The relative abundance of primary producers to consumers, and thus the shape of biomass pyramids are ecological properties characterizing given ecosystems. Although these are shaped mainly by top-down and bottom-up forces within the communities, few studies have examined external factors affecting the shape of pyramids. According to the theory of ecological stoichiometry, balance of light and nutrient inputs affects the elemental ratios of primary producers, which in turn affects on the growth rate of herbivores. Thus, it is hypothesized that the shape of biomass pyramids are affected by the balance of light and nutrient inputs through changes in not only production rate but also elemental ratios of producers.
To test the hypothesis, we performed a field experiment with manipulation of irradiance (33–100%) to P limited ponds with 225 m2 and 1.3 m deep, and monitored abundances of algae (chl-a) and zooplankton with measurements of primary production rate and abundance of planktivorous fishes for 85 days in summer 2016. Then, based on a Lotka-Volterra model of two trophic levels, we evaluated effects of the primary production rate, fish abundance (a surrogate of the zooplankton mortality rate) and C:P raito of the algae on the relative abundance of algae to zooplankton.