A trophic cascade triggers blooms of Asterionella formosa in subtropical eutrophic Lake Taihu, China

The importance of trophic interactions for determining the distributions, abundances, and taxonomic compositions of organisms in ecosystems has long been studied and debated. Here we test the effect of a trophic cascade on diatom (Asterionella formosa) blooms in subtropical, eutrophic Lake Taihu, China. A long-term data series (2005–2015) on planktivorous fish, zooplankton and diatoms has been analysed. Structural equation modelling is used to test our hypotheses about the influences of top-down and bottom-up forces on A. formosa. Since 2009, a spring bloom of A. formosa has occurred in the lake, coinciding with a reduction of Daphnia galeata biomass and of total cladoceran biomass following a marked increase in the stock of planktivorous fish (bighead carp and silver carp). Light, phosphorus (total and soluble reactive phosphate) and silica did not act as limiting factors for the growth of A. formosa. Structural equation modelling analysis showed that top-down effects of cladocerans on A. formosa biomass were more important than bottom-up effects (wind speed and soluble reactive phosphate). Although A. formosa was negatively correlated with total nitrogen, total with other inorganic nitrogen (NO 3 and NH 4) did not restrict the proliferation of diatom blooms after 2007. These results suggested that the substantial reduction of the Daphnia population caused a diatom bloom through a trophic cascade by planktivorous fish. Our study provides new insight into the effects of trophic interactions on diatom bloom formation in natural freshwater ecosystems.