A good understanding of the processes that govern the transfer of energy and matter through the planktonic food web is necessary to predict effects of human impacts such as coastal eutrophication and fisheries on the global ocean. We theoretically explored the consequences of nutrient enrichment and altered fish predation on species persistence and energy transfer in a planktonic food web module that is found throughout the world’s oceans, which we term ‘pentagon module’. It consists of five components: a limiting nutrient, two competing algae (small nanophytoplankton and larger algae represented by diatoms), a ciliate that preys on nanophytoplankton, and a crustacean top consumer (copepod) that preys on ciliates and diatoms. More generally, there are two competing food chains of different length along which energy and nutrients flow from producers to copepods. We describe the dynamics of the pentagon module with Lotka-Volterra-type equations assuming a closed system. Fertilization was modeled by varying the amount of nutrient in the system and altered fish predation was modeled by varying copepod mortality. We investigated three variants of the model: variant 1 assumes linear functional responses; variant 2 assumes type 2 functional responses; variant 3 assumes a type 3 functional response in the copepod and type 2 functional responses in all other species.
Results/Conclusions
The analytical solution of variant 1 reveals non-intuitive responses. With nutrient enrichment all members of the even-linked chain (including nanophytoplankton) increase, whereas the intermediate species of the odd-linked chain (diatoms) decreases and eventually goes extinct. Responses to increased copepod mortality show the reverse pattern. Thus, nutrient enrichment increases energy flow along the longer, less efficient 4-linked food chain, whereas increased copepod mortality has the opposite effect. Analyses of other food web modules consisting of linked food chains of odd and even length reveal that energy flow always increases with enrichment (and decreases with top consumer mortality) along the even-linked chain, regardless of whether the odd-linked chain is longer or shorter. The model with type 2 functional responses shows similar responses to nutrient enrichment and copepod mortality as the linear model but is unstable in much of the parameter space. Assuming a type 3 functional response in the copepod strongly stabilizes the system and significantly enlarges the coexistence area. In conclusion, asymmetries in the lengths of linked food chains have important consequences for energy flow, and the feeding behavior of the top consumer linking the chains has important consequences for stability and coexistence.
