Wavelength nm

The proportioning of products depends on environmental conditions and the plankton's age. This complex process can be summarized by

Different species preferentially absorb different wavelengths of visible light during this process. The absorption spectra for three broad types of phytoplankton are shown in Fig. 4.1. Note that the algae denoted by a colour tend not to absorb that wavelength band.

In §2.1.1 we saw how solar radiation is rapidly absorbed within the ocean so that little such energy is left at a depth of 100 m. The penetrative power of solar radiation is also a function of the water clarity, as shown in Fig. 2.2. This will be a function of location. We further saw in that section that red light is preferentially absorbed by sea water, allowing the blue-green wavelengths to penetrate deeper. Too much radiation also limits photosynthesis. The zone of maximum photosynthesis is thus below the surface, where there is still sufficient radiant energy of the appropriate wavelengths, but not so much that reaction (4.1) is suppressed. These characteristics, and the annual and daily variation in the intensity of the phytoplanktons' energy source, will strongly control the temporal, vertical and geographical distribution and abundance of phyto-plankton.

There are other controls on phytoplankton abundance. Photosynthesis provides the phytoplankton with energy but the basic building block of the physical structure of the plant's cells is nitrogen. Nitrogen as a gas, N2, can be fixed by some species of algae, particularly freshwater species. The blue-green algae that are common in British waterways during hot, dry summers are one such group. However, oceanic species have to rely on absorption through cell walls of nitro-geneous compounds, particularly nitrate, NO3-, nitrite, NO2- and ammonium, visible light nCO2 + 2nH2O ^ nO2 + nH2O + n(CH2O)

respiration

Fig. 4.2. Typical seasonal

North Atlantic cycle in phytoplankton (solid line) and Zooplankton (broken line) communities in the

North Atlantic cycle in phytoplankton (solid line) and Zooplankton (broken line) communities in the from Parsons et al., Biological Oceanographic Processes,

North Atlantic. [Reprinted from Parsons et al., Biological Oceanographic Processes,

Copyright 1984, 330 pp., with kind permission from Elsevier Science Ltd., The Boulevard, Langford Lane, Kidlington

Phosphorus is another control on phytoplankton growth. It is also a component of the cell structure, but is more important as a constituent of the cell's genetic make-up, or DNA. It thus participates in cell reactions. Phosphorus is mostly absorbed by the organism as the phosphate ion, PO42-.

Nitrate and phosphate are known as limiting nutrients for phytoplankton. Even in good lighting conditions there must be adequate supplies of these compounds for phytoplankton growth to occur. Plankton growth shows a strong seasonal cycle away from the tropics, illustrated in Fig. 4.2 (see, however, §4.1.2). This is due to the interaction between light intensity and nutrient supply. During the winter there are low light levels, but the mixed layer is deepened to mix supplies of nutrient from the thermocline into the upper ocean. As spring begins light levels increase and the mixed layer shallows because of the onset of thermal stratification. There is fresh nutrient available and a riot of growth, known as the spring bloom, occurs. This quickly uses up nutrient, and also allows zooplankton - animal plankton that feed on the phytoplankton - to dramatically increase in population. For some months a balance is maintained between nutrient supply, phytoplankton growth, and predation by the zooplankton and other marine life-forms. There may be a second, smaller, bloom in the autumn as deepening of the mixed layer begins, while light levels are still conducive to active photosynthesis. To maintain productivity during the summer a mechanism for re-cycling nutrients is required, as only limited diffusion from beneath the thermocline, or from atmospheric input, replenishes the reservoir of nutrients. Bacteria provide this mechanism by decomposing dead organisms, or the excreta of living organisms, so that their cells become available as nutrients for later generations. The bacteria, of which a number of species are required to fully decompose any matter, secrete enzymes that react chemically with the detrital material. The process of bacterially-induced decomposition is called bacterial demoralization. This cycle is shown schematically in Fig. 4.3. The speed of conversion of nitrogenous or carboniferous material from waste through decomposition to re-incorporation into new organisms is very variable. It can occur over a few hours to days, or take several thousand years. During spring blooms this conversion seems to be particularly fast.

