Role of Micronutrients in HNLC Regions

Question 1

What limits primary productivity?

Answer 1

Nutrient availability, light availability, grazing by zooplankton

Question 2

What is the typical pattern of a spring bloom?

Answer 2

Bloom uses up NO3 in euphotic zone, NO3 replenished by deep mixing over the winter

Question 3

What are the regions where there is no spring bloom and the surplus NO3 never gets used up?

Answer 3

Subarctic Pacific Ocean, Eastern equatorial Pacific Ocean, Southern Ocean

Question 4

What are iron concentrations like in the worlds oceans?

Answer 4

Surface dissolved iron is very low, and is in limiting supply relative to macronutrients. Shows a nutrient-like profile. Open-ocean has lower iron than coastal regions

Question 5

What are some sources of iron in the modern oceans?

Answer 5

Open ocean/coastal: vertical mixing, aeolian deposits of continental dust and volcanic ash, coastal eddies, hydrothermal vents. Coastal waters: river sediments, continental runoff, resuspension of bottom sediments.

Question 6

Why is iron important?

Answer 6

Used in phytoplankton physiology: photosynthesis, nitrogen assimilation (used in synthesis of nitrate reductase and nitrite reductase, nitrogenase enzyme complex for N2 gas fixation in cyanobacteria), synthesis of chl-a

Question 7

What is the iron hypothesis?

Answer 7

Phytoplankton growth in HNLC areas was limited by the availability of iron (John Martin 1986)

Question 8

How was the first experiment of the iron hypothesis conducted?

Answer 8

In Southern Ocean in 1989, test tubes with natural seawater spiked with iron, dramatic increase in chl-a and decrease in [NO3] in Fe treatments after a few days

Question 9

Why were researchers skeptical of Martin’s results?

Answer 9

Containers were too small (no mixing), all zooplankton had been removed, possibility of iron contamination

Question 10

What is some past evidence of the iron hypothesis?

Answer 10

During the ice ages there was less rain and the Earth was a drier, dustier place, which may carry Fe to ocean and cause phytoplankton blooms and drawn down CO2, to help further cool climate during glacial periods

Question 11

What did other tests of the iron hypothesis find?

Answer 11

Using ultra-clean techniques they confirmed Martin’s original findings: Fe stimulates growth and NO3 uptake

Question 12

What kind of growth occurred in these experiments?

Answer 12

Larger organisms (esp. diatoms) did most of the growth and NO3 uptake.

Question 13

What causes the size-based response to Fe?

Answer 13

Small phytoplankton have a lower Ks and greater SA:V ration which helps uptake of Fe, which is good when [Fe] are low, but when Fe is available, diatoms have a much higher growth rate than smaller organisms

Question 14

What were some experiments of the iron hypothesis?

Answer 14

IronEx I/II, EisenEx, EIFEX, SOIREE, SEEDS I/II, SOFeX (north/south), SERIES

Question 15

What did IronEx I (Oct 1993) do?

Answer 15

Single pulse of Fe added to eastern equatorial Pacific HNLC zone from to raise [Fe] ~4nM, tracked changes using chl-a fluorescence and sulfur-hexafluoride

Question 16

What did IronEx I find?

Answer 16

After 2-3 days: tripling of [chl-a], 4x increase in NPP, no measureable drawdown of NO3 or CO2, fertilized patch subducted below pycnocline after 4 days

Question 17

What did IronEx II (June 1995) do?

Answer 17

Patch fertilized 3x over 1 week to prevent rapid sedimentation. Tracked uing chl-a and SF6 and drifter deployed.

Question 18

What were the results of IronEx II?

Answer 18

Patch for 19 days (drifted 10-100km/day), phytoplankton growth rates doubled (NPP increased), [chl-a] increases 25x, [NO3] decreased 50%, uptake incrase 15x, ocean-atmosphere CO2 flux decreased 60%, most growth due to large diatoms, micro- and meso-zooplankton biomass doubled

Question 19

What did SOIREE (Jan-Mar 1999) do and find?

Answer 19

Fe added repeatedly over a few weeks. Dissolved Fe decreased after infusions, photosynthetic competency increased in patch, photosynthetic biomass increased, PP increased, nutrients removed from surface waters, large drawdown of aCO2, DMS increased with enrichment

Question 20

What is DMS?

Answer 20

Dimethylsulphide, a cloud-nucleating agent, so increased DMS may lead to increased cloud cover and cooling (affect climate)

Question 21

What produces DMS?

Answer 21

Haptophytes (i.e Phaeocystis, coccolithophores)

Question 22

What are phaeocystis?

Answer 22

Form large gelatinous colonies or unicellular flagellates, can form extensive blooms in temperate oceans, dominates polar phytoplankton assemblages, can produce 10% of the total global flux of DMS to atmosphere

Question 23

What results did SERIES (July-Aug 2002, subarctic NE Pacific) find?

Answer 23

Diatoms were dominant, specifically pseudo-nitzschia, which can produce toxins

Question 24

What were some common findings in the mesoscale Fe experiments?

Answer 24

Phytoplankton blooms but a wide range in bloom signatures, not all resulted in more C export

Question 25

What did LOHAFEX (Feb/Mar 2009, southern ocean) do?

Answer 25

Fertilized inside the core of an eddy and followed response for 39 days.

Question 26

What did LOHAFEX find?

Answer 26

Chl-a biomass doubled in 2 weeks, heavy grazing pressure (copepods and amphipods), so only a modest amount of C sank out, minor effect of CO2 transfer from atmosphere-ocean, blooms phaeocystis (but not diatoms b/c Si limitation), which can produce DMS

Question 27

How do we know [Fe] were higher during glacial periods?

Answer 27

Greenland and Antarctica ice cores show dust levels during last ice age 30x higher than today, sediment cores from southern ocean back 140000yrs show higher rates of biological productivity during ice ages

Question 28

Why do scientists think fertilizing oceans on a massive scale is risk?

Answer 28

Too many unknowns: diatom sp. which can be toxic increase after Fe fertilization, increase heterotrophic activity may cause higher levels of CO2, but decreased O2 in oceans, increased productivity can also have increase DMS

Question 29

What is the commercial interest in fertilization?

Answer 29

Increase fish production and obtain C credits