Issue: Ecosystem disruptive algal blooms (EDABs) are occurring with
increasing frequency and intensity in coastal waters of the US and other
countries in response to increased anthropogenic nutrient inputs (eutrophication).
These blooms are caused by toxic or unpalatable species that severely
disrupt food web dynamics.
Two of these EDAB species, Auereococcus anophagefferens and Aureoumbra
lagunensis have caused massive, ecologically and economically
harmful blooms in US waters in the past 20 years. These blooms are
usually referred to as “brown tides” because of the characteristic
brown color they impart to the water. Aureococcus caused major
brown tide blooms in Long Island and other northeast coastal waters
beginning in 1985. The blooms killed entire year classes of larval
scallops in the Peconics during 1985-1988 and decimated the scallop
industry there. These blooms also severely impacted the clam industry
and destroyed ecologically sensitive sea grass beds. Aureoumbra caused
a massive brown tide bloom in Laguna Madre, Texas in 1990, which continued
unabated for eight years. This bloom caused a substantial decline in
sea grass communities and dramatically decreased the abundance and
diversity of clams and other benthic invertebrates. Despite a considerable
research effort, the factors and mechanisms responsible for the initiation
and propagation of these blooms are poorly understood.
The brown tide species, Aureococcus and Aureoumbra belong
to the group of microscopic algae known as pelagophytes. Both species
are very small, averaging 2 and 5 microns in diameter, respectively.
They have low maximum growth rates and appear to be well adapted to growth
at low nutrient concentrations and low light levels. However, because
the blooms are associated with eutrophication, one is left with a paradox:
Why would the growth of low-nutrient-adapted species be stimulated by
increased nutrient inputs? Researchers at CCFHR have proposed a new conceptual
model that solves this paradox. They note that brown tide blooms are
usually preceded by a “pre-bloom” of a non-toxic species, whose growth
is stimulated by nutrient inputs. These algae are capable of rapid growth
rates and their large size allows them to initially avoid grazing by
zooplankton. As the bloom progresses these algae consume essential nutrients
and/or attenuate light to limiting levels. This in turn selects for smaller
phytoplankton species that are capable of growth at low levels of limiting
resources, but are efficiently grazed by protozoans and other micro-zooplankton.
Eventually a steady-state evolves in which algal growth and grazing rates
roughly balance, and nutrient concentrations remain stable because nutrient
loss from algal uptake is balanced by recycled inputs from zooplankton
grazing (see A). Such systems, however, are susceptible to disruption
by small species adapted for growth at low light and/or nutrients that
are also toxic or unpalatable to grazers. Low grazing rates allow these
species to proliferate, and as the brown tide bloom progresses, zooplankton
grazers diminish through poisoning or starvation. The decrease in grazing
pressure leads to even higher algal biomass and demand for nutrients,
and less recycling. This combination causes even more severe nutrient
limitation, which further selects for the EDAB species (see B). Eventually
this species may occur as a virtual monoculture with little transfer
of nutrients and energy to higher trophic levels.
Approach The above model is consistent with existing
data on bloom development and propagation. However, additional experiments
are still necessary to test a key tenet of the model: that the two brown
tide species are well adapted for growth at very low levels of nutrients
and light. To test this hypothesis we have initiated light-limitation
and nutrient-limitation growth experiments with cultures of Aureoumbra and Aureococcus.
The growth rate of these two species at varying light intensities and
varying nutrient levels will be compared with that of competing non-toxic
species that inhabit healthy ecosystems.
The culture experiments will be used to test the validity of our conceptual
model and to modify it as required. Once this has been done, we will
use the conceptual model to construct numerical ecosystem models that
simulate the dynamics of EDAB blooms. These models initially will be
simple time dependent models, but will eventually be refined to include
spatial components as well. The models will allow us to further probe
the sensitivity of bloom behavior in response to different environmental
and biological variables. These models should eventually provide the
capability to predict when and where EDAB blooms may develop, how severe
they will be, and how long they may last. They also may be used to determine
what measures may be taken for bloom mitigation.
Outcome for Users: The results of this work will be used by researchers
and regulators alike. The conceptual model we have proposed, if verified,
will represent a major advance in our understanding of the dynamics of
brown tides and similar ecosystem disruptive blooms. The model provides
a framework for the construction of bloom simulation models whose predictions
can be tested by other researchers. Such models will provide managers
with the capability to better forecast the occurrence, severity, and
longevity of ecosystem disruptive algal blooms. They also should provide
regulators with insights into to fundamental environmental factors (both
natural and man-induced) that promote bloom development so that these
factors may be mitigated.