There are other factors important in phytoplankton growth such as temperature, salinity, pH, availability of iron, carbon and silicon. Iron and silicon may act as limiting factors for particular species. The iron required by phytoplankton cannot be assimilated from its commonest state in oceanic waters, as part of particles or colloids. It is obtained from dissolved iron compounds involving

Fig. 4.3. Schematic illustration of the cycling of nutrients within the mixed layer.

Fig. 4.3. Schematic illustration of the cycling of nutrients within the mixed layer.

the hydroxyl or OH- ion, or radical} Numerous field experiments suggest that productivity in the Southern Ocean, the equatorial Pacific and the sub-arctic North Pacific is significantly limited by iron deficiency. The dissolution of iron from its colloidal form is enhanced by photolysis involving light with blue-green wavelengths (300-400 nm). Iron may therefore be made available by this process for plankton uptake deep in the euphotic zone, and light restriction in the winter may be an important co-limiter of productivity.

Silicon functions in a similar way to nitrogen for some species, for instance diatoms. Silicate (SiO42-) is abundant in coastal waters because it comes from dissolved clays in run-off. Diatoms are thus often restricted to these areas, or appear in number in much of the ocean only during the spring bloom, when the winter supply of entrained silicate is still relatively abundant.

The oceanic concentration of the basic building block of the phytoplankton cells - carbon - is two or three orders of magnitude greater than nitrate or phosphate. Carbon dioxide, through photosynthesis, is vital for this conversion. Atmospheric levels of CO2 seem high enough not to limit phytoplankton growth but laboratory-based experiments have indicated that sufficiently low oceanic partial pressures may hinder cell growth. Biological productivity may therefore have been CO2-limited in some areas under the significantly lower atmospheric carbon dioxide levels of glacial times (~180 ppm - parts per million). The interaction of atmospheric CO2 and marine productivity in glacial climates will be considered further in §6.2.1.

Notwithstanding this discussion, however, the limiting factors for the growth of most species and regions are the supply of nutrients, iron and light, while temperature acts as a regulator of the speed of growth.

4.1.2 Geographical variation

The growth cycle of phytoplankton given in Fig. 4.2 is very much a North Atlantic picture. In fact there are radically different seasonal cycles in different parts of the world ocean, depending on the background light, nutrient and temperature patterns. Four different regimes are shown in Fig. 4.4, for

1 These compounds contain Felll, or iron in compounds where the oxidation state is such that there are three fewer electrons associated with the iron atom than in the elemental atom.

Fig. 4.4. Summary of seasonal cycles in plankton communities in different parts of the world. Phytoplankton biomass is shown by a solid line, zooplankton with a broken line. The scales are relative and not necessarily comparable from one site to another. [Reprinted from Parsons et al., Biological Oceanographic Processes, Copyright 1984, 330 pp., with kind permission from Elsevier Science Ltd., The Boulevard, Langford Lane, Kidlington OX5 1GB.]

polar waters, North Atlantic, North Pacific and tropical areas. In the polar oceans there is only sufficient light for one bloom during the summer. This occurs rather later in the season than the spring bloom of the North Atlantic double-bloom cycle. In the North Pacific, zooplankton of the species Calanus pulchrus and Calanus cristatus hatch from eggs laid by adults in late winter just as conditions become propitious for phytoplankton growth. No bloom is seen because these young zooplankton consume any new growth. Late in the summer the zooplankton population may decline sufficiently for a small autumn bloom in phytoplankton. In the tropics there is sufficient light and heating throughout the year, so that there are continual small rises and falls of population as zooplankton and phytoplankton populations, and nutrient levels, interact.

There are also changes from year to year in plankton population. These can be linked to changes in the climate, through winter mixing variability, temperature and light availability. For instance, during the cooling of western Europe in the 1960s (see §6.4.5) there was a marked reduction in the size of phytoplankton blooms in the North Sea, as well as a shortening of the growth season. Large variations are also experienced in tropical Pacific primary production during El Nino events (§5.2). There may also be changes in the supply of nutrients from land run-off. Disease and fishing practices may also alter the balances within the ocean that affect phytoplankton stocks.

