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

Why do we care about elemental composition?

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

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

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

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

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)

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

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

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?

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

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)

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

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

Small phytoplankton dominate biomass in the oligotrophic gyres

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

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

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

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

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

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

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

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?

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

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

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

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

Detritus is an amalgam of zooplankton and phytoplankton C:N

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

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

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

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

Cell models with appropriate fluxes link metabolism with large scale processes

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

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.

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

Katechakis et al., 2006

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

Heterotrophs usually show more stoichiometric homeostasis than autotrophs