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Understanding Phytoplankton Bloom Development

Bess Ward* and Mary Jane Perry

WHAT QUESTIONS REMAIN UNANSWERED?

Historical biogeographical data sets document global and regional patterns in phytoplankton and zooplankton species distributions. Although correlations with, for example, temperature and nutrient concentrations, are strong, they remain descriptive. At a very fundamental level, we still do not know what controls the species composition of phytoplankton assemblages and what key environmental variables determine the success or failure of different groups under different conditions. Even biogeochemical models that include functional groups do so with low resolution and depend upon simple variables such as size to distinguish groups. Although size sounds like an objective variable, it’s not obvious that actual phytoplankton fall into ecologically meaningful size categories. Thus we need to link observations of phytoplankton species to measured and model outcomes to determine what factors matter to phytoplankton, and thence to predictive power in response to anthropogenic changes such as nutrient enrichment and global warming.

WHY FOCUS ON PHYTOPLANKTON BLOOMS?

Molecular ecological investigations, first of prokaryotic plankton and more recently of eukaryotic phytoplankton, have revealed a vast diversity of species in natural assemblages. The diversity at the molecular level is

*

Princeton University

School of Marine Sciences, University of Maine



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Understanding Phytoplankton Bloom Development Bess Ward* and Mary Jane Perry† WHAT qUESTIONS REMAIN UNANSWERED? Historical biogeographical data sets document global and regional patterns in phytoplankton and zooplankton species distributions. Although correlations with, for example, temperature and nutrient con- centrations, are strong, they remain descriptive. At a very fundamental level, we still do not know what controls the species composition of phytoplankton assemblages and what key environmental variables deter- mine the success or failure of different groups under different conditions. Even biogeochemical models that include functional groups do so with low resolution and depend upon simple variables such as size to distin- guish groups. Although size sounds like an objective variable, it’s not obvious that actual phytoplankton fall into ecologically meaningful size categories. Thus we need to link observations of phytoplankton species to measured and model outcomes to determine what factors matter to phy- toplankton, and thence to predictive power in response to anthropogenic changes such as nutrient enrichment and global warming. WHY FOCUS ON PHYTOPLANKTON BLOOMS? Molecular ecological investigations, first of prokaryotic plankton and more recently of eukaryotic phytoplankton, have revealed a vast diversity of species in natural assemblages. The diversity at the molecular level is * Princeton University † School of Marine Sciences, University of Maine 115

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11 OCEANOGRAPHY IN 2025 even greater than can be determined by the best microscopist, and mod- ern methods such as DNA/RNA microarrays can evaluate diversity and abundance rapidly in relatively high throughput mode. The fact remains, however, that despite the large background diversity, only a very small number of types ever reaches really high abundance in blooms. Irigoien et al. (2004) showed that the highest biomass/production occurs mostly in high biomass blooms and these blooms are due to only four phytoplank- ton types: diatoms, coccolithophorids, Phaeocystis and dinoflagellates. Thus blooms, although rare and geographically small, are inordinately important to overall marine productivity and are dominated by a few types. WHAT CONTROLS THE SPECIES COMPOSITION OF PHYTOPLANKTON BLOOMS? The answer to this question depends on the factors that cause a high diversity, low abundance assemblage to develop into a low diversity, high abundance bloom. We suspect that the critical events occur long before the typical oceanographic measurement—chlorophyll or some other mea- sure of biomass—can detect changes in the assemblage. The important responses that allow some species to win and cause others to lose must occur very soon after conditions change to allow a bloom: introduction of new nutrients by advection or upwelling, cessation of mixing due to surface warming, etc. Responses to environmental changes occur at the level of gene expression, probably often in genes that we have not yet identified or whose ecological significance we have not yet grasped. Thus fundamental molecular biological research is required to identify targets for response assay development. WHAT NEW TECHNOLOGIES COULD BE DEvELOPED AND WHAT qUESTIONS ANSWERED? In order to investigate the early development of phytoplankton blooms, it is necessary to begin sampling even before there is indication that changes are occurring. This can be done in manipulative experiments with large volume incubations at sea or on land, or by using remote observations to predict likely bloom development and then undertake in situ sampling. In order to determine the appropriate time scales for investigation of phytoplankton responses, some additional work with pure cultures is probably warranted. Then both experimental and in situ sampling can be scaled to catch, e.g., changes in gene expression that can occur on the order of minutes to hours. The development of large phytoplankton blooms depends partly on the absence or lag of grazing,

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11 BESS WARD AND MARY JANE PERRY and overall production is thus subject to top-down control. The initial response at the genetic level that allows some species to take advantage of episodic changes in growth conditions, however, is fundamentally bottom-up. We must be able to identify and interpret such responses long before food web interactions become obvious. WHAT TECHNOLOGIES ARE REqUIRED? The two basic needs are development of technology to evaluate gene diversity and expression for many different phytoplankton species rap- idly and specifically, and to deploy those methods on samples collected at appropriate time and space scales. It’s hard to imagine being able to carry out the biological measurements required to assess early bottom-up responses outside the laboratory, but some progress has been made. In situ quantitative polymerase chain reaction (Q-PCR) and hybridization meth- odologies are under development (John Paul, Chris Scholin). By selecting a few well-characterized genes, very specific assays can be developed to identify major phylotypes and assess gene expression among them. Functional gene microarrays offer a way to analyze the relative abun- dance and level of gene expression of many different kinds of genes from a multitude of different organisms simultaneously. At the very least, fre- quent sample collection and preservation can be done so that laboratory based analyses can be linked to remotely measured chemical and physical variables. Remote sampling and sample processing are engineering chal- lenges beyond my insights. Clearly it’s not just technology, but communi- cation between e.g., molecular biologists and engineers, which will make important advances possible. HOW WILL THE RESEARCH BE CONDUCTED? In the analysis of actual blooms, high frequency sampling and rela- tively rapid analysis is required. This probably requires a combination of remotely operated vehicles, ships and laboratory analysis. Real time analysis is not necessary for everything as long as real time sampling can be coordinated with the necessary physical and chemical measure- ments. In order to develop molecular assays for the key genetic responses involved in bloom development, it is also necessary to support laboratory based molecular biological research. This can go on in parallel, as genetic samples can often be reanalyzed when new tools become available. REFERENCE Irigoien, X., J. Huisman, and R.P. Harris. 2004. Global Diversity Patterns of Marine Phyto- plankton and Zooplankton. Nature. 429(6994): 863-867.