Nutrients

In addition to dissolved carbon dioxide, which is present in seawater in ample quantities to support the most prolific naturally occurring plant growth, there are other substances, the nutrients, which plants also extract from the water and which are essential for their growth. Many of these are minor constituents of seawater, present only in very low concentration, and their supply exerts a dominant control over production. Nitrate and phosphate are of special importance. Where the quantities of these ions are known, theoretical estimates of the potential productivity of the water generally accord well with observed values. Iron, manganese, zinc and copper are other essential nutrients, silicon is required by diatoms, and molybdenum and cobalt and probably other elements

Compensation Depth

Figure 5.6 Generalized diagram to illustrate changes in depth distribution of phytoplankton, compensation depth and critical depth between (A) late summer and (B) late autumn. With onset of winter, further decline of illumination causes the compensation and critical depths to ascend further until extinguished at the surface. The biomass of phytoplankton decreases rapidly.

Figure 5.6 Generalized diagram to illustrate changes in depth distribution of phytoplankton, compensation depth and critical depth between (A) late summer and (B) late autumn. With onset of winter, further decline of illumination causes the compensation and critical depths to ascend further until extinguished at the surface. The biomass of phytoplankton decreases rapidly.

are necessary for some plants. Organic compounds dissolved in the water (DOM) may be important in some cases (see Section 4.3.3).

The absorption of nutrients by the phytoplankton reduces the concentration of these substances in the surface layers, and this limits the extent to which the plant population can increase. A certain amount of the nutrients absorbed by phytoplankton may be regenerated and recycled within the euphotic zone, but in deep water plants are continually being lost from the surface layers through sinking and by consumption by zooplankton which moves to deeper levels during the daytime. Many of the nutrients absorbed from the surface layers are therefore regenerated in the deeper and darker layers of water where plants cannot grow. Consequently, nutrients accumulate at deep levels due to the continuous transfer of material from the surface. This loss of nutrients from the productive layer of the sea to deep levels contrasts with the nutrient cycle of the land surface. In soil the breakdown of organic compounds releases nutrients where they are quickly available for reabsorption by plant roots, thereby maintaining the fertility of the land. In the sea the continuance of plant growth depends to a great extent upon the rate at which nutrients are restored to the euphotic zone by mixing with the nutrient-rich water from below. The lower overall productivity of the sea compared with the land is largely a consequence of the regeneration of nutrients in the sea far below the zone of plant growth, with recycling dependent upon relatively slow processes of water movement. The greater productivity of coastal areas compared with deep water is partly a consequence of more rapid recycling of nutrients where the sea bottom is closer to the productive layer.

Some of the vertical mixing processes which restore fertility to the surface layer of the sea are summarized below.

Upwelling

Offshore winds, by setting the surface water in motion, may cause water from deeper levels to be drawn up to the surface (Figure 5.7). We have mentioned earlier the upwelling which occurs in low latitudes along the western coasts of the continents to replace the westward-flowing surface water in the equatorial currents (see Section 1.3.3). Although this upwelling water probably does not rise from depths greater than some 100-200 m, this is deep enough to supply nutrients to the Canaries Current, Benguela Current, Peru Current, California Current and West Australia Current, and these are all areas of high fertility. In the Southern Ocean, continuous upwelling ensures that, even during the highly productive period of the Antarctic summer, plant growth is probably never limited by shortage of nutrients. In the Arctic, where upwelling is less, there is some depletion of surface nutrients during the summer months and production is correspondingly reduced (Smith, 1968).

Upwelling also occurs at divergences, i.e. areas where adjacent surface currents

--Prevailing wind

Surface layers set in motion by wind

Deep layers upwelling replace surface water

Sea surface

Deep layers upwelling replace surface water

Figure 5.7 Upwelling due to wind action at the surface.

move in different directions. In low latitudes, divergences between the equatorial currents and countercurrents cause upwelling close to the Equator and along the northern boundary of the equatorial counter-currents, and these are important in maintaining the productivity of tropical waters.

Turbulence (eddy diffusion)

Turbulence is a term loosely applied to various complex and irregular movements of the water in which different layers become mixed by vertical eddies. The effects of turbulence on production depend upon the circumstances. It may promote production by bringing nutrients to the surface, or it may sometimes reduce production by carrying down a considerable part of the plant population below the compensation depth. Broadly, high production is likely to follow a limited period of turbulence once the water column becomes sufficiently stabilized for the phytoplankton to flourish in the replenished surface layer without undue loss of plants by downward water movements. In temperate latitudes these conditions occur at the end of winter.

The following are examples of how turbulent water movements may arise:

• Convection When surface water cools, its density increases and convectional mixing commences once the density of the surface layers begins to exceed that of underlying water, the surface water sinking and being replaced by less dense water from below. In high latitudes, convectional mixing is virtually continuous because heat is continually being lost from the surface. In temperate latitudes, convectional mixing occurs during the winter months to depths of some 75-200 m, but ceases during the summer. The corresponding seasonal changes in production are discussed later (see Section 5.4). In low latitudes where the surface waters remain warm throughout the year there is little if any convectional mixing, and the concentration of nutrients at the surface is generally low unless vertical mixing is occurring from some other cause, such as wind action on the surface causing upwelling.

• Currents Vertical eddies may arise where adjacent layers of water are moving at different speeds, or where currents flow over irregularities on the sea-bed. On the continental shelf, especially where the bottom is uneven, strong tidal currents may cause severe turbulence and keep the water well mixed throughout its depth. Tidal flow in the eastern part of the English Channel and the southern part of the North Sea produces sufficient turbulence to mix almost the full depth of water, preventing the development of seasonal thermoclines and helping to maintain the fertility of the area throughout the summer months.

