Harmful algal blooms that disrupt and degrade coastal aquatic ecosystems are occurring with greater frequency. We developed a model that shows these events are not only the result of nutrient loading of near-shore waters, but also involve interactions among competing algal species, grazing on those species, and changes in nutrient cycling linked to algal grazing. Understanding these processes will help coastal officials better predict and manage the adverse effects of harmful algal blooms.
Why We Care
Harmful algal blooms often are accompanied by severe impacts to coastal resources, local economies, and public health. Fishery closures, reductions in tourism, and medical treatments and advisories result in the loss of millions of dollars per outbreak. In the United States, the frequency of harmful algal blooms is increasing, and their geographic distribution now includes all coastal states.
What We Are Doing
Many factors—such as nutrient availability, herbivore grazing, and nutrient regeneration—have been proposed to separately influence ecosystem disruptive algal bloom dynamics, but interactions among these factors have less often been considered. We created a mathematical model that can be used to examine these interactions and predict how ecosystem disruptive algae respond to changes in nutrient inputs and other environmental factors.
The project team included partners from NOAA/NCCOS and NOAA’s National Marine Fisheries Service.
What We Found and Benefits of our Work
Ecosystem disruptive algal blooms often are caused by toxic or unpalatable algal species that tend to decrease herbivore grazing rates. This decrease in grazing leads to a proliferation of the ecosystem disruptive algae and a reduction in nutrient availability, due to a reduction in grazer-mediated recycling of nutrients. Since many species of ecosystem disruptive algae are well-adapted to low nutrient conditions and exhibit increased toxin production and toxicity under such stress, positive feedbacks are established that can greatly increase bloom development and toxicity.
Our model is helping scientists and coastal managers better understand the factors that govern the development and toxicity of harmful algae and plan accordingly. For example, the model has been used to predict how toxicity blooms in the Gulf of Mexico vary with nutrient availability, and recommendations are being provided to managers in the region detailing how this information can be used to improve early warning and mitigation strategies. Specifically, the model indicates that a pre-bloom of diatoms is a critical triggering factor that modulates nutrient supply rates such that toxic Karenia blooms are favored. Thus, an intense diatom bloom may foreshadow a harmful algal bloom two to three months later.