Environmental effects including climate change

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Until now, we have discussed food-web properties independently of environmental conditions. Do these conditions affect the metabolic rates and the food-web position of pelagic microbes? It is well known that the metabolic rates of organisms depend on both their characteristics (e.g. surface/volume ratio) and the environmental conditions (e.g. temperature, nutrients). In addition, because microbes access resources from both the bottom and the top of the food web, their position in the food web depends on food-web structure. The latter is largely controlled by environmental conditions. We have reported above the modelling result of Legendre and Rivkin (2008) that food-web complexity largely determines the role of microbes (see also Fig. 1). It follows that environmental conditions have strong effects on pelagic microbes.

The next point concerns the effects of environmental changes on pelagic microbes and biogeochemical fluxes. Such changes can be bottom-up or top-down. Examples of bottom-up environmental changes are the increasing ocean temperature and the decreasing nutrient concentrations caused by increased stratification. As a response to increasing water temperature, the metabolic rates of heterotrophic microbes will increase faster than those of both phytoplankton (Lopez-Urrutia et al. 2006) and metazoans (because of higher surface/volume ratio), causing more recycling among microbes and less transfer to other compartments. As a response to decreasing nutrients, bacteria may compete more efficiently for inorganic nutrients than phytoplankton because their affinity for nutrients is generally lower than that of phytoplankton (Kirchman 2000). Examples of top-down environmental changes are the drastic reduction in the numbers of top predators by commercial fisheries, and the introduction of invasive species. As a consequence, future marine food webs may be different than presently, e.g. the often dominant crustaceans may be replaced by other organisms, which would cause changes in the chemical composition of export (e.g. the C:N of marine seston is positively related to salp and appendicularian abundance, and negatively to calanoid copepods; Hassett et al. 1997). Also, future marine food webs may be less complex than presently, e.g. by elimination of top predators, causing less nutrient to be recycled toward microbes, and thus lower PPreg.

Table 3. Relative difference (%) between Rx/Rc at 25°C and 15°C for three food-web compartments. Same model as in Table 2.

Food web (25-15°C)

Rb/Rc

R^z/Rc

Rmet/Rc

Microbial

+6%

+22%

-27%

Herbivorous

+2%

+2%

-3%

In our conceptual analysis, above, we identified temperature as one of the most important bottom-up environmental forcings. Seawater (and air) temperatures are predicted to continue increasing as a result of increases in the concentrations of greenhouse gases in the atmosphere. Increased water temperature would cause the metabolic rates of microbes to increase faster than those of metazoans, causing a shift in recycling from metazoa toward microbes and less transfer of organic matter from microbes toward other compartments. We now examine this conclusion quantitatively, using the same food-web model as previously. Table 3 gives the results of a modelling exercise showing that with increasing temperature, R of microbes increases and R of metazoa decreases, even when MZOO are the dominant grazers of PHYTO-POC (herbivorous food web). This result is consistent with the conclusion of our conceptual analysis that with increasing temperature, there will be more recycling among microbes, and thus less transfer to other compartments.

We further explore the potential effects of changes in temperature and pelagic food-web type on food-web functioning and biogeochemical fluxes using the microbial hub approach proposed by Legendre and Rivkin (2008), where BACT and ^ZOO are grouped into the microbial hub, and larger organisms into a metazoan compartment. Because as stressed by Legendre and Rivkin (2008), only community respiration (RC) is always <PP, it follows that R is the only additive food-web property, and the fraction of PP that is respired by a heterotrophic compartment is the only metric that can be used for comparing compartments. As a consequence in the microbial-hub approach, we calculate five summary R flows among three broad food-web compartments: phytoplankton, the microbial hub and metazoa. The values of the summary R flows are computed from the modelled flows for the food-web model on left side of Fig. 4 (they could be computed from the flows of any model, or obtained by field observations). The five flows in the microbial-hub version of the model correspond to different components of community respiration (RC): metazoan R, Rmet; direct metazoan channelling of PPT toward RC, Rmet(PPT); microbial hub R, Rhub; direct microbial-hub channelling of PPT toward RC, Rhub (PPT); and difference between the transfer of carbon from the hub to metazoan R, and from metazoa to the hub R, Rmet(hub) (this flow can be positive or negative, hence the double arrow in Fig. 4).

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