Primary production at the global scale

The rapid advances in, on the one hand, airborne and, especially, satellite-based remote-sensing techniques and, on the other, the techniques and resolution for analysing the signals thus detected, have verified and greatly amplified our appreciation of the scale of global net primary production (NPP). In barely 20 years, the capability has moved from qualitative observation, to remote quantification of biomass from the air (Hoge and Swift, 1983; Dekker et al., 1995) and on to the detection of analogues of the rate of its assembly and dissembly (Behrenfeld et al., 2002). The newest satellite techniques can provide the means to gather this information in a single overpass. For terrestrial systems, NPP is gauged from the light absorption by the plant canopy (APAR, absorbed photosynthetically active radiation) and an average efficiency of its utilisation (Field et al., 1998). For aquatic systems, sensors are needed to derive the rate of underwater light attenuation (from which the magnitude of the photosynthetically active flux density at depth is estimable) and the rates of light absorption and fluorescence attributable to the phytoplankton (Geider et al., 2001). To do this with confidence requires methodological calibration and the application of interpolative production models (discussed in detail in Behrenfeld et al., 2002). Most of the latter employ traditional functions (such as those reviewed in the previous section); calibration is painstaking and protracted but the remakable progress in interpreting photosynthetic properties of phytoplankton has led to synoptic mapping both of the distribution of phytoplankton at the basin mesoscale and of analogues of the rate of its carbon fixation (Behrenfield and Falkowski, 1997; Joint and Groom, 2000; Behrenfield et al., 2002).

The beauty, the global generality and the simultaneous detail of such imagery are awesome. However, its scientific application has been first to confirm and to consolidate the previous generalised findings of biological oceanographers (e.g., Ryther, 1956; Raymont, 1980; Platt and Sathyendranath, 1988; Kyewalyanga et al., 1992; see also Barber and Hilting, 2002, for review). In essence, the main oceans (Pacific, Atlantic, Indian) are deserts in terms of producer biomass (<50 mg chla m-2) while net primary production is assessed to be generally <200 gC fixed m-2 a-1. In the high latitudes, towards either pole, biomass and production tend to be more seasonal, with maximum production in the six summer months and least in winter. The greatest annual aggregates (200-500 gC fixed m-2 a-1) are detected mainly on the continental shelves. Production 'hotspots' (500-800gC fixed m-2 a-1) are located in particularly shallow areas (e.g., the Baltic Sea, the Sea of Okhotsk), in shelf waters receiving nutrient-rich river outfalls (the Yellow Sea, the Gulf of St Lawrence) and in the upwellings of major cold currents (e.g., the Peru, around Galápagos; and the Benguela, Gulf of Guinea).

Satellite remote sensing has also helped to improve the resolution of global NPP aggregates and their relative contribution to the global carbon cycle. The estimates of total oceanic NPP, based on imagery, converge on values of around 45-50 Pg C a-1. It is interesting that previous estimates, all based on summations and extrapolations of various in-situ measurements, are mostly

Table 3.3 Annual net primary production (NPP) of various parts of the sea and of other major units of the biosphere

Energy invested

Marine domains

Tropical/subtropical trades 13.0 509

Temperate westerlies 16.3 638

Polar 6.4 250

Coastal shelf 10.7 419

Salt marshes, estuarine 1.2 47

Coral reef 0.7 27

Total 48.3 1890 Terrestrial domains

Tropical rainforests 17.8 697

Evergreen needleleaf forest 3.1 121

Deciduous broadleaf forest 1.5 58

Deciduous needleleaf forest 1.4 54

Mixed broad- and needleleaf forest 3.1 121

Savannah 16.8 658

Perennial grassland 2.4 94

Broadleaf scrub 1.0 39

Tundra 0.8 31

Desert 0.5 19

Cultivation 8.0 313

Total 56.4 2205

Source: Based on Geider et al. (2001), using data of Longhurst et al. ( 1995) and Field et al. (1998).

within about 50% of this (20-60 Pg C a-1: see Barber and Hilting, 2002). Only Riley's (1944) estimate of 126 Pg C a-1, based on oxygen exchanges, now seems exaggerated.

It is interesting to compare the estimates for various parts of the ocean and with other major biospheric units (see Table 3.3). Shelf waters contribute nearly a quarter of the total oceanic exchanges despite occupying less than about 1/20 of the area of the seas. Nevertheless, marine photosynthesis is responsible for just under half the global NPP of about 110 Pg (or 1.1 x1017 g) C a-1.

Much of this carbon is recycled in respiration and metabolism and reused within the year (see also Section 8.2.1). Net replenishment of atmospheric carbon dioxide would contribute a steady-state concentration (currently around 370 parts per million by volume, or 0.2gCm-3). Significant natural abiotic exchanges of carbon with the atmosphere include the removal due to carbonate solution and silcate weathering (~0.3 Pg C a-1) but this is probably balanced by releases of CO2 through calcite precipitation, carbonate metamorphism and vulcanism (Falkowski, 2002). As is well known, however, ambient atmospheric carbon dioxide concentration is currently increasing. This is generally attributed to the combustion of fossil fuels (presently around 5.5 ± 0.5 Pg C a-1 and rising) but the oxidation of terrestrial organic carbon as a consequence of land drainage and deforestation also makes significant contributions (some 1.6 ± 1.0 Pg C a-1: data from Sarmiento and Wofsy, 1999, as quoted by Behrenfeld et al., 2002). In spite of this anthropogenic annual addition to the atmosphere of ~7 Pg C, the present annual increment is said to be 'only' 3.3 (±0.2) Pg C a-1; say ~0.6% a-1 relative to an atmospheric pool of ~500 Pg). The 'deficit' (~3.8 Pg C a-1) is explained, in part, by a verified dissolution of atmospheric CO2 into the sea and, in part, by transfers of organic and biotic components of unverified scale.

Considering that it seems more painful (in the short term) to cut anthropogenic carbon emissions, there is currently a great deal of interest in augmenting net annual flux of carbon to the oceanic store of dissolved carbon of around 40 x 1018gC (Margalef, 1997).

Was this article helpful?

0 0

Post a comment