Eloise Ryan 1 , David Hamilton 1 , Vivienne Cassie Cooper 2 & Julie Hall 3

1. Phytoplankton composition & biomass in Rotorua lakes Eloise Ryan1, David Hamilton1, Vivienne Cassie Cooper2 & Julie Hall3 1 Department of Biological Sciences, University of Waikato, Hamilton, New Zealand 2 Landcare Research, Hamilton, New Zealand 3 National Institute of Water and Atmosphere Research, Hamilton, New Zealand

2. Objectives • Vertical variability of phytoplankton • Effects of mixing, light and nutrients To link taxonomy and phytoplankton physiology to understand‘what lives where and why’

3. SCM vs DCM • SCM – Surface Chlorophyll Maxima • chlorophyll in the upper warm, light layer of water • DCM – Deep Chlorophyll Maxima - subsurface, deep layer of chlorophyll Chlorophyll – indicator of phytoplankton biomass

4. Lake Tarawera DCM – 29 m

5. Lake Tarawera Diatoms Fluorescence Chrysophytes 0 0.5 1 1.5 2 2.5 3 3.5 4 Green algae Blue-greens 0 0 10 20 30 40 50 60 10 % composition 20 30 40 Depth (m) Blue-greens Diatoms 50 Chrysophytes Green algae 60 0 20 40 60 80 100 70 % composition 80 Species composition

6. Study sites

7. Methods • 3 lakes – Tikitapu, Okareka, Tarawera • Profiles of chlorophyll fluorescence, temperature, DO, pH and PAR • SCM and DCM samples -Chl a, nutrients, phytoplankton and zooplankton

8. Sampling the SCM Tube Sampler

9. Phytoplankton • Enumeration by Utermohl sedimentation technique • Net rates of growth – cell counts • Biovolumes calculated to assess the relative biomass - Using the closest stereometric formula to their shape • Chlorophyll:carbon ratio

10. Fluorescence Lake Tikitapu 0 2 4 6 8 10 12 14 16 0 2 4 6 8 Depth (m) 10 12 14 16 18 20 22 24 26 Lake Okareka Fluorescence 3 4 5 6 7 8 9 10 11 0 2 4 6 8 10 12 14 16 18 20 Low biomass Depth (m)

11. Fluorescence 0 2 4 6 8 10 12 14 16 0 2 4 6 8 10 Depth(m) 12 14 16 18 20 22 24 26 Food webs - Zooplankton phytoplankton → zooplankon → fish? Fish?

12. Physical vs Biological formation • Physical - Water column stability - Transparency - Nutrient supply - Predation pressure • Biological - In situ growth - Sinking of phytoplankton - Physiology and behaviour Biological processes classically a response to physical conditions – consider combined effect

13. Lake Tikitapu *Data from EBOP monitoring programme

14. Stability Regimes Fluorescence 0 2 4 6 8 10 12 14 16 0 10 Depth 20 26 L Tarawera Strong DCM Weak DCM DCM → SCM

15. Fluorescence 0 1 2 3 4 5 6 7 0 5 10 15 20 25 30 35 40 45 Algal bloom effects vs DCM Lake Tarawera - February 2003 Depth Reduction in DCM Reduced light at depth → reduction of DCM

16. Embayment effects Temperature Depth Mapping internal waves and thermocline movement using thermistor chains Fluorescence Depth

17. Limitations of Growth SCM High light levels DCM Control Nutrients Control Nutrients L Tikitapu - Nutrient limited L Tarawera - Light limited

18. Future Questions • DCM vs Algal blooms • Nutrient enrichment →loss of DCM? • To what extent do zooplankton exploit the DCM? • Do DCM layers contribute significantly to fish production?

19. Acknowledgments Environment Bay of Plenty Foundation for Research Science and Technology Andrew Lang Rotorua Habourmaster Fish and Game Dudley Bell, David Burger University of Waikato