1. Microzooplankton regulation of particulate organic matter elemental composition
David Talmy, Adam Martiny, Anna Hickman, Mick Follows
Bigelow, March 2016

2. Why do we care about elemental composition?

3. Animal growth efficiency is influenced by food C:N ratio
Cabbage butterfly caterpillar gross growth efficiency
Biomass C:N in food
Elser 2000
…Ecosystem function depends on elemental stoichiometry

4. Biological pump efficiency depends on C:N ratio of organic material
CO2
Surface layer
Sinking organic material
Deep ocean
Inorganic N

5. Alfred Redfield (1934,1963): C:N of organic matter = 6.625

6. Martiny et al., (2013): large data compilation shows scattered C:N ratios and regional variation

7. Average C:N value in particulates close to Redfield (Martiny et al
Phytoplankton grown in the lab have average C:N values close to Redfield (Geider and La Roche, 2002)

8. Phytoplankton C:N is different to that of bulk partiulates in the western North Atlantic (Martiny et al., 2013)

9. What are the main consumers of phytoplankton?
Phytoplankton losses due to microzooplankton grazing, from dilution experiments (Calbet and Landry, 2004)

10. Questions
What role do phytoplankton play regulating the mean C:N ratio of organic matter?
Does phytoplankton C:N ever diverge from Redfield? If so, what causes these diversions?
Do interactions between producers and consumers influence mean and regional variation in C:N?

11. Outline
Understanding phytoplankton regulation of surface ocean C:N ratio
Exploring microzooplankton C:N ratio in response to varying food sources
Coupling the phytoplankton and zooplankton models in global ocean simulations, and comparing with observations

12. Section 1: Can we understand C:N ratio using a simple model?
Cyanobacteria have narrow range of C:N (Lopez et al., in prep; Elirifi and Turpin, 1985)
Cyanobacteria have narrow range of C:N (Lopez et al., in prep; Elirifi and Turpin, 1985)

13. Large phyto
Ecosystem model with just two functional groups of phytoplankton: small and large.
Question: what is the large scale spatial variation in C:N predicted by this model?
NO3-
Particulate detritus
Small phyto
Dissolved detritus

14. Global ecosystem model: small phytoplankton and large phytoplankton have similar patterns in surface ocean C:N, but different ranges

15. Small phytoplankton dominate biomass in the oligotrophic gyres

16. Model with small and large phytoplankton still has C:N in the gyres close to 10; large variation with latitude

17. Our model of phytoplankton C:N ratio is consistently higher than the C:N of bulk particulates

18. Phytoplankton C:N is different to that of bulk particulates in the western North Atlantic (Martiny et al., 2013)

19. Section 2: How do the main grazers of phytoplankton respond to food with varying C:N ratio?

20. Time (hours)
Starvation experiment: C:N ratio declines over time, probably due to maintenance respiration

21. Food starved zooplankton respire excess C!
Can this model explain changes in Oxyrrhis marina C:N?

22. Section 3: Can we understand microzooplankton regulation of C:N ratio using a model?

23. NO3-
Small phyto
Large phyto
Particulate detritus
Dissolved detritus
Micro-zoo
Ecosystem model with a microzooplankton size class explicit: What does this tell us about microzooplankton control on POC:PON?

24. Highly idealized global ecosystem model – just three functional groups
Large phytoplankton constitute higher portion of carbon biomass in eutrophic environments (e.g. Ward et al. 2013)
Organic carbon distribution

25. Microzooplankton lower C:N in nutrient limited gyres (Talmy et al
Microzooplankton lower C:N in nutrient limited gyres (Talmy et al., GBC, 2016)

26. Martiny et al., 2013
Model with microzooplankton has particulate C:N ratio significantly lower than phytoplankton

27. Modeled phytoplankton and zooplankton C:N ratio as a function of mean light intensity in 12 distinct ocean regimes
Linear increase of phytoplankton C:N ratio with mean light intensity
Not reflected in the modeled aggregate of phytoplankton and zooplankton
…also not evident in observations

28. Detritus is an amalgam of zooplankton and phytoplankton C:N

29. Caveats and limitations
Iron limitation influences on nitrogen fixation and phytoplankton C:N ratio was not modeled
We did not explicitly model microbial remineralization of organic material
Phytoplankton and zooplankton models do not reflect the full diversity of metabolic strategies that influence C:N ratio of organic material

30. Multicellular zooplankton could have C:N ratios higher than Redfield
Forest et al., 2011

31. Conclusions
In the nutrient limited gyres, phytoplankton C:N may be consistently higher than microzooplankton C:N
Microzooplankton respiration may reduce the C:N ratio of bulk particulates, thereby regulating the C:N of bulk particulate material

32. Thank you – Questions?
coauthors: Adam Martiny, Anna Hickman, Chris Hill and Mick Follows
Acknowledgements to: Oliver Jahn, Steph Dutkiewicz

33. Cell models with appropriate fluxes link metabolism with large scale processes

34. Question: can we use the model to understand costs associated with calcification?
Virus
(e.g. EhV)

35. Can we understand the costs and benefits associated with calcification, e.g.:
Costs: energy carbon
Benefits: Reduced grazer metabolic rates (Harvey 2015), or resistance to viral infection.

36. Sensitivity of the model to different assumptions about the respiratory cost of calcification
Low cost of calcification
Moderate cost of calcification
High cost of calcification

37. Katechakis et al., 2006

38. Invation of a plankton community by a mixotroph does not significantly change C:N (Katechakis et al., 2006)

39. Heterotrophs usually show more stoichiometric homeostasis than autotrophs