Ecosystem structural transition between macro-cycle and microbial-loop dominance.
Ishii, Reiichiro1, Nakajima, Hisao2, Yachi, Shigeo1, Higashi, Masahiko3, 1 Research Institute for Humanity and Nature, Kyoto, Kyoto, Japan2 Ritsumiekan University, Kyoto, Kyoto, Japan3 Kyoto University, Kyoto, Kyoto, Japan
ABSTRACT- A recent hot topic in aquatic ecology is the case where the relationship between phytoplankton (primary producer) and bacteria (decomposer), a textbook example of mutualism mediating nutrient-cycling, may become competitive under limited nutrient supply and high light intensity. Here, using a theoretical model, we examine the conditions, mechanism and consequences of this transition. First, the model predicts a paradox: an improvement in light condition with nutrient condition unchanged may lead to a drastic decline of phytoplankton through its losing competition with bacteria. This is caused by the following positive feedback: The increase in the photosynthesis of phytoplankton causes its C-N imbalance, which should increase the EOC production of phytoplankton. This increase in EOC will enhance the bacteria production and make them to consume the nutrient pool rather than contribute to it, causing a depletion of nutrient pool, which will in turn widen the C-N imbalance of phytoplankton, and the further increase the EOC production. Thus the model reveals the transition of the ecosystem from macro-cycle dominant structure to microbial-loop dominant structure. Further, the model shows that a decrease in light intensity with nutrient condition unchanged would cause the reverse transition: from microbial-loop dominant structure to macro-cycle dominant structure. Based on this result, we investigate the effect of seasonality in light intensity on blooming pattern through the structural transition mechanism. We show that, with a sufficient seasonal light oscillation, a system might experience the structural transition so that phytoplankton population exhibits abrupt growth or depletion in a year even with nutrient loading unchanged. This implies the possibility to reduce the magnitude of eutrophication in a water system not by cutting the total amount of nutrient loading but by controlling the timing of the loading to make the microbial-loop more dominant.
Key words: structure transition, C-N balance, microbial loop
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