Stable model structures for representing biogeochemical diversity and size spectra in plankton communities

Abstract

There is a fundamental duality in the way we view planktonic communities. On one hand, we see these communities as composed of a diverse mixture of taxonomically and biogeochemically distinct species; pelagic ecosystem models capable of predicting the ocean's role in the global carbon cycle will need to resolve these multiple plankton taxa. However, we have also come to appreciate that superimposed on this diversity are regular patterns of size structure, where plots of abundance within size classes typically show a power-law dependence on size. A truly complete model descrip- tion of planktonic community structure must reflect both aspects of this duality. Here I examine two classes of ecosystem models that are capable of representing this duality. The first structure is based on multiple (n) couplets of phytoplankton (P) and zooplankton (Z) (an 'n(PZ)' model). Within certain regions of parameter space, this model structure can produce anomalous oscillatory behaviors, the mathematical origin of which is explored in detail. I then examine an alternative food web structure in which total herbivore abundance is represented by a single state variable, G, and where grazing pressure is distributed among taxa in proportion to their abundances. This '(nP)G' system is stabilized by the redistribution of grazing intensity among taxa in response to changes in their densities. This 'distributed grazing' model also naturally produces size spectra of plankton abundance; this last observation argues that the model with distributed grazing can help in uncovering and repre- senting the mechanistic basis for the genesis and maintenance of planktonic size spectra of phyto- plankton and bacteria.