Are plankton dynamics more stable than we thought ?

Posted on December 9, 2010

How do fish affect the dynamics of plankton in lakes?  Theory predicts that predator-prey systems in lakes should alternate abruptly between two distinct states: one where fish control zooplankton, which leads to high  phytoplankton abundance as grazing pressure is relaxed; and another where zooplankton are not controlled by fish and are free to crop phytoplankton to low levels.  One of the assumptions underpinning the dual-state theory is that fish are size-selective visual predators of zooplankton.  However, in practice, many fish also rely on unselective filter-feeding to exploit both zooplankton and phytoplankton, and the potential implications of this feeding mode for predator-prey dynamics have been largely ignored.  Including filter-feeding in a model of a typical lake food chain (plant plankton, water fleas, tilapia) had the effect of dampening population cycles, increasing the persistence of zooplankton, and preventing catastrophic shifts in dynamics.  Omnivorous filter-feeding allows more prey switching, which stops zooplankton being overexploited by predatory fish, and this means that the amount of fish that the system can support is increased without causing zooplankton collapse.  The simulation results indicate that even low levels of omnivory can stabilise plankton dynamics.  On the applied front, they also suggest that phytoplankton biomass is best controlled by keeping stocks of filter-feeding fish at either very low or very high levels. 

Reference:  Attayde, J.L., van Nes, E.H., Araujo, A.I.L., Corso, G. & Scheffer, M.  2010.  Omnivory by planktivores stabilizes plankton dynamics, but may either promote or reduce algal biomass.  Ecosystems 13, 410–420.

Posted in: fish, food webs, plankton