Plankton and nutrient dynamics in marine waters

Biological oceanographers generally distinguish between two contrasting trophic pathways in the pelagic environment, i.e. the herbivorous and the microbial food webs. The former goes from large phytoplankton and zooplankton to fish, whereas the latter comprises small eukaryotic algae and cyanobacteria as well as heterotrophic bacteria and protozoa. The present paper describes a continuum of trophic pathways, between systems dominated by the herbivorous food web and those dominated by the microbial loop (i.e. almost closed system of heterotrophic bacteria and zooflagel-late grazers, the latter releasing dissolved organic matter used as substrate by the bacteria). It is proposed that the continuum goes from the herbivorous web (or chain) to a “multivorous food web”, to the microbial web, and finally the microbial loop. Characteristics of the various pathways maybe summarized as a series of interconnected ratios. It is hypothesized that systems dominated by the herbivorous food web or the microbial loop are of transient nature and thus inherently unstable, whereas the multivorous and microbial food webs have higher stability and are thus longer lasting. This view is supported by a review of properties of several systems, that include herbivorous webs of the spring phytoplankton bloom and in upwelling areas, the multivorous web in the “high nutrient low chlorophyll” region of the North Pacific Ocean, a microbial web at a retreating ice edge off Antarctica, and the microbial loop in oligotrophic waters where the biomass of bacteria significantly exceeds that of phytoplankton.