6.04 The Biological Pump C. L. de la Rocha University of Cambridge, UK NOMENCLATURE 84 6.04.1 INTRODUCTION 84 6.04.2 DESCRIPTION OF THE BIOLOGICAL PUMP 84 6.04.2.1 Photosynthesis and Nutrient Uptake 86 6.04.2.1.1 Levels of primary production 87 6.04.2.1.2 Patterns in time and space 87 6.04.2.1.3 Nutrient limitation 88 6.04.2.2 Flocculation and Sinking 88 6.04.2.2.1 Marine snow 88 6.04.2.2.2 Aggregation and exopolymers 89 6.04.2.2.3 Sinking 89 6.04.2.3 Particle Decomposition and Repackaging 90 6.04.2.3.1 Zooplankton grazing 90 6.04.2.3.2 Bacterial hydrolysis 90 6.04.2.3.3 Geochemistry of decomposition 90 6.04.2.4 Sedimentation and Burial 91 6.04.2.5 Dissolved Organic Matter 91 6.04.2.6 New, Export, and Regenerated Production 92 6.04.3 IMPACT OF THE BIOLOGICAL PUMP ON GEOCHEMICAL CYCLING 92 6.04.3.1 Macronutrients 92 6.04.3.1.1 Carbon 92 6.04.3.1.2 Nitrogen 92 6.04.3.1.3 Phosphorus 93 6.04.3.1.4 Silicon 94 6.04.3.2 Trace Elements 96 6.04.3.2.1 Barium 96 6.04.3.2.2 Zinc 96 6.04.3.2.3 Cadmium 98 6.04.3.2.4 Iron 98 6.04.4 QUANTIFYING THE BIOLOGICAL PUMP 99 6.04.4.1 Measurement of New Production 99 6.04.4.2 Measurement of Particle Flux 100 6.04.4.2.1 Sediment traps 100 6.04.4.2.2 Particle-reactive nuclides 101 6.04.4.2.3 Oxygen utilization rates 101 6.04.5 THE EFFICIENCY OF THE BIOLOGICAL PUMP 102 6.04.5.1 Altering the Efficiency of the Biological Pump 102 6.04.5.1.1 In HNLC areas 102 6.04.5.1.2 Through changes in community composition 104 6.04.5.1.3 By varying the C : N : P ratios of sinking material 104 6.04.5.1.4 By enhancing particle transport 105 6.04.6 THE BIOLOGICAL PUMP IN THE IMMEDIATE FUTURE 105 6.04.6.1 Response to Increased CO2 105 6.04.6.2 Response to Agricultural Runoff 106 6.04.6.2.1 Shift towards silicon limitation 106 6.04.6.2.2 Shifts in export production and deep ocean C : N : P 106 6.04.6.3 Carbon Sequestration via Ocean Fertilization and the Biological Pump 106 ACKNOWLEDGMENTS 107 REFERENCES 107 83 84 The Biological Pump NOMENCLATURE One side-effect of the biological pump is that CO is shunted from the surface ocean and into * 2 H2CO3 dissolved CO2 þ H2CO3 (mM) the deep sea, thus lowering the amount in the J flux of organic C to depth Corg atmosphere. For many years it has been (g C m22 yr21) recognized that pre-Industrial CO2 levels in the z depth (m) atmosphere were about one-third of what they PP primary production (g C m22 yr21) 2 21 would be in the absence of a biological pump D diffusivity of CO2 in seawater (m s ) (Broecker, 1982). It is also known that the r radius of phytoplankton cell (mm) biological pump is not operating at its full Ce extracellular CO2 concentration (mM) capacity. In so-called “high-nutrient, low- Ci intracellular CO2 concentration (mM) 0 21 chlorophyll” (HNLC) areas of the ocean, a k rate constant for HCO3 ! CO2 (s ) considerable portion of the nutrients supplied F flux of CO to cell surface (mmol s21) CO2 2 to the surface waters is not utilized in support of Pt residence time (yr) primary production, most likely due to the CO2 total CO2 (mM) limitation of phytoplankton growth by an inadequate supply of trace elements (e.g., Martin and Fitzwater, 1988). The possibility that the 6.04.1 INTRODUCTION biological pump in HNLC areas might be Despite having residence times (t) that exceed stimulated by massive additions of iron both the ,1,000 yr mixing time of the ocean (Broecker artificially as a means of removing anthropogenic and Peng, 1982), many dissolved constituents of CO2 from the atmosphere and naturally as a seawater have distributions that vary with depth cause for the lower glacial atmospheric CO2 and from place to place. For instance, silicic levels (Martin, 1990) is the focus of much acid (t ¼ 1.5 £ 104 yr), nitrate (t ¼ 3,000 yr), research and debate (e.g., Martin et al., 1994; phosphate (t ¼ (1–5) £ 104 yr), and dissolved Coale et al., 1996; Boyd et al., 2000; Watson inorganic carbon (DIC; t ¼ 8.3 £ 104 yr) are et al., 2000). generally present in low concentrations in surface Although the biological pump is most popu- waters and at much higher concentrations below larly known for its impact on the cycling of the thermocline (Figure 1). Additionally, their carbon and major nutrients, it also has profound concentrations are higher in older deep waters impacts on the geochemistry of many other than they are in the younger waters of the deep elements and compounds. The biological pump sea (Figure 2). This is the general distribution heavily influences the cycling, concentrations, exhibited by elements and compounds taking and residence times of trace elements—such as part in biological processes in the ocean and cadmium, germanium, zinc, nickel, iron, arsenic, is