The Ecology of the Plankton Communities of Two Desert Reservoirs by Tyler R Sawyer A Thesis Presented in Partial Fulfillment of the Requirements for the Degree Master of Science Approved July 2011 by the Graduate Supervisory Committee: Susanne Neuer, Chair Daniel L. Childers Milton Sommerfeld ARIZONA STATE UNIVERSITY August 2011 ABSTRACT In 2010, a monthly sampling regimen was established to examine ecological differences in Saguaro Lake and Lake Pleasant, two Central Arizona reservoirs. Lake Pleasant is relatively deep and clear, while Saguaro Lake is relatively shallow and turbid. Preliminary results indicated that phytoplankton biomass was greater by an order of magnitude in Saguaro Lake, and that community structure differed. The purpose of this investigation was to determine why the reservoirs are different, and focused on physical characteristics of the water column, nutrient concentration, community structure of phytoplankton and zooplankton, and trophic cascades induced by fish populations. I formulated the following hypotheses: 1) Top-down control varies between the two reservoirs. The presence of piscivore fish in Lake Pleasant results in high grazer and low primary producer biomass through trophic cascades. Conversely, Saguaro Lake is controlled from the bottom-up. This hypothesis was tested through monthly analysis of zooplankton and phytoplankton communities in each reservoir. Analyses of the nutritional value of phytoplankton and DNA based molecular prey preference of zooplankton provided insight on trophic interactions between phytoplankton and zooplankton. Data from the Arizona Game and Fish Department (AZGFD) provided information on the fish communities of the two reservoirs. 2) Nutrient loads differ for each reservoir. Greater nutrient concentrations yield greater primary producer biomass; I hypothesize that Saguaro Lake is more eutrophic, while Lake Pleasant is more oligotrophic. Lake Pleasant had a larger zooplankton abundance and biomass, a larger piscivore fish community, and smaller phytoplankton abundance compared i to Saguaro Lake. Thus, I conclude that Lake Pleasant was controlled top-down by the large piscivore fish population and Saguaro Lake was controlled from the bottom-up by the nutrient load in the reservoir. Hypothesis 2 stated that Saguaro Lake contains more nutrients than Lake Pleasant. However, Lake Pleasant had higher concentrations of dissolved nitrogen and phosphorus than Saguaro Lake. Additionally, an extended period of low dissolved N:P ratios in Saguaro Lake indicated N limitation, favoring dominance of N-fixing filamentous cyanobacteria in the phytoplankton community in that reservoir. ii ACKNOWLEDGEMENTS Funding for this project came in the form of The Research and Training Initiatives Office Facilities Initiative Grant for Grads (RTI FIGG) and from the Arizona State University School of Life Sciences as funding and support as a research and teaching assistant. Field sampling and laboratory assistance was contributed by Jessica Amacher, Charles Baysinger, and Aaron Robinson. Alissa Rickborn carried out the DNA analysis of the zooplankton guts. Natasha Zolotova provided assistance with the preparation and analysis of particulate nutrient samples in the W.M. Keck Laboratory. Analysis of dissolved nutrient samples was handled by Bill Clinton at the University of California Santa Barbara Marine Science Institute Analytical Lab. The staff of Scorpion Bay Marina at Lake Pleasant provided helpful information on reservoir levels, weather, and other extraneous data in addition to un-equaled enthusiasm about the project. Finally, I express gratitude and respect for my advisor, Susanne Neuer, and to my supervisory committee for their insight and consideration. iii TABLE OF CONTENTS Page LIST OF TABLES…………………………………………………………….…………vi LIST OF FIGURES………………………………………………………………….…vii INTRODUCTION………………………………….............................………….…….1 Field Data Collection Sites………………………………………………….…2 Previous Work…………………………………………………………….…….4 Hypotheses…………………………………………………………………...…8 METHODS…………………………………………………..………………………….10 Conductivity, Temperature, and Dissolved Oxygen……………………….10 Water Collection……………………………………………………………....11 Dissolved Constituents…………………………………………………….…12 Particulate Constituents……………………………..……………………….12 Secchi Depth………………………………………………………………..…13 Reservoir Depth………………………...……………………………………..14 Chlorophyll a…………………………………………………………………..14 Phytoplankton Abundance……………...……………………………………14 Zooplankton Abundance………………………..……………………………15 Zooplankton Biomass……………………………….………………………..16 Rotifer Abundance…………………………………………………………….16 DNA Based Water Column and Gut Content Examination……………….16 RESULTS…………………..…………………………………………………………..19 Temperature, Dissolved Oxygen, and Conductivity….