Oceanography set 4

What does it mean that plankton have the same phenology as land plants?

They follow similar seasonal cycles of growth and productivity, with blooms during periods of maximum light and nutrients

When does the phytoplankton bloom occur in the Northern Hemisphere?

Around May

Describe phytoplankton productivity in tropical oceans

It remains fairly constant year-round

Describe phytoplankton productivity in polar oceans

Blooms occur during maximum sunlight and ice melt (June/July), with little stratification

Describe phytoplankton productivity in temperate oceans

Two blooms: one in spring (March/April) and one in fall (September/October). Strong stratification develops in spring as nutrients are used up

What is the photic zone?

The upper 0-200 m of the ocean where sunlight supports photosynthesis

What is the compensation depth?

About 125 m deep, where the rate of photosynthesis equals the rate of respiration

What happens above the compensation depth?

The ecosystem produces oxygen and consumes carbon dioxide

What happens below the compensation depth?

Respiration dominates, oxygen is consumed, and carbon dioxide is produced

Does respiration occur at all depths?

Yes

What type of organisms are diatoms?

Eukaryotic protists (unicellular algae)

Where are diatoms most important?

In coastal areas, especially during spring blooms

What are diatom shells made of?

Silica/glass, called frustules

What nutrients do diatoms require?

Silica, nitrate, and phosphate

Can diatoms swim?

No, but they can regulate buoyancy and sometimes migrate vertically

What are coccolithophores?

Single-celled eukaryotic protists encased in calcium carbonate disks (coccoliths)

Where do coccolithophores live?

In the photic zone, since they are photosynthetic

What do coccolithophores’ calcium carbonate disks do?

They may help shade the cell from excessive sunlight

What can large coccolithophore blooms cause?

Milky-white seawater

What are dinoflagellates?

Eukaryotic protists with cellulose plates and two flagella for movement

How do dinoflagellates move?

With flagella that allow vertical migration toward light

What feeding strategies do dinoflagellates use?

They can be autotrophic (photosynthetic), heterotrophic (consume others), or mixotrophic (both)

Where do dinoflagellates thrive?

In nutrient-rich (eutrophic) waters

What are red tides and what causes them?

Toxic blooms caused by certain dinoflagellates

What is Noctiluca?

A bioluminescent dinoflagellate (“light at night”)

What type of organisms are cyanobacteria?

Photosynthetic bacteria

What is the size range of cyanobacteria?

0.2-2 microns (picoplankton)

When did oceanographers realize cyanobacteria’s abundance and importance?

In the 1980s

What is the modern ecological role of cyanobacteria?

They likely dominate photosynthesis and oxygen production in today’s oceans

What major event are cyanobacteria responsible for in Earth’s history?

The initial oxygenation of Earth’s atmosphere about 2 billion years ago

What does “algae” refer to?

Photosynthetic eukaryotes (includes most phytoplankton, but not cyanobacteria)

What are macroalgae?

Large algae like seaweeds and kelp

What are protists?

Mostly unicellular eukaryotes that can reproduce sexually or asexually and have great genetic diversity

What type of dinoflagellates live in symbiosis with coral polyps?

Zooxanthellae

What do dinoflagellates provide to coral?

Food (through photosynthesis)

What do corals provide to dinoflagellates?

Protection from predation

What causes coral’s bright colors?

Pigments in zooxanthellae

What is coral bleaching?

When stressed zooxanthellae die or leave the coral host, often due to high temperature

What organism causes red tides in Florida?

Karenia brevis, a red-pigmented dinoflagellate

What diatom is known for harmful blooms in Rhode Island?

Pseudo-nitzschia

Why are HABs harmful?

Some algae produce toxins that bioaccumulate in higher trophic levels

Which organisms can accumulate algal toxins?

Clams, fish, and humans

Have HABs always existed?

Yes, they date back to ancient times and appear in historical texts

Why might HABs be increasing in frequency?

More nutrient pollution, species introductions, and increased monitoring

What two factors most commonly limit primary production in the ocean?

Light and nutrients

What is fixed nitrogen used for?

Making proteins

What is phosphate used for?

Cell membranes and DNA/RNA

Which organisms require silicate?

Diatoms and some plants

Who discovered that nutrient ratios in seawater match phytoplankton composition?

Alfred Redfield

What is the Redfield ratio for phytoplankton?

106 C : 16 N : 1 P

What is the extended Redfield ratio for diatoms?

