Phytoplankton and the Carbon Cycle

Question 1

Carbon pumps

Answer 1

Act to maintain a gradient of carbon dioxide between the upper and lower ocean
This stores carbon dioxide in the deep ocean out of contact with the atmosphere and hence reduces atmospheric carbon
Maintain a sharp gradient of carbon dioxide between atmosphere and deep ocean where 38000 Gt C is stored

Question 2

Solubility pump

Answer 2

Carbon dioxide dissolves into surface waters (solubility is temp dependent), when waters sink into interior take C with them

Question 3

Biological pump

Answer 3

Organic C is produced in the surface, when a proportion of this C sinks into the interior it takes C with it.

Question 4

Biological Carbon Pump mechanisms

Answer 4

POC sinks to the deep ocean where it is remineralised by bacteria.
Net result is transport of carbon dioxide to the deep ocean for ~1000 years
Amount of POC produced based on food web and relative abundance of species.
Availability of inorganic nutrients plays a large role

Question 5

New production

Answer 5

Component of upper ocean OM production which is supported by nutrients which are newly supplied to the euphotic zone

Question 6

Export

Answer 6

The sinking of organic C to the aphotic zone
Over long timescales, new production and export must balance

Question 7

Export production

Answer 7

Continuous rain of particles settles down through the ocean, representing a downward flux of POM.
Made up of:
- ungrazed dead phytos
- faecal pellets from copepods and larger orgs
- organic aggregates or marine snow
- dead metazoan orgs and moults
This export of C from the surface ocean represents the bio pump.
Marine snow can be collected

Question 8

Moored sediment traps

Answer 8

Funnel collects sinking particles and funnel these into collection cups.
Collection cups are rotated to provide a time series of material.
Traps are typically attached to a bottom weight with a release mechanism
Vertical spacing of sediment traps allows measurements of flux at different depths.

Question 9

Free drifiting traps

Answer 9

Tethered or moored sediment traps can be influenced by artifacts

Question 10

Variations in export flux, depth

Answer 10

Downward flux of organic C decreases with depth as the POC is degraded
Zooplankton and bacteria use the supply of organic C ‘raining’ down from above for energy
In doing so they respire a proportion back to carbon dioxide
Only small amount reaches sea floor
Seasonal variability

Question 11

Export variations, production gradients

Answer 11

Higher export in high productivity systems, more C.
Proportion of C produced and then exported to depth can also be higher
Variations can occur over multiple scales in response to physical forcing perturbing the environment
Changes in PP and new production result in altered export

Question 12

Regional production gradients

Answer 12

In low production systems, autotrophic-heterotrophic coupling is strong, so little OM is available for export
In higher productivity systems coupling is wealer with higher export

Question 13

Dissolved OM

Answer 13

Organic C <0.2um
Spectrum of material from AA and sufars to large molecules, viruses and sub-micron detrital remnants of previously consumed biota.
DOM includes DOC, DON and DOP
It can be easily metabolised or less easily assimilated and decomposed by bacteria and other microbes

Question 14

DOM Sources

Answer 14

Natural sources of terrestrial or benthic origin
Autochthonous DOM can be anthropogenic
Most from PP by phyto or grazing:
- phyto excretion or leakage of molecular weight
-Zooplankton feeding and excretion
-Sloppy or messy feeding of large consumers
- viral lytic cycle when infected cell bursts and releases DOM

Question 15

Marine heterotrophic bacteria

Answer 15

Marine bacteria are unicellular prokaryotic plankton (<0.5-1um in length)
Highly abundant but small sizes mean they dont dominate biomass
Abundance can be measured using radiotracer methods.
Diversity can be investigated using modern molecular biological techniques

Question 16

Microbial loop

Answer 16

Cycling of C and nutrients through DOM and bacteria
Mixotrophy:
- combines hetero and autotrophic metabolisms has come to be recognised as a key characteristic of oceanic microbial systems
- mixotrophy has influenced the evolution of phyto and the biogeochem of marine systems
microbial C and nutrient cycling involve multiple complex interacting transformations
However the net effect appears to be a pumping of some of the DOM into refractory forms

Question 17

Biological and microbial pump

Answer 17

biological:
Production of new OM in surface by phyto.
- Packaging of particulate material into sinking forms.
- Sinking of POC to depth.
- Remineralisation of POC back to CO2 and storage in the deep ocean.
Microbial:
Production of new OM in surface by phyto.
Transformation of OM into refractory dissolved OM, allowing long term storage.