1 June 2007.
Working Group 1: Heterotroph working group

The format of this working group is a bit different from the other working groups, because during the 4th an 5th Green Ocean Workshops informal working groups have already been held. The present working group therefore is more a progress meeting, evaluating which of the goals have been achieved since the last meeting and what still needs to be done. See also Minutes zooplankton meeting, Norwich & Cambridge 27-30 November 2006.

Summary: Data synthesis of zooplankton rates/parameters and concentrations/evaluation data is well under way. The work on bacteria/monera has only just started. Working group output: When the macro- meso- and micro- zooplankton (and possibly also nanozooplankton and monera) have been parameterised and evaluated, we will write a synthesis paper on all three (five) size classes. So far, contributing authors will be Erik, Richard, Christine, Roisin, Sevrine, and Corinne.

• Micro:
• Particulate egestion. Richard: there is (almost) no data, but there are a few papers on assimilation efficiency. Louis: AE does not include DOC exudation. Erik:Therefore 1-AE-DOC egestion might be used as a measure of particulate egestion, though not a perfect one.
• DOC: Richard since the synthesis of Nagata (2000) there has been some work on DOC production.
• Respiration. Richard: there are about 5 papers on respiration, including by Verity. This will still need to be converted to basal respiration.
• Mortality. Mortality other than grazing mortality is probably low. Sevrine: there is some laboratory work on starving mortality. Whether that is important will be influenced by the threshold food concentration for ingestion. Sevrine will derive this threshold from her ingestion database. This could be then compared to food concentrations in the ocean to see if this is potentially important and needs to be included in the model. In addition, micro grazing on micro, especially ciliates on flagellates needs to be taken into account. The present strategy of using a high basal respiration rate to account for this would underestimate the production of DOC.
• Q10 ingestion. Richards in situ data have a lower Q10 (2.2) than Sevrines laboratory data (2.9). Christine thinks this could be because laboratory experiments tend to use 1 predator and 1 prey, in which shifts of optimal prey size with temperature could obscure the actual temperature dependence. Such a shift has been documented. During the plenary discussion Corinne remarks that the model should be parameterised with the actual behaviour/physiology and that we should check that a different apparent Q10 is found as an emergent property under the appropriate conditions. This point has wider implications, as mesozooplankton have been parameterised with field data, and it should be checked whether their apparent Q10 in the model is different from the parameterised one.
• Meso:
• We have only discussed food selection, which is presently only qualitatively based on observations. Louis remarks that the well studied quality of diatoms as copepod food have shown some low quality diatom species, but this effect is far from universal and no special treatment of diatoms appears to be justified. There seems to be a consistent preference for zooplankton prey, which seems logical from their similar biochemical composition to the predator. Sevrine:In addition, different zoo seem to have different preferred predator:prey size ratios, roughly 1 for (dino)flagellates, ~3 for ciliates and ~8 for copepods.
• Macro:
• Though the macro parameterisation is not yet complete, Roisin has found at least one paper for all parameters except food selection.
• Bacteria:
• Erik presents the first version of the equations for explicit bacteria in PlankTOM10. Richard criticizes this formulation as in contradiction with the available evidence of relative preferences of bacteria for DOM relative to POM. Erik explains that with the present fixed C:N:P ratios and a single DOC pool it is not possible to take into account the changes in lability of DOC and that it is therefore not possible at the same time to use this bioenenergenitical constraint and achieve the known higher concentration of DOM than of POM. Helle and Christine clarify that POM seems to have a fairly constant turnover rate. It appears that a correct model must either be simpler and directly specify turnover rates of POM and DOM (as a function of temperature) without modelling bacterial biomass (as in PlankTOM5), or more complicated and model changes in DOM lability (e.g. due to changes in DOC:DON).

• Richard has made a database of microzooplankton biomass with about 5000 points.
• Roisin has received krill and salp databases for the Southern Ocean from Angus Atkinson. Potentially there are data for appendicularians with Gorski (Villefranche), Deibel (USA), the Japanese and Cunya. Michelle: There may be data on Ctenophores?
• Richard has a database with about 10000 points of bacterial biomass.

• Louis: Macro PFT is included for its importance in big fecal pellets and export, but this mainly comes from the crustaceans/euphausiids and (semi)gelatinous filter feeders. Erik: should the other macro phyla be functionally included with the meso PFT?
• Richard: micro and meso are similar in that they mainly lead to remineralisation/shallow export of organic material; should they be aggregated into a single PFT? Dieter and Corinne caution that the present strategy hasn't been evaluated yet, and that while they are open to the need for simplification if particular complexity is dysfunctional, they cite the long development times of DGVMs that Colin mentions as a need for patience and an appropriate evaluation strategy.