California Association of Chemistry Teachers
John W. Vanlandingham
Bureau of Commercial Fisheries Washington, D. C.
I n this modern age of space consciousness and sub-space exploration, most of us remain unaware of a less spectacular but equally determined thrust into the inner space of the great oceans and seas covering more than two thirds of the earth's surface. The pressure harrier encountered in this submarine world has been just a ; ~effective in limiting man's exploration of the ocean depths as the thermal barrier has been in postponing his direct entry into the fringe areas of interplanetary space, long after he had achieved the rocket capability to do so. Today the major governments of the world are pouring great sums of money and expending much time and effort into a study of the chemical, physical, and biological properties of the marine environment. The purpose of the discussion here presented is threefold: to illustrate briefly the practical factors which make such research necessary, to discuss the inter-relationship of the above mentioned properties, and to present a semiquantitative, tridimensional study of the distribution of representative inorganic nutrient materials. The most representative of these nutrients are phosphate-phosphorous and nitrite or uitratenitrogen. The scope of this paper will he limited to inorganic phosphate and dissolved oxygen. Dissolved oxygen is being discussed for three reasons: firstly, it is just as vital to the welfare of the marine organism as are the nutrients; secondly, because it is important to the physical oceanographer in his study of the structure and transport of water masses; and finally, because there is a unique relationship between the distribution of dissolved oxygen and that of the nutrients. Most people might suppose the oceans to be rather homogenous masses of water in which fish and other organisms are randomly distributed. However, years of chemical, physical, and biological research have shown such conceptions to be erroneous. We now know that the chemical and physical properties of sea water vary considerably with respect to both the horizontal and vertical dimensions of the oceans. Paralleling this variability in the properties of sea water we frequently find a systematic diversification (or in some cases limitation) of the bio-types encountered in any particular environment ($2, 3). Since the harvest of the sea is a vital factor in our economy it has become increasingly important to study this problem of nutrient distribution and its relationship 272
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to the biological population on an ever expanding scale. Any such study of nutrient distribution must first consider the structure of the oceanic water masses. The most important aspect of this structure involves the vertical profile of the water from the surface to the bottom. This column may be divided into three zones, the uppermost of which is called the euphotic or photosynthetic zone. This is the layer of the water mass in which the intensity of the penetrating sunlight is sufficient to provide the photochemical energy required by the chlorophyll hearing organisms. This zone extends to a depth of 100 or 200 m ( 1 , 4 ) depending upon the clarity of the water and the intensity of the ambient sunlight. The euphotic zone is the most heavily populated region in the water column and is the one which is most seriously affected by climatic conditions a t the surface. The predominant life forms in this zone are the phytoplankton and zooplankton, the microscopic plants and animals of the sea. Far below the euphotic zone lie the abyssal depths where conditions are quite different from those near the surface. Here turbulence and light penetration are practically nil, and abyssal current velocities are almost imperceptible compared to those in the upper layers. Abyssal water is isolated from the sunlit euphotic zone by the midwater region, a zone of gradual but continuous change in physical properties and in nutrient concentration. The mid-water zone is usually preceded by a distinct layer of water which is characterized by rapid changes in its physical properties. The abrupt nature of these changes is responsible for the descriptive name "discontinuity layer" (5) which has been applied to this zone. The thickness of the discontinuity layer as well as the depths of its upper and lower boundaries are quite variable. The upper limit frequently reaches well up into the euphotic zone and the lower boundary may extend down to 200 m (8). This zone is just as importaut to the marine community as is the euphotic zone. Many of the larger forms of marine animals appear to prefer the lower temperature beneath the euphotic zone hut in many cases will ascend into this region to forage for food (11). Vertical Transport of Water and Nutrients
In Figure 1 we have a schematic representation of the life cycle in the sea; beginning with the nutrient, chemicals at the bottom of the chart, progressing upward through the phytoplankton, branchiug on around
to include the zooplankton slid fish and finally returuing again to the nutrient materials to complete the cycle. The ~mtrientcycle actually begins in the land masses of thc cart,h, and as a rule, in the higher altitudes of these land masses. Here we have heavy rainfall situations predominating, with concnrrent leaching of the soluhle nutrient ~natnrialsfrom the soil which run off, through the streams and rivers, and eventually drain into the sea. This alluvial irn-off, by reasou of its greater freshness, is less dense than sea water. Therefore it tends to splnad out on the surface of the sea and is carried into the open ocean by the prevailing surface currents. When these nutrients con~e in contact v i t h the marirw cnvironniei~t they are event,ually consumed h.v t,he phytoplankton and assimilated into ~-
LIFE CYCLE IN THE SEA
zone. Another type of upnelliug occurs a-heir a n ahyssal current impinges upon a submarine hauk (Fig. 3). On the current side of the shoal ahyssal mater is deflected up toward the surface and if the banks are high enough and the velocity of the current is great enoligh this deep mater reaches the euphotic zone. Notice also that on the leen.ard side of the shoal water is pulled upward by an eddy effect. A similar conditio~~ occurs when a surface curreut impinges upon an island mass (1:ig. 4). Here, however, the nutrient layer is deflected away from the surface on the current side of the land mass, hut agai~ion the leeward is forced upward hy aneddy effect. Vertical mater transport sometimes occurs far from the iuflue~rceof land masses or sea mounts. Such a situation develops wheu a warm tropical water mass converges with oue of significantly lower temperature. Here the t,hermal difference between the two water masses creates a great deal of vertical turhulence and mixing all along the points of juncture (9). The several zones of diffusion are represented in 1:igure 5 hy the varicolored bands. Another mechanism for vertical transport of nutrients is called vertical eddy diffusion (fi). This is a cyclical phenomenon produced by wave action on the surface ~vhichcreates a series of
. Figure 1 .
