Systems Without Blooms

Question 1

If provided with a mix of N
compounds, which would be preferentially
utilized by phytoplankton? Why?

Answer 1

In order to use nitrogen to make amino acids and other compounds they must first reduce it to NH4+

So they will preferentially take up NH4+

Question 2

Major difference between P and N cycling

Answer 2

Nitrogen cycling occurs in many steps that can produce N of different types that may be unusable to plankton. This means it is often limiting in an environment

Phosphorus is readily respired back into the system and cycles through the system very quickly

Question 3

Silicon cycle

Answer 3

This cycle only involves inorganic forms
Dissolution of biogenic Si is solubility driven
Ocean is undersaturated therefore corrosive to Si
Skeletal material dissolves at all depths following death of organism and decay of organic material

Question 4

Importance of nutrient recylcing

Answer 4

increase productivity of food webs

enhances and extends phytoplankton productivity

influences composition of phytoplankton assemblages

a key component of the microbial loop

works with other biotic and abiotic processes to create biogeochemical cycles

Question 5

Why is iron important to phytoplankton

Answer 5

Photosynthesis: photosystems I&II and cytochrome complex require iron

Nitrogen Assimilation: Nitrogen assimilation, used in synthesis of nitrate reductase

Synthesis of chl a

Question 6

Sources of Iron to the ocean

Answer 6

Margin sediments
Dust from atmosphere
Hydrothermal vents
Karin Ridge

Question 7

HNLC Regions

Answer 7

Areas with high macronutrients but very low chlorophyll

Subarctic NE Pacific
Eastern Equatorial Pacific
Southern Ocean

Question 8

Development of Iron Hypothesis

Answer 8

Fe is not very soluble in oxygenated water and most ocean far from Fe sources

Open ocean is very low in Fe and in limiting supply compared to other nutrients

Preliminary bottle experiments showed that Fe stimulated growth of large diatoms led to nutrient drawdown

Indirect evidence from Island Effects

Question 9

Glacial Fe Hypothesis

Answer 9

Links dust flux to the strength of the biological carbon pump

Fe rich dust enters ocean: increase PP and decrease carbon in atmosphere

Temperature decreases: decrease precipitation, increased desertification and more dust

Question 10

Importance of Vostok Ice Core

Answer 10

Provided evidence that Fe supply may affect atmospheric CO2

Question 11

What were some arguments against the iron hypothesis

Answer 11

Artifacts associated with bottle experiments hinder extrapolation to ocean temporal and spatial scales

Bottle experiments are not a true representation of phytoplankton communities

Possible Fe contamination or improper analytical technique could lead to faulty conclusions

Question 12

Testing the iron hypotheisis

Answer 12

Fertilizing paths in the ocean and observing the effects
Mesoscale iron enrichment expermints showed significant increase in Chl a and drawdown of CO2

Most of the phytoplankton growth and N uptake is dominated by the large phytoplankton (diatoms)

Question 13

What we know so far about HNLC and Fe

Answer 13

About 30% of the ocean is HNLC

Mesoscale fertilizations have shown that Fe is the limiting nutrient in these cases

Some scientists have suggested that large scale iron fertilization could reverse the effects of climate change

Question 14

Kasatochi Eruption Event Effect

Answer 14

Caused serious phytoplankton bloom
Removed ~0.01 Pg-C, but volcano produced about the same

Question 15

Kasatochi Bloom Event Impact on Sockeye Salmon

Answer 15

The juveniles effected by the bloom went on to have record-breaking runs in 2010

However the bloom mostly occurred where juveniles were not, and productivity in 2011 was similar to 2010

Question 16

Haida Salmon Restoration

Answer 16

Massive Iron Fertilization event in 2012, and saw record-breaking returns in 2014

Question 17

Ocean Station Papa Trends (NE Pacific)

Answer 17

No obvious cycle of chl, occasional peaks, mostly just noise

Subarctic NE pacific lower and more random that North Atlantic

Question 18

Subtropical North pacific trends

Answer 18

No seasonal cycle
Slow steady production
Very low chl

Question 19

What does the euphotic zone depth at Ocean Station papa tell us about light availability

Answer 19

It tells us nothing
Only gives an attenuation coefficient, not surface irradiance

Question 20

Light Variability between OSP, North Atlantic and Subtropical pacific

Answer 20

OSP factor of 4 between winter and summer

North Atlantic similar difference to OSP

Subtropical pacific has very little variation

Question 21

North Atlantic Mixed Layer and Crit depth

Answer 21

Has very deep mixed layers (300m)
mixed layer must be less than crit depth for blooms
Low light in winter and deep mixed layer, N Atlantic is very light limited in winter

Question 22

Subarctic NE pacific mixed layer

Answer 22

has a layer of FW on its surface

The mixed layer shallow in summer, and limited to 100m in winter

similar winter light availability, but with shallower mixed layer light limitation is not much of an issue

Question 23

Subtropical Pacific Mixed layer

Answer 23

Shallow mixed layer summer, winter limited to 100m

High light year round so never light limited

Question 24

Macronutrient cycle at OSP

Answer 24

Half of macronutrient levels utilized year round
Lower in summer but not low
High all year round (HNLC)

Question 25

Macronutrient cycle in North Atlantic

Answer 25

Following spring bloom nitrate drawn down to zero

Macronutrient limited all summer

Replenished in the winter by deep mixing

Question 26

Macronutrient cycle in Subtropical pacific

Answer 26

Very low nutrients year-round
Classically macronutrient limited system
Low levels of productivity
supported by tight nutrient
recycling in the euphotic zone

Question 27

Fe concentrations at OSP

Answer 27

Fe is low in surface waters (100m) all year round
Increase with depth below 100m
Mixed layer sometimes reaches 100m but not enough to replenish Fe in surface waters

Question 28

Simulated Surface Fe concentrations

Answer 28

Limited at Subarctic NE pacific and antarctic regions
Subtropical Pacific probably Fe limited too, but has low nutrients so doesn’t matter

N Atlantic is lowish, but not limiting or as low as OSP
High near coasts and wind deposits

Question 29

Iron Fertilization Bottle Experiments at OSP

Answer 29

Smallest phytoplankton dont respond to iron addition

Medium have a small response

Largest have a very strong response

Question 30

Why don’t we get big blooms of small phytoplankton?
What could be limiting small phytoplankton in the subarctic NE
Pacific if not iron

Answer 30

Likely grazing of the small phytoplankton

Question 31

Grazing Hypothesis

Answer 31

Not nutrient or light limited over the winter for small phytoplankton

Maintains a population of small zooplankton over winter
This allows zooplankton to respond quickly and graze down small phytoplankton

Question 32

Evidence for grazing hypotheis

Answer 32

Phytoplankton growing 2-3x more in summer than in winter

They are growing more in summer, but their biomass is not increasing

Less zooplankton in winter and their biomass increases in summer

Grazing rate increases as phytoplankton grow more
Prevents phytoplankton bloom in summer of small phyto

Question 33

Microbial Loop in NE pacific

Answer 33

Microzooplankton graze on small phytoplankton or heterotrophic bacteria

Zooplankton produce NH4+, to increase recycling efficiency (still very low)

Small phyto use low levels of NH4+ to grow

Larger phyto need Fe in order to utilize the abundant NO3 here

Question 34

Which size
class of phytoplankton
are likely responsible
for the occasional
peaks in chlorophyll?

Answer 34

Occasional large blooms of
diatoms caused by Fe inputs

Question 35

What provides the Fe input to allow large phytoto grow

Answer 35

volcanic ash
dust storms
glacial flour
coastal water transported by eddies