Microbial loop, microbial food web and classical food chain: Their significance in pelagic marine ecosystems

The dependency of metabolic rates on cell size and the effect of temperature are demonstrated on a hypothetical size spectrum for a phytoplankton population in an oligotrophic open ocean region under steady-state conditions. The model spectrum illustrates the preponderance of small cells over large ones, especially their much higher weight-specific growth rates. It also shows that the allometric advantage of small cells, for instance in competing for limiting nutrients, is further enhanced by temperature increase. These basic facts are used to characterize the microbial and classical food web in relation to cold- and warm-water ecosystems and to new and regenerated production. It appears that the microbial food web is controlled by grazing and the classical food chain by nutrients. The former is present in all environments but is best adapted to oligotrophic warmwater ecosystems where regenerated production predominates. The microbial food web may be regarded as the original and normal form of life in the pelagic zone and the classical food web more as an exceptional feature depending on new production, which in turn is governed by the changing wind and temperature regime over the world ocean