1. The Deep Chlorophyll Maximum in Lake Superior M.T. Auer, L.A. Bub and K.D. Elenbaas Department of Civil and Environmental Engineering Michigan Technological University 2000 HN210 Chl

2. seasonal development of the DCM HN Transect - Chl (µgL -1 ) 4/28/006/22/007/30/008/25/0010/20/00

3. Reported in both freshwater and marine systems and related to a variety of environmental factors SystemFactorReference Adriatic Sea Zooplankton grazing Nutrient levels, distribution Revelante and Gilmartin 1995 Baltic Sea Ciliate grazing Sedimentation Kononen et al. 1998 Mediterranean Sea Zooplankton grazing Nutrient levels, distribution Physical factors Gould and Wisenburg 1990 Lake Tahoe In situ growth Sedimentation Zooplankton grazing Coon et al. 1987 Lake Michigan In situ growth Sedimentation Shade adaptation Brooks and Torke 1977; Fahnenstiel and Scavia 1987 Finger Lakes of New York, New York City Reservoirs, et al.

4. In Lake Superior YearInvestigatorFactor 1966 Olson and Odlaug Putnam and Olson Light Temperature Zooplankton grazing 1975 Watson et al.Factors other than depth- variable settling differences 1978 Munawar and MunawarIn situ growth 1983 Fahenstiel and GlimeIn situ growth 2001 Zhou et al.Light – nutrients Thermocline tilting

5. The Ecological Enhanced growth due to conditions of light, temperature, nutrients, grazing, particularly suitable for some or all taxa The Physical Accumulation of phytoplankton due to differences in settling velocities over the metalimnion But it comes down to two camps … Or more simply, “Is the DCM a great place to live … or are they just hanging around?

6. The Ecological For phytoplankton: nutrient supply temperature optima light efficiency chlorophyll adaptation grazing pressure chemoautotrophs

7. nutrient supply

8. ELF Gloeocystis ELF Fragilaria ELF (Enzyme-Labeled Fluorescence) is an alkaline phosphatase substrate which yields a bright yellow-green fluorescent precipitate upon enzymatic cleavage. ELF-APA Data from Vickie Morris, MTU

9. nutrient supply ELF Activity % of cells 81% 61% 65% 0.073 0.037 0.025 APA V max µM/min

10. temperature optima Optimum T @ ~15 °C T in DCM ~ 4-5 °C

11. light efficiency -50 -40 -30 -20 -10 0 300350400450500550600650700 1% light level, m wavelength, nm HN090 - JD 239 3-5% 1-3% 0.6-1%

12. chlorophyll adaptation Shade adaptation does account for some of the DCM, but certainly not all of it. The maximum is also present in the carbon data. carbon data courtesy of Noel Urban July and August Offshore Stations

13. grazing pressure OPC scans provided by David J. Osterberg, MTU temperature (0-20 m) zooplankton (15-30 m) chlorophyll (25-40 m) 0 m 30 m 60 m 10 m 20 m 40 m 50 m

14. 0.00.51.01.52.0 0.000.050.100.150.20 Chlorophyll (  g L -1 ) Production (mg C m -3 h -1 ) August 1999 HN210 PC2 PC1 5m 22m 50m Carbon Source Utilization let’s ask the bacteria

15. … and furthermore Carbon excretion by actively photosynthesizing phytoplankton is able to satisfy the bacterial carbon requirement (BCR) only in the surface waters.

16. The Physical

17. The Expanding Universe 6/22/00 7/30/00 9/24/00 T °C Chl (µg/L)

18. Conversation Overheard at 30m … Hey! Is this a great place to live? Naw. We’re just hangin’ around.

19. acknowledgements: For field support: Captain & Crew of the R/V Laurentian For assistance in data analysis: Jae Jeong, MTU For data: Angela Cates, LLO Judy Budd, MTU Kim Elenbaas, MTU Sarah Green, MTU Vickie Morris, MTU Dave Osterberg, MTU Noel Urban, MTU For financial support: National Science Foundation