Fig. 10-13 Schematic representation of biological processes in the marine euphotic zone.
of zooplankton grazing and bacterial respiration. Oxygen is consumed and organic-N is released as ammonia according to
The released ammonia is preferentially taken up by phytoplankton relative to nitrate (Dortch, 1990) to drive regenerated production. The /-ratio is used to describe the relative amounts of new and regenerated production (Dugdale and Goering, 1967) where
The /-ratio typically varies from values as low as 0.03 to 0.30 in the open ocean (e.g., McCarthy et al., 1996) to values greater than 0.80 in the coastal ocean (Piatt and Harrison, 1985).
If we define the sum of new plus regenerated production as gross production (P) and the difference of gross minus regenerated production as net production (P — R) then we can also express
Specific examples of marine ecosystem models can be seen in Frost (1987), Fasham et al. (1990), Frost and Franzen (1992), and Loukos et al. (1997).
As shown in Fig. 10-13, there is also a flux of 02 produced during net photosynthesis from the ocean to the atmosphere and an export flux of particulate and dissolved organic matter out of the euphotic zone. For a steady-state system, new production should equal the flux of 02 to the atmosphere and the export of organic carbon (Eppley and Peterson, 1979) (when all are expressed in the same units, e.g., moles of carbon). Such an ideal state probably rarely exists because the euphotic zone is a dynamic place. Unfortunately, there have been no studies where all three fluxes were measured at the same time. Part of the difficulty is that each flux needs to be integrated over different time scales. The oxygen flux approach has been applied in the subarctic north Pacific (Emerson et al., 1991) and subtropical Pacific (Emerson et al., 1995, 1997) and Atlantic (Jenkins and Goldman, 1985). The organic carbon export approach has been evaluated in the equatorial Pacific (Murray et al., 1996), subtropical Atlantic (Michaels et al, 1994) and several locations by Buesseler (1998) and Hansell and Carlson (1998).
Integrated, interdisciplinary studies of elemental cycling in the euphotic zone have been one main focus of the Joint Global Ocean Flux Study (JGOFS) and these have contributed greatly to our understanding of carbon cycling in specific ocean regions. Multi-investigator JGOFS process studies have been conducted in the North Atlantic (North Atlantic Bloom Experiment; NABE) (Ducklow and Harris, 1993), Equatorial Pacific (EqPac) (Murray, 1995, 1996; Murray et al, 1994, 1997), Subtropical Atlantic and Pacific (HOT and BATS) (Karl and Michaels, 1996), Arabian Sea (van Weering et al, 1997; Smith, 1998), and Southern Ocean (Turner et al, 1995; Gaillard and Treguer, 1997; Smetacek et al, 1997).
10.3.3 Factors Affecting the Rate of Plankton Productivity
Liebig's Law of the Minimum states that under equal conditions of temperature and light, the nutrient available in the smallest quantity relative to the requirement of a plant will limit productivity. The "classic" approach for evaluating nutrient limitation is to compare the requirements of "average" marine plankton with "average" seawater. For example, plots of PO4 versus N03 in deep ocean seawater show a very tight correlation with a slope of slightly less than 6.0 and a small but significant intercept on the PO4 axis (e.g., Fanning, 1992; Gruber and Sarmiento, 1997). When these waters are upwelled nutrient uptake takes place with RKR proportions and N03 will run out first and become the limiting nutrient. Biological oceano-graphers have repeatedly demonstrated through enrichment experiments and observations of nutrient distribution that throughout the most of the coastal and open oceans phytoplankton productivity is most often limited by the availability of fixed inorganic N (Falkowski et al, 1998). There is approximately a ten-fold excess of inorganic carbon (largely as HCO3") in deep seawater relative to the availability of phosphorus and nitrogen availability which implies that carbon is never limiting in the ocean. In addition, except under very intense bloom conditions, the carbon fixed by plankton is provided by upwelling and not from the atmosphere. The reason deep ocean sea water is slightly depleted in N relative to RKR probably reflects nitrogen loss due to denitrification, which occurs primarily in the intense oxygen minimum zones of the eastern tropical north and south Pacific, the Arabian Sea and continental margin sediments throughout the world's oceans (Christensen et al., 1987).
In the subtropical ocean gyres the situation is more complicated (Perry, 1976). These regions are considered to be the marine analogs of terrestrial deserts because all nutrients are greatly depleted and biological biomass is small. A recent time-series study (The Hawaiian Ocean Time-series or HOT) has revealed that the ecosystem of the north Pacific subtropical gyre is temporally and spatially variable (Karl et al., 1995). This variability appears tied to the El Niño southern oscillation (ENSO) cycle. Increased stratification and decreased upper-ocean mixing during the 1991-92 El Niño event resulted in increased abundance and growth of nitrogen-fixing blue-green microorganisms called Trichodesmium. This resulted in a shift from the primarily nitrogen-limited regime that existed in 1981-90 to a phosphorus-limited condition in 1991-92. Growth of Trichodesmium spp. in subtropical habitats is favored under calm ocean conditions. Their ability to reduce N2 can remove the fixed-nitrogen limitation. N2 fixation may contribute up to half of the N required to sustain total annual organic matter export in this region (Karl et al., 1997). N2 fixation appears to be an important source of "new" nitrogen, especially under El Niño conditions. Thus the ecosystem in the subtropical gyres may switch periodically between N-limitation and P-limitation.
