Chapter 10: Biolimiting Elements James W. Murray - vertical and horizontal segregation Univ. Washington - case studies (Fe and N) (10/29/01) By definition, biolimiting elements are those: necessary to sustain life and exist in low concentrations For the most part the prototypical macro biolimiting elements are: P as PO 4 Soft Parts N as NO 3 " Si as H 4 SiO 4 Hard Parts Several trace elements can be limiting, most notably iron. I. Vertical distributions Box models can be used to determine the rates of transfer between reservoirs and transformations within a reservoir. Fig 10-1 Schematic of 2-box model Advantages - Easy to conceptualize - Provide an overview of fluxes, reservoir sizes and turnover or residence times - Provide the basis for more detailed models Disadvantages - the analysis is superficial - little insight is gained into processes (e.g. transport) - we usually assume homogeneous distributions within reservoirs 1
Broecker (1971) Two-Box Model (Fig 10-2) V river C river V river x 0 Surface Box Vup = V down = Vmix = Annual water transport = 300 cm y -1 Vmix/Vriver = 30 V mix B (1-f) x B V mix C deep Deep Box f x B B = Vriver Criver + Vmix Cdeep - Vmix Csurf f B = Vriver Criver (1-f) B = Vmix Cdeep - Vmix Csurf Assumptions: 2 boxes - each well mixed rivers the only source sediments the only sink removal is by biogenic particles steady state 2
Mass Balance - Surface Box Inputs Outputs Vriver Criver + Vmix Cdeep = Vmix Csurf + B Mass Balance - Deep Box Vmix Csurf + (1-f) B = Vmix Cdeep Mass Balance - Whole Ocean Vriver Criver = f B Two important properties 1. g = efficiency of bioremoval from the surface as particles. = bioremoval/inputs = B / (VrCr + Vmix Cdeep) Element g N, P 0.95 Si 1.00 C 0.20 Ca 0.01 2. f = fraction of particles that don't dissolve. This is the efficiency of ultimate removal. f B = Vriver Criver f = Vriver Criver / B = Vriver Criver / (Vriver Criver + Vmix Cdeep - Vmix Csurf) = 1 / (1 + Vmix/Vriver (Cdeep/Criver - Csurf/Criver)) Element f N,P,Si 0.01 C 0.02 Ca 0.12 3
The fraction of an element removed to the sediments for each visit to the surface box is given by the product of f x g. For PO 4 f x g = 0.01 x 0.95 = 0.0095 Particle flux total particle particle flux to sediments flux to sediments Total particle Total input to total input to Flux surface box surface box So f x g = 0.01 which means that 1% of the PO4 introduced to the surface ocean is removed during each mixing cycle. If the mixing time of the ocean is 1000 y, then PO4 goes through 1 / f x g = 105 cycles of 1000 y / 105 cycles = 9.5 years / cycle Or 1 / f x g = the average number of cycles (through the thermocline) that an element cycles through the ocean before being removed permanently to the sediments. = Fig 10-3 4
Table 10-1 Table 10-2 5
II. Perturbation analysis 1. The simple model can be used to evaluate the outcome of different physical and biological perturbations. In this case we ask how the concentration of PO4 in the deep ocean will change when we double the rate of physical exchange (e.g. V m ) once the controlling process (P burial) is known. Fig 10-4 6
Fig 10-5 7
III. Horizontal distributions The superposition of the vertical flux of biologically produced particles on the horizontal circulation of the ocean results in: a) Low surface nutrient concentrations b) High nutrient concentrations in the deep ocean c) Higher nutrient concentrations in the deep Pacific than the deep Atlantic d) a shallower calcium carbonate compensation depth in the Pacific than the Atlantic Fig 10-6 Conveyor Belt 8
Fig 10-7 9
Trace elements in seawater Definition: Those elements that do not contribute to the salinity All elements less than 1 mg kg -1. Why: 1. many are micronutrients (e.g. Fe, Cu) 2. others are toxic (e.g. Cu, Hg) 3. can be tracers for redox conditions (Cr, I, Mn, Re, Mo, V, U) 4. can be enriched in economic deposits such as manganese nodules (e.g. Cu, Co, Ni, Cd) 5. some are tracers of pollution (e.g. Pb, Pu, Ag) Difficult to collect samples for without contamination and to analyze. Most data available since 1975. Oceanographic consistency (Boyle and Edmond (1975) Nature, 253, 107-109) Acceptance of data must satisfy two criteria: 1. Verticle profiles should be smooth 2. Correlations should exist with other elements that share the same controlling mechanisms. See examples in Fig 10-8. Summary of concentrations: See Table 1 in Lecture 1 of notes which gives the composition of seawater Fig 10-9: The "state of play" diagram showing the range of concentrations for different elements. Concentrations are as low as 10-21 M. How many atoms is this? 10
