Nutrientlike behaviour

As in continental waters (see Section 5.5.1), NO-, DIP (dissolved inorganic phosphorus) and silicate are usually considered to be the limiting nutrients for biological production, although in some situations it has been suggested that trace elements, particularly iron, may be limiting (Section 6.6). Excepting high-latitude areas the oceans are so large and deep that they are effectively permanently stratified. The production of biological material removes nutrients from surface waters (Box 6.6). After death, this biological material sinks through the water column, decomposing at depth to re-release the nutrients. The nutrients are then slowly returned to surface waters by deep-ocean mixing processes and diffusion. The net result is that the vertical profiles of nutrients are characterized by low concentrations in surface waters (where biological utilization rates exceed supply rates) and deep-water maxima, where decomposition rates exceed uptake rates because of the absence of light (Fig. 6.20). Nitrogen and DIP are cycled

Molybdenum (nmol l 1) Tungsten (nmol l 1)

Molybdenum (nmol l 1) Tungsten (nmol l 1)

Fig. 6.19 Vertical distribution of dissolved molybdenum and tungsten in the North Pacific. After Sohrin et al. (1987), with permission from Elsevier Science.

with the organic tissue of organisms, while silicon and calcium (Ca) are cycled as skeletal material. The decomposition of organic tissue is mainly by bacterial respiration, a rapid and efficient process. By contrast, skeletal material is dissolved slowly (Sections 6.4.4 & 6.4.5). The effect of these different decomposition rates is that the NO- and DIP concentration profiles show rapid increase with depth, implying shallower regeneration of material in the water column than silicon. Nitrate and DIP distributions are therefore closely correlated (Fig. 6.21) with a slope of approximately 16:1. This ratio reflects the relative proportions of nitrate and DIP regeneration and utilization by phytoplankton. This ratio is often referred to as the Redfield Ratio in honour of Alfred Redfield who first described the close linking of these two ions in the ocean.

Biological cycling not only removes some ions from surface waters, it also transforms them. The stable form of iodine (I) in seawater is iodate (IO-), but biological cycling results in the formation of iodide (I-) in surface waters, because the production rate of the reduced species is faster than the rate of its oxidation. Biological uptake of IO- in surface water results in a nutrient-like profile, contrasting with the conservative behaviour of most halide ions, for example Cl- and Br-. The biological demand for NO- also involves transformation. Phytoplankton take up NO- and reduce it to the -3 oxidation state (see Box 4.3 & Fig. 5.12) for

Box 6.6 Oceanic primary productivity

The rate of growth of phytoplankton (primary productivity) in the oceans is mainly limited by the availability of light and the rate of supply of limiting nutrients (usually accepted to be nitrogen (N), phosphorus (P), silicon (Si) and iron (Fe)).The need for light confines productivity to the upper layers of oceans. Also, in polar waters there will be no phytoplankton growth during the dark winter months.

In temperate oceans there is little winter productivity because cooling of surface waters destroys the thermal stratification and winds mix waters and phytoplankton to depths of hundreds of metres. This deep mixing, which occurs in all temperate and polar oceans, also means that vertical gradients in nutrient concentrations are temporarily eliminated, allowing a corresponding rise in surface-water nutrient concentrations. In spring, surface-water stratification is re-established as the waters warm and winds decrease. Once enough light is available, vigorous plankton growth begins. Nutrient concentrations are high, having built up over the winter, resulting in the 'spring bloom' of phytoplankton. The bloom is generally later

Fig. 1 Seasonal cycles of productivity and global average rates of primary production. Reprinted from Koblentz-Mishke et al. (1970), with permission from Scientific Exploration of the South Pacific, Courtesy of the National Academy of Sciences, Washington, DC.

(continued)

in the spring — and larger — moving polewards.

The duration of the spring bloom is limited by nutrient availability and/or grazing by zooplankton. Phytoplankton growth and abundance then decline to lower levels, which are maintained throughout the summer by nutrient recycling within the euphotic zone. In some locations, limited mixing in autumn can stimulate another small bloom, before deep winter mixing returns the system to its winter condition.

In tropical waters, vertical stratification persists throughout the year and production is permanently limited by nutrient supply rates, which are controlled by internal recycling and slow upward diffusion from deep water. Under these conditions, productivity is low throughout the year.