• Internal waves The depth to which surface waves appreciably move the water is rather less than their wavelength. Although swell waves in deep water occasionally exceed 100 m in wavelength, most surface waves are considerably shorter than this and do not mix the water column to any great depth. However, where the water column is not homogeneous, it is evident that very large internal waves can exist far below the surface. Over a great part of the ocean the water is stratified in layers of different density - density increasing with depth - and these layers do not remain still but oscillate about a mean level. Internal wave movements of at least 200 m in height have been detected in the deep ocean.

Internal waves may be produced in several ways. For example, strong onshore winds driving light surface water towards the continental slope will bend the equal-density layers downwards. When the wind ceases, the low-density water which has been forced down the continental slope will return to the surface, and the displaced density layers may not return simply to the horizontal but will probably oscillate up and down, transmitting their motion over great distances as waves in the deep layers.

Oscillations in the deep layers of the North Atlantic may also arise from irregularities in the rate of southward flow of deep water from the Arctic over the north Atlantic ridges, due perhaps to changes of Arctic climate influencing the rate of sinking of surface water. Cooper (1961) has suggested that this flow is intermittent, and that sometimes enormous 'boluses' of cold water flood down the south side of the ridge, displacing the deep density layers and setting up internal waves.

Cooper (1952) maintains that internal waves can cause vertical mixing where they impinge upon the continental slope, their motion here becoming translated in a manner comparable with that of waves breaking on the shore, carrying deep water up the continental slope much as surface waves run up a sloping beach. In this way, oceanic deep layers rich in nutrients may sometimes spill over the contintental edge, mixing with and increasing the fertility of shelf water.

If onshore winds are sufficiently strong and continuous, the displacement of the density layers may become so severe that the water column becomes unstable. A profound disturbance may then ensue which Cooper has termed 'capsizing' or 'culbute mixing'. There is no exact English equivalent for the French word 'culbuter', meaning to upset violently resulting in a confused heap or jumble, which well describes this process.

Capsizing or 'culbution' is a cataclysmic disturbance of the water column which is believed to occur if low-density water is forced so far down the continental slope that it eventually comes to lie beneath water of greater density. This unstable, topsy-turvy condition is thought to resolve by the low-density water, which has been forced downwards, bursting up towards the surface, i.e. the water column capsizes. The ensuing upheaval must produce a homogeneous mass of water from surface to bottom. This water mass, being a mixture of surface and deep water, is denser than adjacent surface water and will subsequently subside to its appropriate density level. The process must be continuous as long as onshore winds of sufficient strength persist, the line of capsizing gradually receding seawards and involving progressively deeper water. The water blown over the continental shelf will be capsized water containing a component of deeper water, and therefore richer in plant nutrients than ordinary oceanic surface water. Its nutrient content may be expected to increase as the sequence proceeds and extends to deeper levels. This process is described in detail in Figure 5.8.

• Frontal mixing (Pingree et al., 1975; Pingree, Holligan and Mardell, 1978, 1979). During the summer months, blooms of phytoplankton are often associated with boundary mixing zones between bodies of colder, vertically

Figure 5.8 From Cooper (1952), by courtesy of Cambridge University Press.

(a) A frequent pattern of isosteres south of the Celtic Sea in winter when the uppermost 75-100 m of water in the ocean is homogeneous and is overlying water with density and content of nutrient salts increasing downwards.

(b) A pattern of isosteres in winter over the continental slope, drawn to a scale of 1:4, with no forces operative. The amount of light water which will later lie to windward is considered unlimited.

(c) and (d) Cushioning of light oceanic surface water against the continental slope brought about by on-slope gales. The resistance of the solid slope to further progress of the foot of the light water is, however, absolute causing the isosteres to curl and ultimately to become vertical. Restoring Archimedian forces in (d) have then become zero.

(e) The drag of the surface wind current will draw the upper strata of stratified water with it leading to an unstable density inversion as illustrated.

(f) The unstable tongue of heavier water will capsize violently, leading to a homogeneous mass of mixed water extending from the surface to the depth of the capsizing water mass. New isosteres, bracketing the capsized water mass, are so created.

How Draw Continental Slop

(g) The newly formed surface water will be heavier than the surface water inshore and to seaward and will subside to form a lens of homogeneous water at its appropriate density level and will be replaced by fresh oceanic surface water blown in from seaward. Processes (d)-(g) must be considered as simultaneous parts of a continuous process, with the line of capsizing receding from the continental slope and the maximum depth of the phenomenon increasing as the gale proceeds. Consequently, the water blown on to the continental shelf of the Celtic Sea by a strong southerly gale should be entirely capsized water and richer in nutrients than it would be if it were solely oceanic surface water. Moreover, the nutrient content of the water passing on to the shelf should increase as the gale proceeds, and the depth of capsizing becomes greater.

mixed water and warmer, strongly stratified water. Around the British Isles such zones occur mainly at the western end of the English Channel and across the central North Sea. These thermal fronts move irregularly under the influence of tides and wind, mixing the water in complex ways, sometimes characterized by extensive cyclonic eddies which persist for several days. In such transitional areas, stratified and unstratified water are closely intermingled in changing, unstable relationships. Over deep water these conditions appear to favour rapid phytoplankton growth by combining sufficient stratification to prevent excessive loss of plants below the compensation depth while at the same time effecting sufficient vertical mixing to supply ample nutrients.

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  • rosamunda
    Why are marine plant nutrients lost from the surface?
    8 years ago

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