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Who’s Blooming? Toward an Understanding of Why Certain Species Dominate Phytoplankton Blooms Mary Jane Perry,* Michael Sieracki,* Bess Ward,† Alan Weidemann‡ What questions remain unanswered? A critical question in biological oceanography is “what controls the species composition of phytoplankton assemblages?” While the gen- eral patterns of development of mid- and high-latitude spring blooms, up­welling blooms, and storm-induced blooms are reasonably well known—at least with respect to biomass—the specifics that determine which species first become abundant, as well as the species that replace them over time, are not. Because phytoplankton respond directly to physical forcings, it is likely that both species composition and timing of blooms will change in response to climate change. While ecosystem models have evolved from parameterization of phytoplankton as total biomass to functional groups and individual species, observational assessment of phytoplankton species on appropriate space and time scales remains a technological challenge. Why care about phytoplankton species? Some phytoplankton species are toxic—either to humans (harmful algal toxins are concentrated by filter feeding bivalves), copepods (some * School of Marine Sciences, University of Maine †  Princeton University ‡  Naval Research Laboratory 111

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112 OCEANOGRAPHY IN 2025 diatom aldehydes lead to birth defects in some copepods), or birds and marine mammals (domoic acid is transferred up marine food webs). Some phytoplankton species provide better nutrition to zooplankton and fish than other species; nutrition plays an important role in successful growth and development of larval fishes. Some phytoplankton species are more efficient in transporting carbon to the benthos, by virtue of heavier cell coverings (e.g., silica) that results in more rapid sinking. Some species scatter light more effectively as a consequence of spines or calcite cocco- liths, thereby changing the relationship between chlorophyll concentra- tion and light attenuation. What controls the species composition of phytoplankton blooms? Part of the answer lies in understanding how phytoplankton are introduced (or inoculated) into a local surface mixed layer. Horizontal transport is an important mechanism for introduction of species into a local water mass. Does interannual variability in the advective introduc- tion of species, either due to changes in source waters or relative transport rates of currents, lead to interannual variations in the species composi- tion of blooms? In shallow waters overlying continental shelves some phytoplankton species, particularly diatoms and dinoflagellates, can form resting stages or cysts that lie dormant in the sediment for months to years. These cysts and resting stages can be either triggered to germinate by exposure to very low levels of light at the end of the winter or can be reintroduced into the euphotic zone by vertical mixing events. The second part of the answer to what controls the species composi- tion of phytoplankton blooms lies in relative net growth of individual species: dP = P(gain – loss) dt where P is abundance of a phytoplankton species, gain is primarily growth rate, loss is primarily grazing rate, and t is time (from Gordon Riley). If the identity of species that might bloom were better known, it would be possible to carry out focused laboratory experiments to deter- mine how growth rates of individual species are controlled by light, temperature, nutrients and mixing. Grazing rates depend on who the consumers are, whether grazer abundances vary interannually, and if grazers are selective for or against certain species of phytoplankton. Again, with knowledge of potential bloomers, grazer selectivity experi- ments could be carried out.

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Mary Jane Perry et al. 113 What new technologies could be developed and what questions answered? Although current technologies can provide highly resolved time series of total phytoplankton biomass (typically as chlorophyll a con- centration) prior to and during blooms in both Eulerian and Lagrangian frames of reference, there are very few observations of phytoplankton species collected either in a Lagrangian frame of reference or with suffi- cient temporal frequency to resolve changes in organisms that can double population size on the order of once per day. The notable exceptions are measurements with flow cytometers and FlowCAMs from ships and a few shallow water moorings. In order to truly understand bottom-up regulation of marine eco- systems (i.e., to what extent species composition, abundance and timing of the primary producers exert control over the rest of the food web), the field needs to move beyond observing and assessing phytoplank- ton primarily as chlorophyll. To understand why certain species become abundant, the capability to easily assess phytoplankton species on the appropriate space and time scales needs to be developed. What technologies are required? To address the questions raised above, systems for unattended mea- surement on both mobile (Lagrangian) and fixed (Eulerian) platforms need to be developed. Two likely methodologies are optical imaging and molecular analysis. Early examples of both of these technologies exist today, but will require serious and significant technological investment if easy assessment of species is to be enabled by 2025. Optical imaging of individual phytoplankton-size particles today is laser based, with image analysis of larger cells (flow cytometry and FlowCAM). Molecular tech- nologies, including microarrays, are rapidly developing and need to be coupled with water sampling and microfluidics. In addition to the need for serious reduction in sensor size, other issues include sensor robust- ness, depth rating, power consumption, battery life, sensing frequency including conditional sampling, sensing duration of weeks to months, on-board manipulation of water samples, on-board data analysis and compression, data storage and transmission, and ability to be integrated into mobile platforms and moorings. How will the research be conducted? Experiments could be conducted with a combination of platforms that move actively (e.g., long-duration AUVs or combination gliders),

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114 OCEANOGRAPHY IN 2025 covering potential source water downstream of an area of interest, and platforms that follow the water (e.g., Lagrangian mixed layer floats). High frequency sampling would provide a picture of what really happens during a bloom—similar to a walk through the garden to see what plant species are there and who grows the fastest; high frequency identification of species would provide an answer to who’s blooming and the beginning of the answer to why.