Enhancement of poorly edible phytoplankton by allochthonous dissolved organic material: a modelling study

The effect of different input rates of dissolved inorganic (DIN) and organic (DON) nitrogen on a microbial food web was studied with the help of a mathematical model. Three species of phytoplankton were modelled, with trade-off characteristics for maximum per capita growth rate, half-saturation constant and edibility. Bacteria used only DON, while 2 different types of microzooplankton grazed on the bacteria and phytoplankton species. DON stimulated bacterial growth and phytoplankton growth via mineralization of nitrogen by the bacteria. At moderate and high DIN and/or DON input rates, the poorly edible phytoplankton species became the dominant phototroph. However, the biomass of the poorly edible phytoplankton species, and the proportional contribution of this species to total phytoplankton biomass, was more correlated with changes in DON input rates than DIN input rates. Furthermore, the abundance of the poorly edible phytoplankton was most sensitive to changes in the maximum grazing rate of the ciliate. The enhanced mass flow through the heterotrophic members of the food web enhanced the grazing pressure on all members of the microbial community, including phytoplankton, and caused the shift towards a grazing-resistant phytoplankton community. We formulated the testable hypothesis that input of allochthonous dissolved organic nutrients may contribute to blooms of poorly edible algae through food web interactions.