This project will evaluate whether harmful algal bloom (HAB) species may have a competitive advantage over other phytoplankton in response to coastal acidification and climate changes occurring in northeast US waters. Some phytoplankton move up and down in the water column – cells travel down to absorb nutrients at depth and move up to absorb light closer to the surface. Thus, they are well adapted to increasingly stratified coastal habitats. During these vertical migrations, the cells encounter carbonate system gradients in the water column that may influence or be exploited by different species in different ways. This project is the first study of the behavior and response of co-occurring HAB and non-HAB phytoplankton over a broad range of environmental conditions. This information will be used to improve HAB model predictions, early warnings, and forecasts.

Why We Care
Many HAB species are dinoflagellates that vertically migrate to absorb nutrients at depth and light closer to the surface. These vertical migrations cause the organisms to encounter carbonate system gradients in the water column. Little is known regarding how these gradients influence, or are exploited by, the dinoflagellates’ vertical movement. This project will address current gaps in understanding the interactions among acidification, vertical migration behavior, and nutrient physiology in the development of toxin-producing HABs. Study results will provide a mechanistic understanding of how vertically migrating HAB species respond over a broad range of gradient conditions. This information will be used to improve HAB model predictions, early warnings, and forecasts.

What We Are Doing
This project will advance understanding of the effects of acidification conditions on dinoflagellate vertical migrations and inform projections of how these HABs may respond under future acidification scenarios. Two frequently co-occurring dinoflagellate species are used as models to study the relationships among acidification, vertical migration, and nutrient physiology in the development of toxic dinoflagellate blooms. Alexandrium catenella is the causative species of paralytic shellfish poisoning (PSP) and perhaps the most widespread toxic HAB globally. The non-HAB species Heterocapsa triquetra is also globally distributed and occurs commonly in coastal waters.

The project team will characterize growth and toxicity of these organisms through a range of conditions to address gaps in understanding the relationship between dinoflagellate behavior and acidification.

1. How do tolerances to acidification parameters vary between these two co-occurring, vertically migrating dinoflagellate species?
2. How do the depth ranges of each species affect exposure to differing acidification conditions?
3. To what extent do the different species balance acidification-related stress and optimization of light exposure, carbon uptake, and nutrient assimilation under different water column conditions?

The project team will conduct laboratory culture, field, and modeling studies to investigate how the two species balance opposing needs for nutrient uptake and avoidance of acidification stress and examine how specific conditions may favor one or the other of these species. Experiments will measure nutrient uptake under different CO2 and nutrient conditions and study how the species may use these conditions as cues for vertical migration in vertically-stratified test chambers. Two sentinel field sites, in Massachusetts and Maine, regularly experience early season A. catenella blooms are often closed before other shellfish harvest sites each year due to the presence of paralytic shellfish toxins. Field research at these sites will leverage the ongoing deployment of a regional HAB sensor network (HAB Observing Network – New England), and will use a combination of previously developed sensor platforms to make continuous observations of HAB cell abundance and physiology alongside time series hydrographic profile measurements of acidification parameters (DIC, pH/pCO2) and nutrients using in situ chemical sensors. Research findings will be translated into an educational infographic and shared broadly through the Northeast Coastal Acidification Network (NECAN).

Impact/Benefits of our Work
This project is the first to directly compare the relative competitiveness of a toxin-producing HAB species with a non-HAB species in response to coastal acidification under a combination of realistic laboratory and actual field conditions. Project results will provide insight into whether co-occuring HABs and acidification will have synergistic negative effects (e.g., increased A. catenella bloom magnitude or toxicity) under future climate change conditions, and the knowledge gained will be readily incorporated into predictive models of A.catenella blooms and toxicity to inform coastal managers and shellfish industry partners in the northeast US.

Dr. Michael Brosnahan of Woods Hole Oceanographic Institution (WHOI) leads this project. Co-investigators are Dr. Zhaohui Aleck Wang (WHOI), Dr. Rubao Ji (WHOI), Dr. Collin Roesler (Bowdoin College), Dr. Charles Culbertson (US Geological Survey), and Dr. Jake Kritzer (Northeastern Regional Association of Coastal Ocean Observing Systems).

The project is funded through the NCCOS Competitive Research Program, in partnership with NOAA’s Ocean Acidification Program.