Primary Production of the Biosphere: Integrating Terrestrial and Oceanic

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Primary Production of the Biosphere: Integrating Terrestrial and Oceanic R EPORTS strongly connected to global-scale observa- tions. For the oceans, APAR can be related to Primary Production of the satellite-derived measurements of surface chlo- rophyll (Csat)(14), and for terrestrial systems, it Biosphere: Integrating can be determined from satellite-based esti- mates of vegetation greenness, often the nor- Terrestrial and Oceanic malized difference vegetation index (NDVI) (15). APAR depends on the amount and distri- Components bution of photosynthetic biomass (the primary source of variability in Csat and NDVI), as well Christopher B. Field,* Michael J. Behrenfeld, James T. Randerson, as the amount of downwelling solar radiation and the fraction that is in the visible (photosyn- Paul Falkowski thetically active) wavelengths. « is an effective photon yield for growth that converts the bio- Integrating conceptually similar models of the growth of marine and terrestrial mass-dependent variable (APAR) into a flux of primary producers yielded an estimated global net primary production (NPP) organic compounds (NPP). For both terrestrial of 104.9 petagrams of carbon per year, with roughly equal contributions from and oceanic models, « cannot be directly mea- land and oceans. Approaches based on satellite indices of absorbed solar ra- sured from space and must be parameterized diation indicate marked heterogeneity in NPP for both land and oceans, re- with field measurements. flecting the influence of physical and ecological processes. The spatial and For marine systems, « can be parameter- temporal distributions of ocean NPP are consistent with primary limitation by ized from thousands of 14C-based field mea- light, nutrients, and temperature. On land, water limitation imposes additional surements of NPP (16–18). Terrestrial values constraints. On land and ocean, progressive changes in NPP can result in altered are less abundant, largely because « depends carbon storage, although contrasts in mechanisms of carbon storage and rates on time-consuming determinations of NPP of organic matter turnover result in a range of relations between carbon storage and APAR (19, 20). Uncertainty in « is a and changes in NPP. primary source of error in land and ocean NPP estimates. With few exceptions, ocean Biological processes on land and in the NPP, originally defined as the amount of NPP models estimate « solely as a function of oceans strongly affect the global carbon cycle photosynthetically fixed carbon available to sea-surface temperature (11, 16, 21–23). In on August 9, 2010 on all time scales (1–4). In both components the first heterotrophic level in an ecosystem terrestrial ecosystems, « varies with ecosys- of the biosphere, oxygenic photosynthesis is (5), is also the difference between autotrophic tem type and with stresses from unfavorable responsible for virtually all of the biochemi- photosynthesis and respiration (6). NPP is a levels of temperature, nutrients, and water cal production of organic matter. Mecha- major determinant of carbon sinks on land (20, 24, 25). nisms of and constraints on photosynthesis on and in the ocean (7, 8) and a key regulator of In this study, we combined results from land and in the oceans are similar in many ecological processes, including interactions conceptually similar land and ocean NPP respects, but past syntheses of primary pro- among trophic levels (9, 10). Because ocean models, the Carnegie-Ames-Stanford ap- duction from photosynthesis have focused on NPP is dominated by phytoplankton, nearly proach (CASA) (26) for land and the Verti- the terrestrial or ocean components individu- all of the plant biomass is photosynthetic. cally Generalized Production Model (VGPM) www.sciencemag.org ally. Consequently, models of the global car- Therefore, relatively short-term measure- (16) for the oceans (27). Both of these models bon cycle are compartmentalized, with limit- ments (24 hours) can account for both pho- are simple formulations designed with an em- ed opportunities for comprehensive or com- tosynthesis and respiration. In contrast, the phasis on integrating spatially extensive sat- parative analyses. Here, we present integrated major components of terrestrial plant biomass ellite observations rather than describing the estimates of primary production based on are roots and stems, which respire but do not mechanistic details of NPP. In essence, both satellite measurements for both oceanic and generally photosynthesize. In terrestrial eco- models use versions of Eq. 1, expanded to terrestrial ecosystems. This integrated ap- systems, it is relatively straightforward, in provide an effective interface with observed Downloaded from proach builds from parallel data sets and principle, to determine NPP from incremental variables. The fundamental relation in the model formulations toward a truly biospheric increases in biomass plus litter fall over CASA model is view. weeks, months, or years. Below-ground pro- 5 3 3 ε 3 ~ ! 