These seasonal cycles of productivity are shown schematically in Fig. 1, along with a map of current estimates of primary production rates. In the north Pacific zooplankton population growth supresses the spring bloom.

Since production rates vary with time and place, the data on the figure are uncertain, but are consistent with satellite-derived maps of chlorophyll concentrations (see Plate 6.3, facing p. 138). These maps show that, on an annual basis, the short, high-production seasons of temperate and polar areas fix more carbon in organic tissue than organisms in tropical waters. There are a few exceptions to this, in so-called upwelling areas, for example the Peruvian, Californian, Namibian and North African coasts and along the line of the equator (Plate 6.3, facing p. 138 )& Fig. 1). In upwelling areas, ocean currents bring deep water to the surface, providing a large supply of nutrients in an area with abundant light. Very high rates of primary production ensue and the phytoplankton are the basis of a food chain that supports commercially important fisheries.

The spring bloom and upwelling areas are not simply times and regions of higher productivity; changes in the structure of the whole ecosystem result. For example, the phytoplankton community in areas of higher production is usually rich in diatoms, organisms which, upon death, efficiently export carbon and nutrients to deep waters. This contrasts with the tropics, where the phytoplankton community has adapted to the low-nutrient waters by recycling nutrients very efficiently, with little export to deep waters. Phytoplankton are able to live in low-nutrient tropical waters because they are typically smaller, with larger surface area to volume ratios that increase their efficiency in diffusing nutrients across the cell wall.

utilization in proteins. When phytoplankton die they decompose, releasing the nitrogen as ammonium (NH+) hence N is still in its -3 oxidation state. Similarly, when phytoplankton are eaten by zooplankton, the consumers excrete nitrogen primarily as NH+. This NH+ is then available for reuse by phytoplankton: NH+ is the preferred form of available nitrogen since there is no energy requirement in its uptake and utilization. Alternatively, the ammonium is oxidized via nitrite (NO-) to NO-, the thermodynamically favoured stable species. These rapid recycling processes maintain euphotic zone NH+ at low concentrations. In the deep ocean, the only NH+ source is from the breakdown of organic matter sinking from the surface waters. The amount of NH+ released from this source is small and rapidly oxidized, maintaining very low NH+ concentrations.

Elements showing nutrient-like distribution often have long oceanic residence times, although shorter than conservative elements. The residence times of NO3-silicon and DIP have been estimated to be 57000, 20000 and 69000 years respectively (Table 6.9). The vast reservoirs of nutrients in the deep ocean mean that increases in the concentrations of NO3- in riverwaters due to human activity (see

Fig. 6.20 Vertical distribution of dissolved nitrate (a), phosphorus (b) and silicon (c) in the Atlantic, Pacific and Indian Oceans. After Svedrup et al. (1941), reprinted by permission of Pearson Education Inc., Upper Saddle River, NJ.

Section 5.5.1) have little effect on oceanic NO- concentration, assuming that NO- is effectively mixed throughout the ocean volume (Section 6.8.3).

In addition to the actual nutrient elements, many other elements show nutrient-like behaviour in the oceans, i.e. low concentrations in surface waters and high concentrations at depth (Fig. 6.22). This distribution implies that biological removal rates from surface waters are rapid, although it does not prove that these elements are limiting, or even essential, to biological processes. In the case of some metals (e.g. zinc (Zn)), a clear biological function has been established. However, for other metals (e.g. cadmium (Cd)), there is less evidence for a biological role; cadmium is usually thought of as a poison, although not at the extremely low concentrations (<0.1 nmoll-1) found in seawater. Elements like cadmium probably show nutrient-like behaviour (Fig. 6.22) because they are inadvertently taken up during biological processes, or substitute for other elements when these are in short supply. The Cd2+ ion has chemical similarities to Zn2+, thus the nutrient-like cycling of cadmium may reflect inadvertent biological uptake with—or substitution for—zinc.

Finally, we should be aware that even metals with a clear biological role (e.g. Zn2+) can be toxic at sufficiently high concentrations (see Box 5.5). This reminds

Fig. 6.21 Global relationship between dissolved nitrate and phosphate in seawater based on US Geochemical Ocean Sections (GEOSECS) programme data.

us that all elements are potentially toxic, making terms like nutrient impossible to apply in an absolute sense.

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