Modeling ecosystem disruptive algal blooms: positive feedback mechanisms

Harmful blooms of algae that disrupt and degrade ecosystems (ecosystem disruptive algal blooms, EDABs) are occurring with increasing frequency with eutrophication and other adverse anthropogenic alterations of coastal systems. EDAB events have been hypothesized to be caused by positive feedback interactions involving differential growth of competing algal species, low grazing mortality rates on EDAB species, and resulting decreases in nutrient inputs from grazer-mediated nutrient cycling as the EDAB event progresses. Here we develop a nutrient-phytoplankton-zooplankton model to test the conceptual positive feedback hypothesis. In this model we compete the low-nutrient adapted brown tide EDAB species Aureoumbra lagunensis and 2 high-nutrient-adapted diatoms (Thalassiosira pseudonana and T. weissflogii) using published data for growth rate versus limiting nutrient (ammonium) concentration. The model results support the positive feedback hypothesis for EDAB formation, and verify that bloom formation requires low grazing rates on the EDAB species. The model predicts that because of the positive feedback, the harmful bloom should persist once formed. The model further shows that the likelihood and biomass intensity of an EDAB event is increased by greater residence time of water in a coastal system and that increased nutrient supply increases its severity. Our results demonstrate that EDAB events do not simply involve a direct stimulation of growth of harmful species by increased nutrients, but rather involve complex interactions among the growth of competing algal species, differential grazing on those species, and changes in nutrient cycling that are directly linked to algal grazing.