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Seasonal and Interannual Variability of the Sargasso Sea

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Seasonal and Interannual Variability of the Sargasso Sea View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by ASU Digital Repository Seasonal and Interannual Variability of the Sargasso Sea Plankton Community by Amy Hansen A Thesis Presented in Partial Fulfillment of the Requirements for the Degree Master of Science Approved November 2010 by the Graduate Supervisory Committee: Susanne Neuer, Chair Rosa Krajmalnik-Brown Milton Sommerfeld ARIZONA STATE UNIVERSITY December 2010 ABSTRACT Phytoplankton comprise the base of the marine food web, and, along with heterotrophic protists, they are key players in the biological pump that transports carbon from the surface to the deep ocean. In the world’s subtropical oligotrophic gyres, plankton communities exhibit strong seasonality. Winter storms vent deep water into the euphotic zone, triggering a surge in primary productivity in the form of a spring phytoplankton bloom. Although the hydrographic trends of this “boom and bust” cycle have been well studied for decades, community composition and its seasonal and annual variability remains an integral subject of research. It is hypothesized here that proportions of different phytoplankton and protistan taxa vary dramatically between seasons and years, and that picoplankton represent an important component of this community and contributor to carbon in the surface ocean. Monthly samples from the Bermuda Atlantic Time-series Study (BATS) site were analyzed by epifluorescence microscopy, which permits classification by morphology, size, and trophic type. Epifluorescence counts were supplemented with flow cytometric quantification of Synechococcus, Prochlorococcus, and autotrophic pico- and nanoeukaryotes. Results from this study indicate Synechococcus and Prochlorococcus, prymnesiophytes, and hetero- and mixotrophic nano- and dinoflagellates were the major players in the BATS region plankton community. Ciliates, cryptophytes, diatoms, unidentified phototrophs, and other taxa represented rarer groups. Both flow cytometry and epifluorescence microscopy revealed Synechococcus to be i most prevalent during the spring bloom. Prymnesiophytes likewise displayed distinct seasonality, with the highest concentrations again being noted during the bloom. Heterotrophic nano- and dinoflagellates, however, were most common in fall and winter. Mixotrophic dinoflagellates, while less abundant than their heterotrophic counterparts, displayed similar seasonality. A key finding of this study was the interannual variability revealed between the two years. While most taxa were more abundant in the first year, prymnesiophytes experienced much greater abundance in the second year bloom. Analyses of integrated carbon revealed further stark contrasts between the two years, both in terms of total carbon and the contributions of different groups. Total integrated carbon varied widely in the first study year but displayed less fluctuation after June 2009, and values were noticeably reduced in the second year. ii ACKNOWLEDGMENTS This research was supported by NSF BIO-OCE Grant 0752592 and TA/RA funding from the Arizona State University School of Life Sciences. I thank Jessica Amacher, Alexandra Freibott, and Rebecca Mestek for their assistance with microscopy identification and, in the case of Ms. Freibott, for sharing her preliminary results on ciliate taxonomy and abundance at the BATS station. Sincere thanks go to Dr. Michael Lomas of the Bermuda Institute of Ocean Sciences for providing hydrography and FCM data, and for helpful discussions on the context and relevance of this study. This research would not have been possible if not for the tireless efforts of the scientists and crew aboard the R/V Atlantic Explorer. Finally, I am extremely grateful to Dr. Susanne Neuer for her insights into and support of my research during my time at ASU. iii TABLE OF CONTENTS Page LIST OF TABLES ....................................................................................................... v LIST OF FIGURES .................................................................................................... vi INTRODUCTION ....................................................................................................... 1 METHODS .................................................................................................................. 8 At-Sea Sampling and Processing ................................................................ 8 Flow Cytometry Sample Processing ........................................................ 10 Epifluorescence Microscope Slide Counting ........................................... 10 Conversion of Cell Abundance to Biovolume ......................................... 14 Conversion of Biovolume to Carbon ........................................................ 15 Calculating Integrated Chlorophyll a and Carbon ................................... 15 Contour Plots ............................................................................................. 17 RESULTS .................................................................................................................. 18 Hydrographic Data .................................................................................... 18 Flow Cytometry Data ................................................................................ 22 Epifluorescence Microscopy Counts, Biovolume, and Carbon ............... 25 DISCUSSION ............................................................................................................ 42 REFERENCES .......................................................................................................... 58 APPENDIX A ABBREVIATIONS AND ACRONYMS USED .............................. 62 B CRUISE DATES AND DEPTHS ...................................................... 64 iv LIST OF TABLES Table Page 1. Organisms or taxonomic groups counted in epifluorescence microscopy slide counts ...................................................................... 12 2. Carbon conversion equations compiled from the literature and used in this study ................................................................................. 16 v LIST OF FIGURES Figure Page 1. Location of Bermuda and the BATS site ............................................... 8 2. Contour plot of temperature .................................................................. 18 3. Contour plot of Chl a ............................................................................ 19 4. Integrated Chl a from 0-150 m ............................................................. 20 5. Contour plot of primary production ...................................................... 21 6. Contour plots of nutrient concentrations .............................................. 22 7. Contour plots of Synechococcus and Prochlorococcus abundance from FCM data .................................................................................... 23 8. Contour plots of picoeukaryote and nanoeukaryote abundance from FCM data .................................................................................... 25 9. Contour plot of Synechococcus abundance .......................................... 26 10. Contour plot of prymnesiophyte abundance ...................................... 27 11. Average prymnesiophyte abundance per month .................................. 29 12. Contour plots of HNF and HDF abundance and C .............................. 30 13. Total HNF and HDF abundance per month ......................................... 32 14. Contour plot of MDF abundance .......................................................... 33 15. Total MDF abundance per month ......................................................... 34 16. Contour plots of total, centric, and pennate diatom abundance ........... 36 17. Contour plot of cryptophyte abundance ............................................... 37 18. Integrated C per month from 0-150 m .................................................. 38 vi Figure Page 19. Percent contributions of different groups to integrated C per month ................................................................................................... 39 20. Relationship of integrated Chl a to integrated autotrophic C ............ 41 vii INTRODUCTION The deep, clear open ocean appears empty at first glance. These blue waters are so oligotrophic, or bereft of nutrients, that oceanographers refer to them as “deserts.” With none of the hustle and bustle that characterizes coastal kelp forests or coral reefs, it is easy to fall prey to the illusion that these seas are practically lifeless. Peer at them through a microscope, however, and a different picture emerges. Microorganisms dominate the world’s oligotrophic ocean gyres—those vast, slowly swirling bodies of water between continents set in motion by the Coriolis effect—although they are significantly less abundant here than in productive, eutrophic (nutrient-rich) zones. One such gyre is the North Atlantic Subtropical Gyre (NASTG). Also called the Sargasso Sea, the western portion of the NASTG is home to the Bermuda Atlantic Time-series Study (BATS). Decades of research at the BATS site and nearby Hydrostation “S” have led to a firm understanding of the hydrography of this region (e.g. Durand et al. 2001; Steinberg et al. 2001; Nelson et al. 2004). Winter storms bring deep, cold, nutrient-rich water to the surface. This mixing event triggers a phytoplankton bloom in the euphotic zone, the upper region of the ocean where light is sufficient to allow photosynthesis.
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