106 C : 16 N : 1 P : 16 Si

What does the Redfield ratio suggest about nutrients in seawater?

They are largely recycled from the decomposition of phytoplankton

What does Liebig’s Law of the Minimum state?

Growth is limited by the scarcest resource, not the total available amount

Examples of limiting nutrients in the ocean?

N, P, Si, and micronutrients like Fe, Mn, Cu, Zn, B, Mo

What organisms supply new nitrate to the ocean?

Nitrogen-fixing bacteria

Where does most phosphate come from?

Runoff from continents

What are major anthropogenic sources of nutrients?

Wastewater treatment and agricultural runoff

What does respiration consume?

Oxygen

What does respiration produce?

Carbon dioxide

How does respiration contribute to nutrient cycling?

It regenerates nutrients

What is marine snow?

Sinking biomass produced by plankton, representing dead phytoplankton

Why is marine snow important?

It helps scientists understand how nutrients are recycled in the ocean

What tool is used to study sinking organic matter?

Sediment traps

What happens to nutrients and oxygen at the surface of the ocean?

Nutrients are depleted by photosynthesis, oxygen is high

How do nutrients and oxygen change with depth?

Oxygen decreases while nutrients increase due to remineralization

Why are deep waters below the thermocline high in both nutrients and oxygen?

They inherit oxygen and nutrient-rich water from deep water formation in polar regions

Who fixes nutrients into biomass?

Autotrophs (primary producers)

How do nutrients move through marine ecosystems?

From producers → consumers → decomposers → back to seawater

How are deep nutrients returned to the surface?

upwelling

What CO₂ levels correspond to warm interglacial periods?

High CO2

What CO₂ levels correspond to cold glacial periods?

Low CO2

What causes glacial–interglacial changes in atmospheric CO₂?

Changes in deep ocean storage of dissolved inorganic carbon

During glacial periods, where does carbon shift?

From the atmosphere → into the ocean

How fast is atmospheric CO₂ currently increasing?

About 2-2.5 ppm per year

What fraction of human CO₂ emissions does the ocean absorb?

About one-third to one-half

Why is ocean CO₂ especially high in the North Atlantic?

Deep water formation brings down CO2 from the surface

How much more inorganic CO₂ does the ocean contain compared to the atmosphere?

About 50 times more

What are the two major processes that regulate atmospheric CO₂ via the ocean?

The solubility pump and the biological pump

What does the solubility pump do?

Moves inorganic carbon from the atmosphere into the deep ocean via physical and chemical processes

Where is CO₂ most soluble?

In cold water, especially at the poles (CO2 sinks)

What determines air-sea CO₂ flux?

CO2 solubility (temperature, chemistry) and thermohaline circulation

Which form of dissolved inorganic carbon is dominant in seawater?

Bicarbonate

What happens when carbonate upwells to the surface?

It reacts with atmospheric CO2 to form bicarbonate

What drives the biological pump?

Primary producers (phytoplankton)

Why is carbon below the thermocline important for climate?

It’s isolated from the atmosphere and acts as a long-term CO2 sink

What controls the strength of the biological pump?

Nutrient availability

Why is carbon export inefficient?

Most organic carbon is respired as it sinks

What percentage of organic matter leaves the epipelagic zone?

5-10%

What percentage reaches the deep ocean?

1-5%

What percentage reaches the seafloor sediment?

About 0.1%

Q: What did John Martin (1990) propose about low CO₂ during glacial periods?

That low CO2 was caused by a stronger biological pump driven by more iron-rich dust fertilizing the ocean

How did glacial winds contribute to stronger biological pumping?

They transported iron-rich soil dust into the ocean, stimulating phytoplankton growth

What does HNLC stand for?

High Nutrient, Low Chlorophyll

Why are nutrients “left on the table” in HNLC regions?

Because phytoplankton growth is limited by micronutrients, especially iron

What characterizes HNLC regions?

High surface nutrients and low phytoplankton productivity

Why are micronutrients important for primary producers?

They are required in small amounts for enzymes involved in biochemical processes

How do Fe and Zn vertical profiles compare to major nutrients?

Both are depleted in surface waters and abundant in deep waters

Why is iron limited in many parts of the ocean surface?

Because there is low iron input from both upwelling/mixing and atmospheric dust

What do microcosm (bottle) experiments show about iron?

Adding iron to iron-limited seawater increases phytoplankton productivity

How are field iron fertilization experiments performed?

A small ocean patch is enriched with iron and a tracer, then tracked via ships and satellites