1Chapter 8 Primary Productivity - Chemical Tracers (4/18/01) This week we start a discussion of the production and destruction of organic matter by photosynthesis and respiration. Much of this may have been covered in biological oceanography but here we talk about biological oceanography from a chemical oceanography perspective, with an emphasis on chemical tracers and feedbacks. The topic is important for three key reasons: 1. One cannot understand the chemistry of the oceans without considering biological influences. 2. To understand the limits on biological production in the oceans, we need to understand the underlying chemical constraints (especially the macro (e.g. N and P) and micro (e.g. Fe and Zn) nutrients). 3. The balance between ocean productivity and respiration is called export production. Export production is the flux of biologically produced organic carbon from the surface ocean to the deep ocean and is also referred to as the biological pump. This biological pump is a primary control on atmospheric CO2 . Changes in the magnitude of the biological pump are one of the main explanations for why atmospheric CO2 was lower during glacial times than during interglacials. It is important for us to understand how the biological pump might change in response to increases in anthropogenic CO2 and global warming. Units Many different units are used for primary production. The most common are mmolC m-2 d-1, mgC m-2 d-1 , gC m-2 y-1, and Gt C y-1. This can be very confusing, especially at meetings where in successive talks each speaker uses different units. Chemical Oceanographers always recommend that moles be the preferred unit, i.e. mmol C m-2 d-1. Use of moles makes comparison of stoichiometric ratios between nutrients and carbon easier. Global view of primary production: A map of the distribution of primary production in the global ocean is shown on the following page (Fig 8-1). This version was prepared by Berger et al (1989) and is based on literature data. Note the general patterns: central gyres low equatorial zones high especially toward the eastern boundaries coastal regions high Arabian Sea high! circumpolar region mostly no data but moderatly high when available (>50°S)23There have been many different estimates of the total amount of primary production in the ocean. A few examples are listed here. It is interesting to note that the estimates of the total have doubled in the past decade since satellite data have been available and there is general consensus that the correct value is about 50 Gt C y-1. Pilson (1998) also supports the larger value (see attached table). Table 8-1 Historial estimates of ocean primary production Value (Gt C y-1) Koblents-Mishke et al (1968) 23 Ryther (1969) 20 Eppley and Peterson (1979) 19.1 23.7 **The best value Romankevich (1984) 25 appears to be about Martin et al (1987) 51 50 Gt C y-1** Berger et al (1987) 26.9 Field et al (1998) 48.5 **Note: 1 Gt = 1Pg = 109 tons = 1012 kg = 1015 gms** The total (marine plus terrestrial) global annual net primary production (NPP) has recently been estimated to be 104.9 Pg of C per year (Field et al., 1998), with similar contributions from the terrestrial (56.4 Pg of C (53.8%)) and oceanic (48.5 Pg of C (46.2%)) regimes. NPP is defined as the amount of photosynthetically fixed carbon available to the first heterotrophic level in an ecosystem. It can be expressed as the difference betwen autotrophic photosynthesis and respiration. This estimate was made using satellite date and the co-called CASA-VGPM biosphere model. In general NPP for both land and ocean models is determined from the absorbed photosynthetically active (400 to 700 nm) solar radiation (APAR) and an average light utilization efficiency (ε). NPP = APAR x ε Models based on this approach are diverse in detail but they are all connected to global-scale satellite observations. A color map of net primary production (NPP) for the terrestrial and marine biosphere is shown in Fig 8-2 (from Field et al , 1998). Even though the total amounts are about equal the amounts per area are greater on land than in the ocean. Average NPP on non-ice covered land is 426 g C m-2 yr-1 while in the ocean it is 140 gC m-2 yr-1. The lower NPP per unit area of the ocean largely results from competition for light between phytoplankton and their strongly absorbing medium, seawater. Only about 7% of the incident radiation as photosynthetically active radiation (PAR) is absorbed by the phytoplankton, with the remainder absorbed by water and dissolved organic matter. In contrast terrestrial plants absorb about 31% of the PAR incident on land. Even though primary producers in the ocean are responsible for nearly half the total NPP, they represent only 0.2% of the global producer biomass. Thus, the4Fig 8-2 Global annual net primary production (in grams C m-2 y-1) for the terrestrial and marine biosphere. From Field et al (1998).5turnover time of plan organic carbon in the ocean (average 2 to 6 days) is about a thousand times faster than on land (average 19 years). Spatial Variability Global Productivity - Distribution by Regions On land and in the oceans, spatial heterogeneity in NPP is comparabale, with both systems having large regions with low production and smaller areas of high production. Regions of high NPP are spatially more restricted in the oceans where they are limited to estuarine and upwelling systems. Some examples of the variability of ocean chlorophyll distributions as determined using the Coastal Zone Color Scanner launched in 1987 are shown in the Appendix. One example is for the western North Atlantic and the other is for the west coast of the US. In both examples areas with low temperature tend to have higher chlorophyll. The contribution of eddies to this variability can be seen in both regions. In the western north Atlantic there are both warm and cold eddies associated with the Gulf Stream. Of the west coast of the US, capes and points seem to play an important role generating upwelling zones which propagate offshore. In one estimate by Berger et al (1987), the total production was less than what we now accept as the correct value (26.9 GtC y-1) but their estimates by ocean are may be unchanged. Pacific = 44%