Harmful Algal Blooms (HABs) in the Chesapeake Bay, which threaten human and ecosystem health, are more frequent and severe than decades ago. We are developing a new model that incorporates complex nutrient interactions to improve long-term, seasonal, and scenario predictions for different HAB species, especially by assuming fixed nutrient rations (i.e., classic Redfield ratios) and complex nutrient strategies of different HAB species.
Why We Care
The over-enrichment of Chesapeake Bay with nutrients is well recognized and documented. Among the effects of this over-enrichment is an increasing proliferation of HABs, ranging from toxic dinoflagellates (e.g., Karlodinium veneficum) that cause massive fish kills, to ecosystem disruptive high-biomass dinoflagellates (e.g., Prorocentrum minimum) that prevent growth of early stages of shellfish, as well as toxic and high-biomass cyanobacterial blooms that can threaten human health. HABs in Chesapeake Bay are now more frequent, and of significantly higher densities, than several decades ago.
What We Are Doing
Modeling nutrient relationships (stoichiometry) for specific HABs, including those associated with eutrophication, has almost universally been problematic and complex. Traditional biomass-based models (often single-nutrient, nitrogen [N] or carbon [C]) are relatively simple, and have few variables as they assume fixed proportions (usually classic Redfield ratios). Yet, many HABs grow for extended periods when nutrients are not in Redfield proportion. Traditional biomass-based models also typically operate using “Monod” kinetics that relate growth to the external nutrient concentration.
Monod kinetics may not be the most appropriate for HAB modeling because the nutritional modes of many HABs are complex; most HABs are mixotrophic (nutrition from both photosynthesis and eating plankton). These simple models are unsuitable for descriptions of algal growth under variable nutrient conditions. Thus, there is a great need for multiple nutrient models that incorporate the complexities of HAB physiology (e.g., nutrient preferences; life history, including day-night regulation; nutrient storage; mixotrophy; etc.). Multiple “currency” models (C vs N vs phosphorus [P] vs silicon [Si], for example) are particularly useful in physiological descriptions, as stoichiometry is a reflection of the actual nutrient status of the cell.
A coupled hydrodynamic-biogeochemical model (ROMS-RCA) will be linked to the new HAB model for P. minimum and K. veneficum. The RCA biogeochemical/water-quality model includes three phytoplankton functional groups (cyanobacteria, diatoms, and flagellates), separate cycling of N, P, Si and C, and is coupled to a sediment diagenesis model.
Our objectives are to:
- Develop a 3-D mechanistic model to simulate the dynamics of minimum and K. veneficum and couple this model with existing hydrodynamic and water quality models;
- Apply the new coupled modeling system in the development of spring–summer forecasts of three phytoplankton functional groups (cyanobacteria, diatoms, and flagellates) based on winter–spring nutrient loading conditions and spring-bloom development;
- Develop various scenarios to examine how the phytoplankton functional groups, especially the two targeted HAB groups, respond to nutrient management strategies, including identified total maximum daily loads (TMDLs) of N and P, and climate change, based on climate projections and on management-guided nutrient reduction strategies;
- Conduct physiological experiments on phototrophy and mixotrophy of minimum and K. veneficum in order to obtain the necessary parameters to quantify cell quota, nutrient acquisition and growth for multiple nutrients, and under a range of temperature and light conditions; and
- Work closely with state and local federal managers in scenario development and convey model output and predictions to scientists and the Chesapeake Bay management community through direct interactions, websites, conferences, meetings and workshops, and written articles.
This project is led by Dr. Ming Li and Dr. Patricia M. Glibert, both of the University of Maryland Center for Environmental Science, Horn Point Laboratory. The project is funded through the NCCOS Ecology and Oceanography of Harmful Algal Bloom (ECOHAB) Program.