Microzooplankton regulation of particulate organic matter elemental composition David Talmy, Adam Martiny, Anna Hickman, Mick Follows Ocean Sciences, New Orleans, 2015

Elemental composition of particulate organic matter (e.g. POC:PON) controls biological pump efficiency

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

Questions What causes large scale variation in POC:PON ratio? Can phytoplankton alone explain large scale patterns in POC:PON Do interactions between producers and consumers influence mean and regional variation in C:N?

Average POC:PON close to Redfield (Martiny et al., 2013) Nutrient replete Phytoplankton grown in the lab (Geider and La Roche, 2002) density

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., 2005; Elirifi and Turpin, 1985)

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? NO 3 - Small phyto Large phyto Particulate detritus Dissolved detritus

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 partiulates in the western North Atlantic (Martiny et al., 2013)

Missing piece of the puzzle?

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

NO 3 - Small phyto Large phyto Particulat e detritus Dissolved detritus Micro- zoo Ecosystem model with a microzooplankton size class explicit: What does this tell us about microzooplankton control on POC:PON?

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

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

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

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

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