Outline

1. Outline • Approaches to study ecosystems • What is a “global” biogeochemical cycle? • Why are they studied? • Basics of the C cycle and its links to O • A counterintuitive idea about atmospheric O2 • What role to freshwater systems play in the global C balance? • The regulation of a global cycle depends on the time frame considered.

4. An ecosystem is defined as a spatially explicit unit of the Earth that includes all of the organisms, along with all components of the abiotic environment within its boundaries – Likens 1992

5. When looking at the Earth as an ecosystem, most scientists draw boundaries between the “solid” planet and the atmosphere. Use inputs and outputs from the Earth to atmosphere as ecosystem fluxes.

10. Global C balance in Gt y-1rough numbers after Schimel et al. 2001 • Emissions to atmosphere 6.5 • Increase in atmosphere 3.1 • Oceanic gas exchange -1.5 (physical) • Net “terrestrial sink” -1.9 (biological) • How are these numbers validated?

11. Suess Effect Change in 14C (and 13C) in the atmosphere due to human process. Named for Hans E. Suess What changes and why?

14. Very long Past 1000 y Recent record

15. Past 10,000 years of atmospheric CO2 • Relatively stable. • Not decreasing. (Falkowski and Raven) • If organic C was stored on land, where did CO2come from? • Oceanic source, not larger than ~0.75 Gt y-1 • Maybe terrestrial sink is 0.75 larger than the sink from modern atmospheric budget.

16. 0.09 to 0.75Gt y-1 Post glacial (10,000 y) Atmosphere Increasing slowly 0.02 Gt y-1 0.09 to 0.75Gt y-1 0.09 to 0.75Gt y-1 Ocean ocean sediment Values after Sundquist 1993

17. Modern (50 y) Atmosphere Increasing rapidly 3.1 Gt y-1 1.3Gt y-1 ~1.9Gt y-1 6.3 Gt y-1 Ocean ocean sediment Values after Schimel et al 2001

18. Review standing stocks • Before we get in deep- • What are the large and small reservoirs of C on Earth?

19. Atmosphere 760 Ocean 5 plants 3000 DOC 38,000 HCO3 Land 450 plants 700 detritus Sediments 20,000,000 98/y 100/y 102/y 100/y

21. Components of Productivity NEP CO2 GPP Ra Plant biomass accumulation Rh NPP (Rt = Ra + Rh) Consumers Detritus and exudates Decomposers Not decomposed Exported Buried (Sediments and SOM)

22. GPP review • GPP = total photosynthesis (> 0) • R = total respiration (> 0) • NEP =GPP-R (may be + or -) • When NEP is +, equals burial plus export • When NEP is -, net heterotrophy

23. NEP algebra. • External Import from Outside ( Ie) • Export from ecosystem (E) • Burial (export to sediments) (B) • GPP (gross primary production) • R (total respiration in the system) • Total Inputs = GPP + Ie • Total losses = R+E+B • Total Inputs = Total Losses (conservation of matter)

24. NEP Algebra Continued • Since NEP = GPP-R, we can rearrange • (GPP-R) = E+B-Ie or: • NEP = E + B – Ie • So you do not need to measure GPP or R to get NEP. • If Ie is > (E+B), NEP is NEGATIVE

25. R ~100-0.12 99.8Gt/y river transport 0.4 Gt/y burial 0.12 Gt/y whole ocean net heterotrophy is Burial + Export - Import 0.12 –0.4 = -0.28 Gt/Y or OR ~0.8 g C m Biological parts of the C cycle (after Holland, 1993). Atmosphere 760 Gt ~ 48 Gt/y photosynthesis ~52 Gt/y photosynthesis Marine Biosphere Terrestrial 2 - 4 Gt Biosphere 500 -1000 Gt Marine detritus 500 -1000 Gt Terrestrial detritus 1000-2000 Gt Marine sediments, organic -2 y -1 20,000,000 Gt

26. Terrestrial GPP (slightly) > R; NEP > 0 Aquatic GPP (generally) < R; NEP < to << 0 Positive NEPon land subsidizes aquatic R

28. Forest Lake 100 100 105 Export =9 storage = 1 Increase =4 Subsidized system can be both a net source and a net sink 90

29. GPP R Net gas flux transport sedimentation

32. Hudson River

34. Cole & Caraco 2001, Mar.Freshwat Res

35. Back to global – let’s link C and O cycles

36. mammals Land plants eucaryotes pO2 (atm) cyanobacteria 1 2 3 4 Billions of years before present

37. Where does oxygen come from? • Photosynthesis • Balance between GPP and R • GPP-R=NEP= org C burial. • Atmospheric Oxygen comes from org C burial. • If atmospheric O2 has been “flat” for the past 500,000 years, what does that imply?

38. What ever controls organic C burial controls atmospheric O2 • O2 >> 0.2 atm leads to increased fire. • O2 << 0.2 atm unsuitable for most aerobes • What controls C burial? • Mayer hypothesis • Oxygen hypothesis.

39. Clay rules! Where does clay come from?

41. aerobic anaerobic

43. Hartnett et al, Nature 1998 • What is the debate they bring up? • What is the new twist here. • What is the “experiment”

44. Hartnet et al. 40 30 Burial efficiency % 20 10 0.01 0.1 1 10 100 1000 Oxygen exposure time (yr)

45. Hartnett et al, Nature

46. GAIA (Lovelock, 1991) • Hypothesis: Earth is kept in a state favorable to living organisms by (in part) living organisms. • Theory: sees Earth as system in which evolution of organisms is tightly coupled to evolution of the environment. Self regulation of climate and chemistry are emergent properties of this system

47. Thank you. • Jan 10th- think about Coupled Biogeochemical Cycles and Geoengineering

48. Organic C burial in lakes is large • Natural lake organic C burial • 0.065 Gt y-1 (Mullholland and Elwood 1982) • 0.034 Gt y-1 (Dean and Gorham 1998; Stallard 1998) • Lakes sequester 28 to 54% as much organic C as does the global ocean! • Oceanic organic C burial ~0.12 Gt y-1 • See Cole et al. 2007, Ecosystems

50. Why do lakes bury so much organic C? • Rich theory of C preservation in the sea • Oxygen exposure time hypothesis • Sorptive preservation hypothesis • Poorly developed theory in freshwaters. • Low oxygen (a real possibility) • Low sulfate (especially compared to ocean) • High lignin (plus low O2) – can be dismissed • Certainly not close to a universal law of C burial in freshwaters.