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 environments are: -1. Small algae are better competitors for light and nutrients than larger algae. -2. The potentially high reproduction rate of their predators makes the smaller algae more susceptible to grazing control by microzooplankton than larger algae. -3. Larger algae escape from microzooplankton grazing, due to their size, but experience losses through sedimentation. -4. Microzooplankton is an important food source for mesozooplankton in oligotrophic areas. Basically the maintenance system (also known as the retention or regenerative system), which consists of pico- and nano-algae, microzooplankton, carnivorous mesozooplankton and in which bacteria and heterotrophic nanoflagellates act as decomposers and mineralisers, is typical in consuming most fixed energy within the photic zone. The overflow response, being an extension of the maintenance system with larger micro- and macro-algae and herbi-/omnivorous copepods, shows extended export of fixed energy out of the photic zone through sedimentation. The maintenance system is present in all marine environments, whether light- and/or nutrient-controlled, 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 agreement 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 dominance of large diatoms during the early spring bloom in the Marsdiep area (The Netherlands) was not necessarily caused by their growth characteristics, but was due to their escape from size-selective microzooplankton grazing.