Impacts of toxin production triggers on planktonic food web structure and environmental toxin loads: A theoretical approach.

Harmful algal blooms (HABs) of toxic phytoplankton species are a prevalent and expensive issue worldwide, with toxin production from blooms impacting aquaculture, human health, and higher trophic levels. Despite widespread impacts, it remains uncertain why specific phytoplankton species produce toxins and how toxin production may be induced ('triggered') by ecological conditions. In this study, we develop an idealized theoretical nutrient-phytoplankton-zooplankton model to describe food web dynamics in response to toxin production by phytoplankton. We use our model to analyze three hypothetical cases, motivated by laboratory and field observations: cellular toxicity is constant and unrelated to food web dynamics (Constant Toxins), cellular toxicity is negatively related to toxic phytoplankton abundance (Phyto-triggered), and cellular toxicity is positively related to zooplankton biomass (Zoo-triggered). In all cases, grazing rates are inversely related to cellular toxicity. We examine how the maximum grazing rate of toxic phytoplankton, nutrient supply, and other model parameters impact toxic phytoplankton dominance, zooplankton:phytoplankton ratios, and total toxin load. Our model demonstrates that toxicity triggers can shift the competitive balance between toxic and non-toxic species, with outcomes dependent on total nutrient supply to the system. Total toxin loads were highest under the Constant Toxins and Zoo-triggered cases at high nutrient supply. This idealized model demonstrates how the specific trigger assumed for defense toxicity acts as an ecological feedback that determines food web structure. Our findings highlight the need to better understand the ecological drivers of toxin production in marine food webs before we can predict toxic algal events in natural ecosystems.