Mixed messages on plankton

Posted on March 10, 2014

In traditional descriptions of aquatic food webs, plankton are typically divided into two functional groups: the phytoplankton, which are responsible for primary production, and the zooplankton, which are secondary producers and prey on phytoplankton. However, this functional division is now known to be grossly oversimplistic and can lead to misleading interpretations of food chain transfers, nutrient cycling and the effects of climate change. The error arises from the fact that a significant proportion, if not the majority, of single-celled, nucleated plankton in sunlit waters have characteristics of both of the traditional groups: they can photosynthesise as well as engulf solid particles of food, and therefore should be regarded as mixotrophs (“mixed-feeders”). As an added complication, the total production of mixotrophs isn’t a simple sum of primary and secondary production; instead, there are complex feedback interactions between the two processes. Because mixotrophy hasn’t received a lot of attention from researchers, more studies on this topic are required before the mechanics and implications of aquatic food webs can be fully understood.

Reference: Flynn, K.J. et al. 2013. Misuse of the phytoplankton–zooplankton dichotomy: the need to assign organisms as mixotrophs within plankton functional types. Journal of Plankton Research 35(1), 3–11. http://plankt.oxfordjournals.org/content/35/1/3.full.pdf+html