Version of the model

PHYTOPLANKTON

R f

MICROBIAL HUB

R i V C

Microbial-hub summary respiration flows

Fig. 4. Left side: food-web model of Legendre and Rivkin (2008) with added export flows. The seven food-web compartments are: particulate and dissolved PP (PHYTO-POC and PHYTO-DOC), bacteria (BACT), microzooplankton (^ZOO, <200 urn), mesozooplankton (MZOO, 0.22.0 mm), larger organisms (LARGE, >2 mm), and faecal pellets (DETR, detritus). The two components of export are: faecal pellets, and other organic materials (phytodetritus, etc.) The arrows represent carbon flows into and out of compartments: primary production (PP, particulate, PPp, and dissolved, PPD); heterotrophic detritus consumption (D), excretion (E), egestion (F), production (P) and respiration (R). Right side: application of the microbial-hub approach to the model: PHYTO-POC and PHYTO-DOC are merged into PHYTO, uZOO and BACT make up the microbial hub (HUB), and MZOO are combined with LARGE into the metazoan compartment (METAZ). The arrows represent summary R flows. The HUB consumes PHYTO, receives carbon from METAZ, and redirects the carbon it ingests towards CO2 (respiration) and METAZ. Solid arrows: forward flows; dashed arrows: backward flows; double-headed arrow: net flow possible in both directions. Shaded rectangles: microbial-loop flows, and summary R flows resulting from the microbial-hub approach. Details are given in the text.

Fig. 5. Responses of the five summary R flows (microbial-hub approach, Fig. 4) to temperature for the (A) herbivorous and (B) microbial pelagic food webs. All values are expressed as a percentage of PPT (i.e. maximum potential RC). There are two temperature zones identified: arctic (i.e. -2°C to 8°C) and temperate (i.e. 14-24°C); the minimum temperature for temperate conditions (i.e. 14°C) corresponds to the transition in the climatology of heterotrophic bacteria and the photosynthetic cyanobacterium Synechococcus reported by Li (1998; i.e. direct relationship of annual average abundances to annual average temperature below 14°C, and no relationship above).

Fig. 5. Responses of the five summary R flows (microbial-hub approach, Fig. 4) to temperature for the (A) herbivorous and (B) microbial pelagic food webs. All values are expressed as a percentage of PPT (i.e. maximum potential RC). There are two temperature zones identified: arctic (i.e. -2°C to 8°C) and temperate (i.e. 14-24°C); the minimum temperature for temperate conditions (i.e. 14°C) corresponds to the transition in the climatology of heterotrophic bacteria and the photosynthetic cyanobacterium Synechococcus reported by Li (1998; i.e. direct relationship of annual average abundances to annual average temperature below 14°C, and no relationship above).

Figure 5 shows the values of the five summary R flows for the microbial and the herbivorous pelagic food webs, under changing temperature in Arctic and temperate conditions. All summary flows are expressed as a percentage of PPT (which provides the value of maximum potential RC). For the herbivorous food web (Fig. 5A), the five summary R flows show changes with temperature: in both Arctic and temperate conditions, there are slight increases of the four microbial-hub and metazoan R flows with temperature. The fifth flow, Rmet(hub), is positive in Arctic conditions, meaning that the microbial hub channels more carbon towards Rmet than metazoa channel carbon toward Rhub, and the reverse takes place in temperate conditions. For the microbial food web (Fig. 5B), there are strong increases of microbial-hub R flows with increasing temperature in both Arctic and temperate conditions, whereas there is little change in the other summary R flows. Figure 5 shows that some responses to temperature increase are similar in Arctic and temperate conditions, and others are different. Similar responses to temperature increase in the two conditions are: for the herbivorous food web, small increases of metazoan and microbial-hub R flows, and for the microbial food web, strong increases of microbial-hub R flows. Different responses of the herbivorous food web to temperature increase in the two conditions are: in Arctic conditions that the microbial hub channels more carbon towards Rmet than metazoa channel carbon toward Rhub (decreases with increasing temperature), and the reverse occurs in temperate conditions, i.e. the microbial hub channels less carbon towards Rmet.

We conclude from Fig. 5 that temperature increase in Arctic and temperate waters will have different effects on respiration flows in the euphotic zone depending on the pelagic food web that is present. With the microbial food web present (e.g. non-bloom conditions, including under the ice cover), there will be strong increases of microbial-hub R flows. With the herbivorous food web present (e.g. phytoplankton blooms, in winter-spring in temperate waters and after the melting of the ice-cover in the Arctic), there will be slight increases of the four metazoan and microbial-hub R flows, and a decrease in the channelling of carbon by the microbial hub channels towards Rmet. Concerning the latter, under Arctic conditions, the microbial hub will channel more carbon towards Rmet than metazoa channel carbon toward Rhub, whereas under temperate conditions, the reverse will take place, i.e. the microbial hub will channels less carbon towards Rmet than metazoa channel carbon toward Rhub. Overall, increasing temperature will not only increase respiration in the euphotic zone, but it will reinforce the role of microbes in community respiration. This is consistent with the conclusions of both our conceptual analysis above, and our modelling exercise on the effect of temperature on microbial and metazoan R (Table 3).

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