The total phytoplankton mass produced per year within the world ocean is about 31 x 109 tonnes of carbon. The geographical variation in this is strongly tied to the shape of the ocean basins. As can be seen in Fig. 4.5, most of the ocean's production occurs in coastal zones, where the supply of nutrients is continually renewed from land sources. We saw in §3.5 that airborne nutrient

Fig. 4.5. Distribution of primary production in the World Ocean. A global average over September 1997 to April 2002, from SeaWiFS. Lighter shading indicates more productivity. Note the coastal bias, the equatorial tongues of high productivity and the lower production in the sub-tropical gyres. [Image courtesy of NASA.]

Fig. 4.5. Distribution of primary production in the World Ocean. A global average over September 1997 to April 2002, from SeaWiFS. Lighter shading indicates more productivity. Note the coastal bias, the equatorial tongues of high productivity and the lower production in the sub-tropical gyres. [Image courtesy of NASA.]

deposition into the sea is negligible, other than for iron (see also §4.1.4). Some coastal production, however, is associated with upwelling zones, where nutrients are supplied from beneath the thermocline. Such regions are off West Africa, Arabia, and the western coast of South America. In §2.11.2 we discussed the physical processes responsible for this upwelling.

The centres of the sub-tropical gyres are clearly minima of production. This occurs because of the basic downwelling that is induced by Ekman convergence within these gyres, as discussed in §2.11.1. Tropical regions also tend to have low production, as Fig. 4.4 suggests, because of the intense and continual competition between zooplankton and phytoplankton; there is also little entrain-ment of nutrients from below the mixed layer and in some regions a shortage of available iron. Exceptions to this are found near the equator, particularly in the Atlantic and eastern Pacific, although the latter's production can be limited by iron. The upwelling induced along the equator by Ekman divergence, discussed in §2.11.3, brings supplies of nutrient and carbon dioxide to the surface allowing greater productivity.

There are other regions of the ocean where production seems higher than might be expected. The northern Atlantic and Pacific, the Atlantic sector of the Southern Ocean, and southeast of Australia all have elevated levels of production. This is partly because these areas of ocean are away from the centres of the sub-tropical gyres, but they are also regions of active eddy formation in vigorous currents. When cold, cyclonic eddies are produced by the instabilities in these currents the resulting upwelling feeds nutrients to the surface waters, allowing enhanced production. Such eddies can sustain enhanced production for many months, producing local phytoplankton growth in the middle of the North Atlantic gyre, for instance. Western boundary currents, and the Antarctic Circumpolar Current are prime locations for copious eddy formation. It should also be noted, however, that warm, anticyclonic eddies have the opposite effect. Their downwelling flow tends to decrease productivity by stopping entrainment of deep supplies of nutrients.

Fig. 4.6. Abundance of the Zooplankton species Sagitta pseudoserratodentata along a 3 km track. Note the large-and small-scale structure in the patchiness. [After Parsons et al., 1984.]

Fig. 4.6. Abundance of the Zooplankton species Sagitta pseudoserratodentata along a 3 km track. Note the large-and small-scale structure in the patchiness. [After Parsons et al., 1984.]

Enhanced productivity can also occur in the sea beneath intense atmospheric depressions. These can generate local cyclonic eddies within the water, producing upwelling and sparking a local phytoplankton bloom. This effect is generally relatively limited, however, as deep penetration of mixing needs thermodynamic, as well as wind, forcing.

Superimposed on these larger scale geographical variations are dramatic changes over small distances. Phytoplankton respond to the local conditions, as well as the wider constraints of the general circulation. Thus populations can be very patchy. Fig. 4.6 shows variation in a zooplankton species, Sagitta pseudoserratodentata, over a 3 km distance, revealing changes in population size approaching an order of magnitude. There can also be sharp variations in production near oceanic fronts, separating water of distinctly different characteristics, as these zones tend to be regions of convergence. Langmuir circulation cells are another small-scale feature that can be associated with local productivity variability, as we saw in §2.10.2. Full description of planktonic activity in the ocean is therefore an extremely difficult task and beyond current technology, even using satellite-based instruments discussed in the next section.