Fig 10-8 11
Fig 10-9 Concentration range of trace elements in seawater 12
Examples: Conservative - Cesium (Cs) Molybdenum (Mo) - under oxic conditions Metal Limiting and Toxicity - Copper (Fig 10-10, 10-11) Role of Free Metal Ion Nutrient Like - Shallow and Deep Regeneration Barium (Fig 10-12) Zinc (Fig 10-8) Germanium Cadmium (Fig 10-8; 10-13; 10-14) Iodate Nickel (Fig 10-8) Copper (Fig 10-8; see also paper by Boyle) Surface Enrichment Lead (Fig 10-8; 10-15) Manganese (Fig 10-17) Mercury Mid-depth Maximum Manganese (Fig 10-17) Iron Near Bottom Enrichment North Sea Metals (Cd, Cu, Mn) (Fig 10-19) Deep Depletion Lead-210 (Fig 10-8; 10-18) Aluminum (Fig 10-16) Manganese (Fig 10-8; 10-17) 13
Fig 10-10 Dependency of plankton growth rate on the activity of Cu 2+. Fig 10-11 Activity of the free Cu 2+ ion (pcu) from various sites. 14
Fig 10-12 Barium data from the Atlantic and the Ba-Si relationship (Chan et al, 1977) 15
Fig 10-13 Cadmium profiles and the Cd-PO 4 relationship (Bruland, 1980) 16
Fig 10-14 Cadmium as a tracer for paleophosphate. a) Cd-PO4 from the world's ocean (Hester and Boyle, 1982) b) Cd/Ca in benthic forams versus bottom water PO4. 17
Fig 10-15 a)lead in Greenland Ice cores (Murozumi et al, 1969). b) Lead profiles in seawater and new atmospheric input fluxes (Shen and Boyle, 1987). 18
Fig 10-16 a) Aluminum profiles from the Pacific b)comparison on Atlantic and Pacific. c) 19
Fig 10-17 20
Fig 10-18 Lead scavenged from the deep sea - seen in 210 Pb data, relative to its parent 226 Ra.. 21
d) Fig 10-19 Sediments are a source of metals on the continental shelves. e) 22
f) Biolimiting Elements - Case Studies (4/28/99) The tradtional paradigm has been that nitrogen is the limiting nutrient in the ocean. This has been inferred in many ways, one of which are property-property plots such as NO 3 versus PO 4. An example is shown in Fig 10-20 which shows NO 3 versus PO 4 for all the GEOSECS data from the Pacific. There is a linear relationship which has a slope close to the RKR value of 16 : 1 and there is an intercepy on the Y-axis. This suggests that if this water was upwelled into the euphotic zone, and N and P were consumed in RKR proportions, that NO 3 would go to zero first. 1. N versus P Fig 10-20 23
2. Nitrogen Fixation Table 10-2 24
25
26
3. Silica Limitation 27
28
References: Behrenfeld M.J. and Z.S. Kolber (1999) Widespread iron limitation of phytoplankton in the south Pacific Ocean. Science, 283, 840-843. Capone D.G., J.P. Zehr, H.W. Paerl, B. Bergman and E.J. Carpenter (1997) Trichodesmium, a globally significant marine cyanobacterium. Science, 276, 1221-1229. Codispoti L.A. (1989) Phosphorus vs Nitrogen limitation of mew and export production. In ( W.H. Berger, V.S. Smetacek and G. Wefer, eds) Productivity of the Ocean: Present and Past. Wiley, 377-394. Dugdale R.C., F.P. Wilkerson and H.J. Minas (1995) The role of silicate pump in driving new production. Deep-Sea Research, 42, 697-719. Dugdale R.C. and F.P. Wilkerson (1998) Silicate redulation of new production in the equatorial Pacific upwelling. Nature, 391, 270-273. Karl D.M., R. Letelier, D. Hebel, L.Tupas, J. Dore, J. Christian and C. Winn (1995) Ecosystem changes in the North Pacific subtropical gyre attributed to the 1991-92 El Nino. Nature, 373, 230-234. Karl D., R. Letelier, L. Tupas, J. Dore, J. Christian and D. Hebel (1997) The role of nitrogen fixation in biogeochemical cycling in the subtropical north Pacific Ocean. Nature, 388, 533-538. Perry M.J. (1976) Phosphate utilization by an oceanic diatom in phosphate-limited chemostat culture and in the oligotrophic waters of the central north Pacific. Limnol. Oceanogr. 21, 88-107. 29