generally referred to as a “nutrient-type” selenium—through their incorporation into distribution. organic matter and biominerals (Bruland, 1980; Both the lateral and vertical gradients in the Azam and Volcani, 1981; Elderfield and concentrations of nutrients result from “the Rickaby, 2000). Scavenging by sinking biogenic biological pump” (Figure 3). Dissolved inorganic particles and precipitation of materials in the 2 32 microenvironment of organic aggregates and materials (e.g., CO2,NO3 ,PO4 , Si(OH)4) are fixed into particulate organic matter (carbo- fecal pellets plays a large role in the marine hydrates, lipids, proteins) and biominerals (silica geochemistry of elements such as barium, and calcium carbonate) by phytoplankton in thorium, protactinium, beryllium, rare earth surface waters. Some of these particles are elements (REEs), and yttrium (Dehairs et al., subsequently transported, by sinking, into the 1980; Anderson et al.,1990; Buesseler deep. The bulk of the organic material and et al., 1992; Kumar et al., 1993; Zhang and Nozaki, 1996). Even major elements in seawater biominerals decomposes in the upper ocean via 2þ 2þ dissolution, zooplankton grazing, and microbial such as Ca and Sr display slight surface hydrolysis, but a significant supply of material depletions (Broecker and Peng, 1982; de does survive to reach the deep sea and sediments. Villiers, 1999) as a result of the biological Thus, just as biological uptake removes certain pump, despite their long respective oceanic dissolved inorganic materials in surface waters, residence times of 1 Myr and 5 Myr (Broecker the decomposition of sinking biogenic particles and Peng, 1982; Elderfield and Schultz, 1996). provides a source of dissolved inorganic material to deeper waters. Thus, deeper waters contain higher concentrations of biologically utilized 6.04.2 DESCRIPTION OF THE BIOLOGICAL materials than do surface waters. Older deeper PUMP waters contain higher concentrations of bums compared to newly formed deep waters or surface The biological pump can be sectioned into waters. several major steps: the production of organic Description of the Biological Pump 85 P Figure 1 Depth profiles of: (a) CO2, (b) dissolved CO2, (c) silicic acid, (d) nitrate, and (e) phosphate from the Indian Ocean (278 40 S, 568 580 E; GEOSECS Station 427) (source Weiss et al., 1983). matter and biominerals in surface waters, the bacteria. In fact, most of the primary production sinking of these particles to the deep, and the formed will be recycled within the upper hundred decomposition of the settling (or settled) particles. few meters of the water column (Martin et al., In general, phytoplankton in surface waters take up 1987). Some portion of the primary production DIC and nutrients. Carbon is fixed into organic will, however, be exported to deeper waters or material via photosynthesis and, together with even to the sediments before decomposition and nitrogen, phosphorus, and trace elements, form the may escape remineralization entirely and remain carbohydrates, lipids, and proteins, which all in the sedimentary reservoir. comprise bulk organic matter. Once formed, this It is worth taking a closer look at the various organic matter faces the immediate possibility of steps in the biological pump (Figure 3). Rates, decomposition back to CO2, phosphate, ammonia, overall amounts, and the distribution and char- and other dissolved nutrients through consumption acter of materials produced, transported, and by herbivorous zooplankton and degradation by decomposed vary wildly within the ocean. 86 The Biological Pump Figure 2 Nitrate concentrations along the great ocean conveyor at 2,000 m depth (source Levitus et al., 1994,by way of the LDEO/IRI Data Library). Figure 3 Diagram of the biological pump (after OCTET workshop report). 6.04.2.1 Photosynthesis and Nutrient Uptake The first stable product of carbon fixation by the enzyme, ribulose bisphosphate carboxylase In the initial step of the biological pump, (Rubisco), is glyceraldehyde 3-phosphate, a 3-C phytoplankton in sunlit surface waters convert sugar. This 3-C sugar is fed into biosynthetic CO2 into organic matter via photosynthesis: pathways and forms the basis for all organic compounds produced by photosynthetic organ- CO2 þ H2O þ light –! CH2O þ O2 ð1Þ isms. Fixed carbon and major and trace elements Description of the Biological Pump 87 such as hydrogen, nitrogen, phosphorus, calcium, within cells and are important for osmoregulation silicon, iron, zinc, cadmium, magnesium, iodine, and the maintenance of charge balance (e.g., selenium, and molybdenum are used for the Fagerbakke et al., 1999).