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OC211(OA211) Phytoplankton & Primary Production

OC211(OA211) Phytoplankton & Primary Production. LECTURE 10 Week 11 (i)Marine Bacteria and the Microbial Loop (ii)Export Production (iii)Modelling Phytoplankton Production. Dr Purdie SOC (566/18) email: DAP1@soc.soton.ac.uk. Marine Bacteria.

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OC211(OA211) Phytoplankton & Primary Production

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  1. OC211(OA211) Phytoplankton & Primary Production LECTURE 10 Week 11 (i)Marine Bacteria and the Microbial Loop (ii)Export Production (iii)Modelling Phytoplankton Production Dr Purdie SOC (566/18) email: DAP1@soc.soton.ac.uk

  2. Marine Bacteria Now widely accepted that about 50% of marine primary production is processed by bacteria each day. • Large population of bacteria in sea water • The abundant free bacteria are highly active and productive • High growth efficiency • Are they grazed and by what? • How important are bacteria to the pelagic food chain? Comparison of Biomass and Surface area Organism Group Biomass (% total) Surface Area (%total) (mg dry wt L-1) (cm2 L-1) bacteria 26 (4.6) 25 (69)* protozoa 9 (1.7) 0.3 (0.7) algae (phytoplankton) 310 (56)* 11 (30)* copepods: 43 (8) 0.18 (0.48) larvaceans 1.2 (0.2) 0.01 (0.03) gastrapod veligers 161 (29)* 0.13 (0.36) Zooplankton Total 206 (37)* 0.32 (0.9) Total 551 35.9

  3. Marine Bacteria CO2 30 100 70 • High growth efficiency (70%) • Low growth efficiency (20%) Bacterial Carbon DOC 80 CO2 100 20 Bacterial Carbon DOC

  4. Marine Bacteria • Are bacteria grazed and by what? • Bacteria are ingested by a great diversity of predators but due to small size of the prey most bacteriovores are small single celled protozoans typically <5mm nanoflagellates or ciliates. • Some larger flagellates, small ciliates and some specialised larger predators can also ingest bacteria prey. • Some gelatinous zooplankton like larvaceans, pelagic tunicates, salps and appendicularians use a mucus net to capture bacteria cells sieved from suspension. • Adult copepods cannot ingest bacteria (unless attached to particles) the small flagellates are ingested by larger ciliates or nauplii and these in turn ingested by copepods- the herbivorous zooplankton better now to term them “omnivores” • Bacteria are therefore linked to the main grazing chain via the “Microbial Loop” and this serves to “recover” lost phytoplankton derived DOM potentially back to the main food web of the ocean. • How important are bacteria to the pelagic food chain?

  5. Coupling of Pelagic Grazing Food Chain and Microbial Loop

  6. Coupling of Pelagic Grazing Food Chain and Microbial Loop Fig 5.7 Lalli & Parsons

  7. Conceptual Microbial Food Web Figure 1 Microbial Loops by Landry in Encyclopedia of Ocean Science by Steele, Turekian & Thorpe

  8. The Microbial Loop- Trophic level versus size Figure from Azam et al 1983 Marine Ecology Progress Series 10: 257-263

  9. The Microbial Loop- link versus sink hypothesis The efficiency of the Microbial Loop at returning ‘lost’ DOM to the grazing food chain was a question addressed by an experiment in a mesocosm study. The link versus sink hypothesis (slides)Conclusion now is that the Microbial loop is inefficient at returning DOM to the main grazing chain.Bacterial growth efficiencies on naturally occurring organic substrates is about 20%.Each step in the protozoan food chain is about 30% efficient so only a small % of DOM carbon is transferred past the largest protozoan consumers.The microbial loop therefore is important in remineralization and nutrient cycling.

  10. Hypothetical Microbial Food WebGrowth efficiency of each micro-heterotroph group as indicated Fig from Ducklow; Bioscience 33:494-499

  11. Export Production • There is a continuos ‘rain’ of particles settling down through the ocean a downward flux of particulate organic material • This can be intercepted with sediment traps • This material is made up of: • ungrazed phtoplankton cells • faecal pellets from copepods and larger organisms • organic aggregates or marine snow • dead metazoan organisms and moults • This export of carbon and nitrogen from the surface ocean is known as the BIOLOGICAL PUMP removing CO2 from the atmosphere and exporting it in organic form into the deep ocean Fig 7 in Oceanic Marine Bacteria Ducklow & Carlson, Advances in Microbial Ecology Vol 12 Vertical profiles of particulate carbon flux into sediment traps during spring bloom in North Atlantic. Note decline in flux with depth indicating decomposition of sedimenting particles

  12. Export Production Fig from Lampitt Chapter in Summerhayes & Thorpe Time-series sediment trap

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