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.