It has been argued that phosphorus limits oceanic productivity on the million year time scale (Broecker, 1971). The reason is that essentially all phosphorus in the ocean is introduced by rivers and thus ultimately from the weathering of continental rocks. This flux is, in effect, fixed by the rate of chemical weathering of the continents. By comparison fixed nitrogen can be derived from atmospheric N2 (via nitrogen fixation by Trichodesmium) as well as by weathering of rocks. The reservoir of atmospheric N2 is so large that nitrogen fixation can, over long time periods, adjust the overall supply of fixed nitrogen in seawater to the ratio needed by "average" plankton without significantly depleting the N2 source.
Silicic acid (H4Si04) is a necessary nutrient for diatoms, who build their shells from opal (Si02-«H20). Whether silicic acid becomes limiting for diatoms in seawater depends on the availability of Si relative to N and P. Estimates of diatom uptake of Si relative to P range from 16:1 to 23:1. Dugdale and Wilkerson (1998) and Dunne et al. (1999) have shown that much of the variability in new production in the equatorial Pacific may be tied to variability in diatom production. Diatom control is most important at times of very high nutrient concentrations and during non-steady-state times, perhaps because more iron is available at those times.
Over 20% of the world's open ocean surface waters are replete in light and major nutrients (nitrate, phosphate, and silicate), yet chlorophyll and productivity values remain low. These so-called "high-nitrate low-chlorophyll" or HNLC regimes (Chisholm and Morel, 1991) include the sub-arctic North Pacific (Martin and Fitzwater, 1988; Martin et al., 1989; Miller et al., 1991), the equatorial Pacific (Murray et al., 1994; Fitzwater et al., 1996) and the southern Ocean (Martin et al.,
1990). Iron concentrations are extremely low in these regions (Johnson et al, 1997). The main source is probably particulate iron associated with atmospheric dust (Duce and Tindale,
1991). The equatorial undercurrent appears to be an additional source of the equatorial Pacific.
The results of two successful iron-fertilization experiments in the eastern equatorial Pacific have clearly shown that phytoplankton growth rate is limited by iron at that location (Martin et al, 1994; Coale et al., 1996). The species composition and size distributions of the ecosystem are influenced by iron availability (Landry et al., 1997). In particular, large diatoms do not grow at optimum rates in the absence of sufficient iron. Loukos et al. (1997) used a simple ecosystem model with iron limitation to show that the main process causing the persistence of high surface nutrients was not the low specific growth rate of the phytoplankton assemblage but the efficient recycling of nitrogen as a consequence of the food web structure imposed by iron limitation. The first-order process responsible for low phytoplankton biomass is efficient grazing of the small cells by micrograzers, which is also an indirect consequence of iron limitation (Landry et al., 1997). Grazing balances primary production and controls phytoplankton biomass. Nevertheless, because of its impact on the food web, iron deficiency is the ultimate control of the HNLC condition.
Fluxes of continental dust preserved in ice cores of Greenland and Antarctica suggest a 30fold increase in dust flux during the last Glacial Maximum. Dramatic increases in new biological production in the HNLC regions may have resulted, resulting in the draw-down of atmospheric C02 (Martin, 1990).
Light is always necessary for photosynthesis (Raven and Johnston, 1991; Falkowski et al, 1992) and becomes limiting in the winter at high latitudes. In addition, the depth profiles of productivity and light energy correlate well at locations undergoing bloom conditions (Cullen et al., 1992). This suggests that the decline in productivity with depth reflects light penetration. Chemical constituents of seawater that can inhibit light penetration include dissolved humic substances (Gelbstuff) and suspended particulate matter. Both factors can become important factors in estuaries and other near-shore environments.
Trace metals can serve as essential nutrients and as toxic substances (Sunda et al., 1991; Frausto da Silva and Williams, 1991). For example, cobalt is a component of vitamin B-12. This vitamin is essential for nitrogen fixing algae. In contrast, copper is toxic to marine phytoplankton at free ion concentrations similar to those found in seawater (Sunda and
Guillard, 1976). The possibility that iron availability may limit primary productivity was discussed earlier.
Nickel is required by plants when urea is the source of nitrogen (Price and Morel, 1991). Bicarbonate uptake by cells may be limited by Zn as HCO-) transport involves the zinc metalloenzyme carbonic anhydrase (Morel et ai, 1994). Cadmium is not known to be required by organisms but because it can substitute for Zn in some metalloenzymes it can promote the growth of Zn-limited phytoplankton (Price and Morel, 1990). Cobalt can also substitute for Zn but less efficiently than Cd.
10.3.4 The Geographic Distribution of Primary Productivity
The geographic distribution of primary productivity in the ocean is shown in Fig. 10-14. High productivity is characteristic of marine zones where surface water is replenished with deeper water either by upwelling (as on western continental margins) or by deep mixing (as at high latitudes where stratification is less pronounced). Although upwelling regions are characterized by very high productivities (~300g C/m2 per year) they together contribute less than 1% of the total ocean production (Table 105). Coastal regions have mean productivities of about 100g C/m2 per year, but account for approximately 100 times the surface area of upwelling zones. These coastal regions contribute about 25% of the total primary production with the remaining 75% coming from the wide expanses (90% of total area) of low production (50 g C/m2 per year) open ocean.
Recently, the ocean-basin distribution of marine biomass and productivity has been estimated by satellite remote sensing. Ocean color at different wavelengths is determined and used to estimate near-surface phytoplankton chlorophyll concentration. Production is then estimated from chlorophyll using either in situ calibration relationships or from empirical functional algorithms (e.g., Piatt and Sathyendranth, 1988; Field et al, 1998). Such studies reveal a tremendous amount of temporal and spatial variability in ocean biological production.
Fig. 10-14 Approximate geographical distibution of primary productivity in the oceans (g C/m2 per year).
Percentage of ocean
Area (106 km2)
Mean productivity" (gC/m2/yr)
Total productivity" (1015gC/yr)
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