……………………19 Saguaro Lake……………..…………………………………………..19 Lake Pleasant…………………………………………………………21 Secchi Depth……………..……………………………………………………24 Page Dissolved Inorganic Nutrients………………………………………………..25 Particulate Constituents………………………………………………………29 Chlorophyll a…………………………………….…………………………….33 Phytoplankton Community….………………………………………………..36 Zooplankton Abundance…………………………………..…………………38 Zooplankton Biomass……………...…………………………………………43 Rotifer Abundance…………………………………….………………………44 Lake Pleasant Fish Community (AZGFD 2005 and 2008)……………….45 Saguaro Lake Fish Community…………………………………...…………46 DNA based molecular Gut Analyses……………………..…………………46 Saguaro Lake………………..………………………………………..47 Lake Pleasant…………………………………………………………52 DISCUSSION…………………………………………..………………………………58 1. Hydrographical Context………………………………………………….58 2. Seasonal Zooplankton Variability in the Reservoirs in Relation to Phytoplankton Variability……………………..….…………………..61 3. Food Webs in the Reservoirs Derived from Gut Analyses and Literature………………..……………………………………………..64 4. Controls of Community Structure………………….……………………67 5. Hypothesis Evaluation………………………………….………………..68 6. Future Work…………………………………...…………………………..70 REFERENCES………………………………………………..……………………….71 APPENDIX……………………………………………………………………………..76 DNA Based Molecular Prey Preference Results………..……………..76-91 LIST OF TABLES Page 1. 2010 sampling schedule…………………………………..………………………12 2. Dissolved inorganic nutrient concentrations in Lake Pleasant and Saguaro Lake……….…………………………………………………………25 3. POC/PON/POP in Lake Pleasant and Saguaro Lake……….…………………29 4. Qualitative examination of phytoplankton communities in Lake Pleasant and Saguaro Lake………………………………………………………………….36 5. Eukaryotic organisms found via DNA based molecular analysis of the water column in Saguaro Lake………………………………………….………………………47 6. Eukaryotic organisms found via molecular analysis of the water column in Lake Pleasant………..………………………………………………………..52 vi LIST OF FIGURES Page 1. Surface map of Saguaro Lake………………………………….………………….3 2. Surface map of Lake Pleasant…………………………..…………………………4 3. Phytoplankton community structure of Lake Pleasant and Saguaro Lake in 2009………..…………………………………………………………………….7 4. Chlorophyll a data from Saguaro Lake (2007-2009) and Lake Pleasant (2008-2009)…………………...……………………………………………...…8 5. Temperature, dissolved oxygen, and conductivity of Saguaro Lake in 2010……………...…………………………………………………………19-20 6. Temperature, dissolved oxygen, and conductivity of Saguaro Lake in 2010……………………………………………………………………...…21-22 7. Secchi depth and estimated euphotic zone depth in Lake Pleasant and Saguaro Lake………………………………………………………………….24 8. Dissolved inorganic nutrient concentrations determined in the surface of Lake Pleasant and Saguaro Lake……………………………..………..25-26 9. Molar nitrogen and phosphorus ratios plotted with the 16:1 Redfield Ratio…27 10. Particulate P, N, and C values from the surface of Lake Pleasant and Saguaro Lake……………………………………………….…………………30 11. Particulate ratios of surface Carbon and Nitrogen……………...…………….31 12. Chlorophyll values for Saguaro Lake and Lake Pleasant...………...……33-34 13. Abundance of zooplankton in the upper 5m of Lake Pleasant during 2010..38 14. Copepodite and adult copepod populations from Lake Pleasant……………39 15. Abundance of zooplankton in the upper 5m of Saguaro Lake during 2010..40 vii Page 16. Comparison of abundance estimates derived from casts of different depth intervals from Lake Pleasant and Saguaro Lake in November……....41-42 17. Zooplankton biomass in Lake Pleasant and Saguaro Lake………………….43 18. Rotifer abundance in Saguaro Lake from June to November of 2010……...44 19. Relative distribution of cyclopoid prey organisms in Saguaro Lake obtained from DNA based molecular gut analysis……….…………………..………48 20. Nauplii (cyclopoid) prey organisms in Saguaro Lake………..……………49-50 21. Daphnia prey organisms in Saguaro Lake……………………………….……51 22. Cyclopoid prey organisms in Lake Pleasant from DNA based molecular gut analysis……………………………………………………………………..53-54 23. Calanoid prey organisms in Lake Pleasant from DNA based molecular gut analysis……………………………………….………………………………..55 24. Nauplii (cyclopoid and calanoid) prey organisms in Lake Pleasant from DNA based molecular gut analysis…………………………………………...……………………56 25. Bosmina prey organisms in Lake Pleasant from DNA based molecular gut analysis…………………………………………………………………………57 26. Covariation of temperature, chlorophyll, and nitrogen in Lake Pleasant and Saguaro Lake……………………………….…………………………………59 27. Hypothesized and inferred food web of Lake Pleasant…………………...….64 28. Hypothesized and inferred food web of Saguaro Lake…………………….…66 iix Introduction A community is defined as “the sum of all of the interacting populations in a habitat” (Lampert and Sommer, 2007). In aquatic ecosystems, the community consists of primary producers (phytoplankton),