportion of marine life does the benthic zone support?

At least 95 percent near or on the seafloor

What is the main food source in shallow benthic zones?

Photosynthesizers such as seaweeds and phytoplankton

What is the main food source in deep benthic zones?

Marine snow (falling organic matter)

What is the epipelagic zone?

0-200 meters, sunlit zone where about 90 percent of marine life lives

What is the mesopelagic zone?

200-1,000 meters, twilight zone with less than 1 percent surface light

What is the bathypelagic zone?

1,000-4,000 meters, no detectable light

What is the abyssopelagic zone?

4,000-6,000 meters, deep water near benthic communities

What is the hadalpelagic zone?

6,000-11,000 meters, deepest ocean trenches

What is the photic (euphotic) zone?

Surface zone with enough light for photosynthesis

What is the dysphotic zone?

Twilight zone with less than 1 percent surface light

What is the aphotic zone?

Zone with no light and permanent darkness

Why do most organisms live in the photic zone?

High light availability and food from photosynthesis

What is compensation depth?

Depth where photosynthesis equals respiration for phytoplankton at that depth

What is net growth at the compensation depth?

Zero

What is critical depth?

Depth from the surface where total photosynthesis equals total respiration for the mixed population

What happens if the mixed layer is shallower than the critical depth?

A phytoplankton bloom occurs

What happens if the mixed layer is deeper than the critical depth?

No bloom occurs

Why is winter unfavorable for blooms in temperate and polar regions?

Deep mixed layer and low light

Why does a spring bloom occur?

Shallow mixed layer combined with increasing light

Why is growth limited in summer?

Strong stratification traps nutrients below the mixed layer

Why can a smaller bloom occur in autumn?

Increased mixing brings nutrients upward

Why are blooms limited in the tropics?

Persistent stratification limits nutrient supply

What role does sunlight play in phytoplankton growth?

Controls photosynthesis based on season, depth, and clarity

Why is mixed layer depth important?

It determines light exposure relative to critical depth

Which nutrients commonly limit phytoplankton growth?

Nitrate, phosphate, and iron

How does stratification affect phytoplankton?

It limits nutrient mixing from deep water

What is the Redfield ratio?

106 carbon : 16 nitrogen : 1 phosphorus

Who discovered the Redfield ratio?

Alfred Redfield in the 1930s

What does the Redfield ratio describe?

Nutrient uptake by phytoplankton

Why is the Redfield ratio important?

Ensures balanced growth, metabolism, and reproduction

What happens when a nutrient is limiting?

Phytoplankton growth slows or stops

Why is the Redfield ratio found in seawater nutrients?

Marine life evolved to match ocean chemistry

Which organisms reflect the Redfield ratio?

Phytoplankton, zooplankton, and bacteria

What is nutrient limitation?

When one nutrient is in short supply relative to demand

What is a nutricline?

Depth below the mixed layer where nutrient concentrations increase

How much energy is lost at each trophic level?

About 80-95 percent

How much energy is transferred to the next trophic level?

About 5-20 percent

What is dissolved organic material (DOM)?

Organic material dissolved in water smaller than 1 micrometer

What is another term for DOM?

Dissolved organic carbon (DOC)

What are major sources of DOM?

Viral lysis, sloppy feeding, diffusion from phytoplankton, and waste

Why is DOM important?

It provides carbon and energy for marine bacteria

What is the microbial loop?

Pathway recycling DOM through bacteria to higher trophic levels

What organisms consume DOM?

Heterotrophic bacteria

Who consumes bacteria in the microbial loop?

Nano- and micro-zooplankton

How does energy move through the microbial loop?

DOM → bacteria → zooplankton → higher consumers

Why is the microbial loop important?

Prevents DOM buildup and recycles nutrients

How does the microbial loop support ecosystems?

Maintains productivity and energy flow