SIZE-DIFFERENTIAL CONTROL OF PHYTOPLANKTON AND THE STRUCTURE OF PLANKTON COMMUNITIES

We provide evidence and discuss the possibility that the main factors determining food web structure in oligotrophic and eutrophic marine en vironments are: -1. Small algae are better competitors for light and n utrients than larger algae. -2. The potentially high reproduction rate of their predators makes the smaller algae more susceptible to grazin g control by microzooplankton than larger algae. -3. Larger algae esca pe from microzooplankton grazing, due to their size, but experience lo sses through sedimentation. -4. Microzooplankton is an important food source for mesozooplankton in oligotrophic areas. Basically the mainte nance system (also known as the retention or regenerative system), whi ch consists of pico- and nano-algae, microzooplankton, carnivorous mes ozooplankton and in which bacteria and heterotrophic nanoflagellates a ct as decomposers and mineralisers, is typical in consuming most fixed energy within the photic zone. The overflow response, being an extens ion of the maintenance system with larger micro- and macro-algae and h erbi-/omnivorous copepods, shows extended export of fixed energy out o f the photic zone through sedimentation. The maintenance system is pre sent in all marine environments, whether light- and/or nutrient-contro lled, well-mixed or stratified, and oligotrophic or eutrophic. Typical overflow response is observed during an upward shift of the import of the controlling factor, which is either light or nutrients. In agreem ent with the hypothesis of size-differential control it was found that light-controlled phyto-plankton consisted mainly of small flagellates (<8 mu m) during winter in the well-mixed central North Sea. The domi nance of large diatoms during the early spring bloom in the Marsdiep a rea (The Netherlands) was not necessarily caused by their growth chara cteristics, but was due to their escape from size-selective microzoopl ankton grazing.