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A Conceptual Model for Ecosystem-Disruptive Algal Blooms
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.
plankton
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.