4.1.3 Vertical variation and ocean colour

Primary production is often traced by measuring the chlorophyll concentration, as this chemical is associated with photosynthesis and is well correlated with plant biomass. Peaks in chlorophyll can be found both near the surface, and at significant depth. However, a chlorophyll maximum generally occurs near the base of the euphotic zone. This is in association with the nitracline, the region where nitrate concentrations begin to rapidly increase with depth from the surface minimum.

There are a number of factors contributing to this production. The light levels are often less than 10% of those at the surface, but nutrients are more abundant in this region. Also, there is less vertical motion in the water column near the base of the mixed layer, allowing the plankton more time to take up nutrients. In §4.1.1 it was seen that accessible iron may be more readily available through photolytic reactions at these depths.

The depth of this chlorophyll maximum is variable, being as deep as 100-250 m in the sub-tropical gyres and as shallow as 20 m in coastal seas. This variation is dependent on the clarity of the water, as well as the circulation patterns. There is also a feedback between phytoplankton population and this chlorophyll level. The more production generated the less light penetrates to deep water, because of the shading effect of the organisms. Highly productive regions thus have relatively shallow chlorophyll maxima. This will affect the heat balance of the upper ocean by allowing more or less radiational energy to penetrate to a given depth.

The concentration, and vertical variation, of phytoplankton thus affects the light transmissivity of the ocean. The absorption of radiation with depth will not only depend on production, but also the colour of the ocean with respect to the atmosphere. We have seen, in §2.1.1, that sea water preferentially absorbs at the red end of the solar spectrum. In the absence of algae the water will appear blue. Phytoplankton tend to absorb wavelengths at both ends of the spectrum, leaving the green band to be reflected; hence the green colour of chlorophyll in the sea (and in leaves). Primary production will therefore tend to make the sea appear green, rather than blue. Coastal waters are often a rich mix of production and sediment, becoming in consequence a murky grey colour.

This variation in the properties of light reflected from the ocean surface can be used by satellites in estimating the amount of production occurring in the oceans beneath them. The Coastal Zone Colour Scanner (CZCS) was an instrument aboard NASA's NIMBUS-7 satellite, and was in orbit in the late 1970s and 1980s. More recently, in August 1997, the Sea-view Wide Field-of-View Sensor (SeaWiFS) was launched, and at the time of writing had provided five years of data. The global average chlorophyll a concentration over September 1997 to April 2002 is shown in Fig. 4.5. These two satellite missions have provided considerable information about the distribution of production over the world's oceans.

The variation in the wavelength characteristics of reflected light from the oceans due to changes in biological productivity also has a climatic influence. Pushing the predominant wavelength of reflected light from blue to green, if productivity increased, could lead to a greater reflection of energy from the ocean surface, because there is more energy in the green band of the solar spectrum (Fig. 1.2). Regions of productivity would therefore reflect more incident radiation, and also allow less of it to warm the lower reaches of the upper ocean. A contrasting effect of phytoplankton is the warming of the upper ocean by the increased absorption of radiation as biomass increases. Quantitative estimates of ocean production derived from satellite remote sensing are, however, complicated by the unsolved problem of how to extrapolate the observed surface signature to the peak productivity at depth.

4.1.4 Iron from aerosols

As we saw in §4.1.1, wind-borne, or aeolian, iron appears to be a limiting nutrient in several large regions of the global ocean. There are strong seasonal variations in aeolian iron fluxes in the different oceanic basins. Most of the aeolian iron enters the oceans in the Northern Hemisphere, with the summer flux rates approximately twice those of winter. The high iron fluxes tend to

Fig. 4.7. Global ocean mean optical depth for July 2001, from MODIS. Note the strong aerosol signal, and hence iron flux, downwind of desert regions in the sub-tropics. Polar regions are masked. [Courtesy of NASA.]

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