Foodweb functioning

In this section, we examine the general functioning of the pelagic food web. In order to do so, we group various key food-web processes under three broad food-web functions: photosynthetic production (net of autotrophic respiration), which is due to phytoplankon (i.e. microbes), microbial heterotrophic activity, which is due to bacteria and protozoa, and metazoan activity, which is due to organisms larger than microbes (i.e. metazoa). The resulting conceptual model is shown in Fig. 2.

Net photosynthetic production (Fig. 2A) refers here to oxygenic photosynthesis, which differs from anoxygenic photosynthesis that also takes place in the water column of oceans (e.g. Lami et al. 2007; the ecological significance of aerobic anoxygenic phototrophic bacteria is presently poorly understood). Oxygenic photosynthesis (e.g. Falkowski and Raven 2007) uses as inputs CO2, H2O, inorganic nutrients and the free energy of sunlight (i.e. photons). Its outputs are particulate organic matter (POM; here, phytoplankton biomass), DOM (exudates) and O2.

Microbial heterotrophic activity (Fig. 2B) refers to the heterotrophic microbial metabolism. The inputs are DOM (mostly used by bacteria), inorganic nutrients (for which there is competition between bacteria and phytoplankton), POM (bacteria and protozoa compete with metazoa) and O2. The outputs are POM (i.e. microbial biomass), DOM and inorganic nutrients (resulting from excretion, nutrient regeneration and viral lysis) and CO2.

Metazoan activity (Fig. 2C) refers to the metazoan metabolism. The inputs are POM (for which there is competition with bacteria and protozoa) and O2. The outputs are POM (i.e. metazoan biomass, faecal material and various biological products, e.g. appendicularian houses), DOM and inorganic nutrients (resulting from excretion) and CO2.

Figure 2 illustrates the interconnections among the three broad food-web functions. Outputs from one function are inputs into another. In steady state, net phytoplankton production and heterotrophic metabolism balance each other, i.e. A = B + C.

Fig. 2. Conceptual model that groups key food-web processes under three broad food-web functions: (A) photosynthetic production, (B) microbial heterotrophic activity, and (C) metazoan activity. Arrows: inputs and outputs for each function, and interconnections among functions. The three large rectangles specify the domains of the three food-web functions (A and C: dashed lines; B: shaded area).

Fig. 2. Conceptual model that groups key food-web processes under three broad food-web functions: (A) photosynthetic production, (B) microbial heterotrophic activity, and (C) metazoan activity. Arrows: inputs and outputs for each function, and interconnections among functions. The three large rectangles specify the domains of the three food-web functions (A and C: dashed lines; B: shaded area).

In the euphotic zone of oceans, nutrients are not only recycled, but they can also be imported and exported. Conversely, organic matter (POM and DOM) can be exported, laterally and downwards. Figure 3 schematises the pelagic food web in the euphotic-zone context. The photosynthetic production (PP) that is fuelled by imported nutrients is called "new" (PPnew), and the PP that is based on recycled nutrients is called "regenerated" (PPreg).

If we consider first Fig. 3A only (i.e. the simplest pelagic "food web"; it is not really a food web as it includes only phytoplankton), we see that the import of new nutrients and the export of organic matter balance each other. If we consider next Fig. 3A and B together (i.e. phytoplankton-microbial food web), we see that the import of new nutrients and the export of organic matter balance each other, and that microbial metabolism and nutrient recycling also balance each other. If we consider finally Fig. 3A-C together (i.e. complete pelagic food web), we see that the import of new nutrients and the export of organic matter balance each other, and that heterotrophic metabolism and nutrient recycling also balance each other. The roles of microbes in the balance between food-web processes (metabolism and nutrient recycling) and biogeochemical processes (import of new nutrients and export of organic matter) will be discussed in Section "Biogeochemical roles of microbes".

Army Sustainment Symbols
Fig. 3. Conceptual modem of the pelagic food web in the euphotic-zone context. Same as Fig. 2, with two additional, broad arrows representing the input of nutrients into, and the export of organic matter (POM and DOM) from the euphotic zone.

Our conceptual analysis of pelagic food webs has led us, so far, to the following conclusions concerning food-web functioning. (1) Microbes are key players in food webs because of both high metabolic rates and unique position in food webs. (2) They use almost all dissolved resources, and they have a significant share of particulate resources. (3) Because they monopolise a high share of resources, microbes are the main producers, remineralisers, and conduits of organic matter toward other food-web compartments.

We now briefly address the food-web roles of microbes with a quantitative approach. To do so, we used the food-web model of Legendre and Rivkin (2008) to which we added export flows (Fig. 4, left side). The model is in steady state, meaning that masses of model compartments are constant; its currency is carbon. The food-web model includes seven food-web compartments: particulate and dissolved PP (PHYTO-POC and PHYTO-DOC), bacteria (BACT), microzooplankton (^ZOO), mesozooplankton (MZOO), larger organisms (LARGE), and faecal pellets (DETR, detritus). The compartments are interlinked by food-web carbon flows, and there are two components of export, i.e. faecal pellets and other organic materials (phytodetritus, etc.) The growth efficiencies of BACT, ^ZOO and MZOO are temperature-dependent.

Table 2. Respiration (R) in the euphotic zone for three food-web compartments - i.e. BACT (Rb), ^ZOO (RMz) and metazoans (MZOO + LARGE, Rmet) - expressed as a fraction of heterotrophic community respiration (Rc). The model was run for the microbial and the herbivorous food webs, i.e. the fraction of PHYTO-POC grazed by ^ZOO is larger in the microbial food web (i.e. 0.90) than in the herbivorous food web (i.e. 0.25). Values calculated at 15°C.

Food web (15°C)

Rb/Rc

VRc

Rmet/Rc

Microbial

0.56

0.18

0.27

Herbivorous

0.48

0.07

0.45

Table 2 gives the results of a modelling exercise showing that R of microbes (BACT + ^ZOO) is larger than R of metazoa (MZOO + LARGE), even when MZOO are the dominant grazers of PHYTO-POC (i.e. herbivorous food web). This result is consistent with the conclusion of our conceptual analysis that microbes are the main remineralisers of organic matter in the euphotic zone.

Table 2 gives the results of a modelling exercise showing that R of microbes (BACT + ^ZOO) is larger than R of metazoa (MZOO + LARGE), even when MZOO are the dominant grazers of PHYTO-POC (i.e. herbivorous food web). This result is consistent with the conclusion of our conceptual analysis that microbes are the main remineralisers of organic matter in the euphotic zone.

Getting Started With Solar

Getting Started With Solar

Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.

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