Modeling ecosystem disruptive algal blooms: positive feedback mechanisms

Abstract

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.

Positive feedbacks between bottom-up and top-down controls promote the formation and toxicity of ecosystem disruptive algal blooms: A modeling study

“…The model simulations presented here and previously (Sunda and Shertzer, 2012) reveal an additional positive feedback that promotes the formation of EDABs, linked to decreases in cellular N:C ratios and N recycling rates with N-limitation of algal growth rates. The N:C ratios in diatoms and K.b.…”

“…The system contained a zooplankton grazer present at a C-biomass concentration Z (mmol C l À1 ) that consumed each algal species at different C-specific rates. We quantified this conceptual system using a stoichiometric NPZ model, similar to that described by Sunda and Shertzer (2012 The NPZ model was governed by a system of differential equations:…”

“…where g max,i is the maximum grazing rate on algal species i, P T is the total algal C-biomass (mol C l À1 ; P T = P j P j ), P t is the grazing threshold (detailed below), and K z is the half-saturation constant for grazing, which for simplicity, is assumed to be the same for all algal species (Sunda and Shertzer, 2012).…”

“…They further noted that toxin levels and other grazing defenses often increase with nutrient limitation of growth rate, and that this would further promote positive feedback interactions during bloom formation and would increase bloom toxicity. In a previous paper (Sunda and Shertzer, 2012), we used an NPZ model to examine Sunda et al's (2006) first positive feedback hypothesis in which the EDAB species has robust grazing defenses, which do not vary with nutrient limitation of growth rate. In the present model we examined Sunda et al's (2006) second feedback hypothesis, in which EDAB formation was promoted by grazing defenses related to cellular toxins where the cellular toxin levels (and grazing defenses) increased with nutrient limitation of growth rate.…”

“…Previously, Sunda and Shertzer (2012) developed a stoichiometric nutrient-phytoplankton-zooplankton (NPZ) model to test the positive feedback hypothesis of Sunda et al (2006) for the development of EDAB events where the EDAB species had high levels of the grazing defenses, which did not vary with nutrient limitation of growth rate. In the present model we examined the effect of an additional positive feedback associated with increases cellular toxins with nutrient limitation of growth rate of the EDAB species (Sunda et al, 2006).…”