Influence of Nutrients on Phytoplankton in Estuaries and the Coastal Ocean
Issues: In estuaries and the coastal ocean, increased nutrient inputs have been accompanied by an increase in the incidence and severity of ecosystem disruptive algal blooms (EDABs), which have caused much economic and ecological harm in the US and worldwide in recent decades.
Although increased nutrients ultimately fuel blooms, an analysis of bloom dynamics suggests that a major factor in the proliferation of EDAB species during blooms is the onset of nutrient limitation, brought on by an increase in nutrient demand by high phytoplankton biomass and a decrease in nutrient supply from nutrient cycling linked to decreased grazing by zooplankton. Because of their small size, many EDAB species such as the Texas and Northeast brown tide species (Aureococcus anophagefferens and Aureoumbra lagunensis) may be particularly well adapted to growth at low nutrient availability, and thus be able to out–grow competing species during blooms. The decrease in grazing during blooms often results from production of toxins which poison or deter grazers. Experimental evidence indicates that the toxin content of many EDAB species increases substantially under nutrient limitation. For example production of the neurotoxin domoic acid by species of the diatom genus Pseudo–nitzschia increases considerably under limitation by the macronutrients phosphate and silicic acid and limitation by the micronutrients iron and copper.
The availability of specific limiting nutrients determines the rate of carbon fixation by marine algae and the species composition of phytoplankton communities. These factors in turn regulate fisheries productivity. They also regulate the biological sequestration of CO2 (carbon dioxide) and thus determine the ability of the ocean to serve as a sink for atmospheric CO2. Iron, zinc, nitrogen, and phosphorus have been implicated as important regulators of marine algal productivity and species diversity, and thereby influence atmospheric CO2 concentrations and greenhouse warming. A major factor in the uptake and utilization of nutrients is the chemical composition of the nutrient pool and the ability of different phytoplankton species to access different chemical components of this pool; for example, labile dissolved inorganic zinc or iron species vs organic chelates of these metal nutrients.
Approach: The ability of small–sized EDAB species (e.g., Aureococcus or Aureoumbra) to grow well at low concentrations of major nutrients (ammonia or nitrate) will be studied in continuous laboratory cultures grown at constant temperature and a defined light dark cycle. This ability will be compared with that of competing non–EDAB species such as diatoms. The continuous culture methodology will allow us to vary the level of nutrient limitation and to determine relationships among nutrient–limited growth rate, nutrient content of the cells, and nutrient concentration in the culture medium. The same continuous culture approach will be used to determine the importance of nutrient limitation in promoting toxin production in the toxic diatom Pseudo–nitzschia multiseries and the Florida red tide species Karenia brevis. Toxin production will be measured by high performance liquid chromatography. The toxin content of the algal cells will be determined as a function of cellular nutrient content and nutrient–limited growth rate.
We are also conducting research on the ability of coastal phytoplankton communities to access different chemical forms of zinc. In this field study the uptake of zinc by the natural phytoplankton community in a eutrophic estuary (the Elizabeth River and Hampton Roads, Va.) will be determined and related to variations in the measured concentration of dissolved inorganic zinc species and associated concentrations of zinc bound to organic matter. This research is being conducted in collaboration with Dr. John Donat, Old Dominion University. It it partially funded by the Office of Naval Research.
Outcome for Users: This research will provide detailed information on the role of limitation by specific nutrients (e.g., nitrogen) in the development and toxicity of ecosystem disruptive algal blooms. This information will be used by Academic and NOAA scientists to construct integrated models for the development, toxicity, and persistence of ecosystem disruptive algal blooms. Such models will provide managers and the public with better predictive capabilities for the likelihood, severity, and persistence of EDAB events, and provide information needed to identify causative factors. The research will also provide modelers with a better understanding of the relationship between the chemical forms important micronutrient metals (e.g., zinc) and the uptake and utilization of these metals by marine phytoplankton. Such information will be essential for construction of predictive models for the effect of zinc on ocean carbon cycles and its effect on structure and function of marine phytoplankton communities.