3 ~ ! The biologically mediated parts of the cesses, however, add numerous challenges to NPP f(NDVI) PAR * g T h W (2) carbon cycle in terrestrial and ocean biomes these conceptually simple measurements. involve both production and turnover of or- NPP on land and in the oceans has been where APAR (in megajoules per square ganic matter. At the biochemical level, pho- modeled with a variety of approaches with a meter per month) is a function of NDVI and tosynthesis and the biosynthesis of organic range of fundamental mechanisms, specific downwelling photosynthetically active solar compounds, the processes that result in net details, and levels of integration (11, 12). A radiation (PAR) and « (in grams of C per primary production (NPP), are very similar. common contemporary approach, developed megajoule) is a function of the maximum independently for land and ocean models, achievable light utilization efficiency «* ad- calculates NPP as a function of the driving justed by functions that account for effects of C. B. Field, Department of Plant Biology, Carnegie Institution of Washington, Stanford, CA 94305, USA. energy for photosynthesis, the absorbed pho- temperature g(T) and water h(W) stress (26). M. J. Behrenfeld and P. Falkowski, Institute of Marine tosynthetically active (400 to 700 nm) solar For the VGPM, the fundamental equation is and Coastal Sciences, Rutgers University, New Bruns- radiation (APAR), and an average light utili- NPP 5 C 3 Z 3f~PAR! 3 Pb (T) (3) wick, NJ 08901–8521, USA. J. T. Randerson, De- zation efficiency («)(13, 14) sat eu opt partment of Plant Biology, Carnegie Institution of where C is the satellite-derived, near-sur- Washington, Stanford, CA 94305, USA, and Depart- NPP 5 APAR 3« (1) sat ment of Biological Sciences, Stanford University, face phytoplankton chlorophyll concentration Stanford, CA 94305, USA. Models based on this approach are diverse in (in milligrams per cubic meter), Zeu is the *To whom correspondence should be addressed. terms of mechanistic detail, but they are all depth (in meters) to which light is sufficient www.sciencemag.org SCIENCE VOL 281 10 JULY 1998 237 R EPORTS to support positive NPP, f(PAR) describes depend on continuous, high-quality data, over productive than the vast mid-ocean gyres the fraction of the water column from the extended periods. (Fig. 1). Maximal NPP is similar in both 22 21 surface to Zeu in which photosynthesis is light Using the integrated CASA-VGPM bio- systems (1000 to 1500 g of C m year ), b saturated, and Popt (T) is the maximum, chlo- sphere model, we obtained an annual global but regions of high NPP are spatially more rophyll-specific carbon fixation rate (in mil- NPP of 104.9 Pg of C (Table 1), with similar restricted in the oceans (essentially limited to ligrams of C per milligram of chlorophyll per contributions from the terrestrial [56.4 Pg of estuarine and upwelling regions) than in ter- day), estimated as a function of sea-surface C (53.8%)] and oceanic [48.5 Pg of C restrial systems (for example, humid tropics) temperature (11, 16). For the VGPM, varia- (46.2%)] components (38). This estimate for (Fig. 1). On land, 25.0% of the surface area tion in the fraction of absorbed PAR is a ocean productivity is nearly two times greater without permanent ice (3.3 3 107 km2) sup- function of depth-integrated phytoplankton than estimates made before satellite data (39, ports an NPP greater than 500 g of C m22 3 21 biomass (that is, Csat Zeu). The product of 40). Average NPP on land without permanent year , whereas in the oceans, that figure is b 22 21 3 6 2 P opt and f(PAR) yields an average water ice cover is 426 g of C m year , whereas only 1.7% (5.0 10 km ). Highly produc- column light utilization efficiency, making it that for oceans is 140 g of C m22 year21. The tive (that is, eutrophic) regions in the oceans the corollary of « in Eq. 1. The VGPM op- lower NPP per unit area of the ocean largely contribute less than 18% to total ocean NPP erates with a daily time step, whereas CASA results from competition for light between (Table 1). has a monthly time step. phytoplankton and their strongly absorbing Globally, NPP reaches maxima in three Biospheric NPP was calculated from Eqs. medium. For the average ocean Csat of 0.19 distinct latitudinal bands (Fig. 2). The largest 2 and 3, on the basis of observations averaged mg m23 (16, 41), only 7% of the PAR inci- peak (;1.6 Pg of C per degree of latitude) over several years. Because the satellite data dent on the ocean surface is absorbed by the near the equator and the secondary peak at necessary for estimating APAR cover differ- phytoplankton (14), with the remainder ab- midtemperate latitudes of the Northern Hemi- ent time periods for the oceans and land, the sorbed by water and dissolved organics. In sphere are driven primarily by regional max- averaging periods are different: 1978 to 1983 contrast, leaves of terrestrial plants absorb ima in terrestrial NPP.
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