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~
~
~
LIGHT ZONE
The life cycle in the seo
their ccllular structure. 12rom this point 011 the chemical nutrieuts become all iutegral part of the living food chaiu. .I ~rat,uralresult of the metabolic activity of these orgauimis is to return a portion of the nutrieuts t o the water in the form of either soluhle or particulate organic matter. The particnlate nlatter with its adsorbed ious, together with the skeletal remains of those oryanisms vhich are not conipletely devoured by their predators (15), settle downward under the influence of gravity and eventually drop out of the euphotic and discoutinuity zones into the quiet waters of the midwater and abyssal depths. During the sedimentat,io~~ placess the particulate matter is attacked hy various species of bacteria i~lcludingthose of the chcinoa,v~~theticvariety. As a result of this hact,erial decon~positionsome of the uutrieut materials are regenerated in their soluble inorganic forms. The final st,age i n this nutrient cycle should then he to rzturn the regenerated material to the euphotic zone where it would again become available for conhy t,he phytoplankton. surnptio~~ Such vrrtical transport of mater and ~ ~ u t r i e n t material can take place by several means. Figure 2 illustrates one variety of vertical transport called upwelling. 111 this particular casc a wiud blowing parallel to the coast line of a land mass displaces surface water offshore a t an angle with the direction of the wind. The viscosity effect of this surface current tends to pnll nutrient-rich deep water up the sloping shelf' of the lnnd mass and disperse it in the euphotir
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DARK
ZONE
Figure 2. Upwelling produced b y wind a n d surface current in the vicinity of large land mosses.
SHOALS . -
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.
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LIGHT ZONE
HIGH NUTRIENT
HIGH NUTRIENT
Figure 3. Upwelling produced by o rub-rurfoce current impinging upon o rubmarine bonk.
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vertical eddies alo~igthe water column. The oscillatory motion of the water in these eddies increases the rate of diffusion of nutrient water upward against the force of gravity. However, this process is not nearly as effective as upwelling. A similar cyclical pattern of circulation is produced by convection cells (10). These cells are formed by convection currents arising from a negative thermal gradient, or temperature inversion, where a warm water mass is occluded within or beneath a cold water mass. Any co~iditionwhich inhibits this vertical movement of water is called stability (5). Such a condition is caused by a positive density gradient (i.e., a rapid increase in dcnsity with depth). The density gradient, in turn, is related to either a rapid decrease in temperature with depth, or a rapid increase in salinity, or in some cases a combination of these two effects. However, since this phenomenon is usually associated with a thermal gradient we call it the thermocline or discontinuity layer. Since large expanses of the ocean areas have a reasonably well developed thermocline either briefly during the warm summer season in temperaturn zones, or throughout the year in tropical regions, vertical transport is effectively retarded in most of the oceanic areas. The net effect then, is
ISLANDS
-
Figure 4.
Upwelling produced b y e surface current impinging on on
Figure 5. Upwelling produced by turbulence ot the boundary between two woter morrer.
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depletiou of the nutrients by biological utilization in the euphotic zone of these areas and regeneration and accumulation of nutrients ill the abyssal regions. Surface Distribution of Inorganic Phosphates
Let us now examine the distribution of inorganic phosphate in the surface waters along the north-south plane of the Pacific Ocean. I n theory rye mill take an oceanographic cruise from the equator to the Aleutian Islands, hegim~ingthe expedition in our o ~ island n paradise of Hawaii, which lies a t approximately 20 degrees north latitude. I11 the vicinity of Hawaii. in the immediate inshore and offshore waters, the island mass effect which has hecn discussed earlier is in operation. Phosphate values are generally higher and subject to fluctuations, the peaks of which usually coincide with the rainfall peaks during the winter season. However, as we leave the influence of the Hawaiian Archipelago and proceed northward toward the Aleutians, the surface phosphate concentration rapidly decreases and we find ourselves in a phosphate poor region where the concentration remains relatively constant a t 0.1 to 0.3 microgram atoms per liter. At approximately 30 degrees north latitude we enter a positive phosphate gradient wherp each change in our northerly progress is accompanied hy a corresponding increase in the concentration of surface phosphat~. At about 30 to 45 degrees north, depending upon the season of the year we are struck hy a tremendous change in the physical appearance of the water. The beautiful deep blue of the sub-tropical regions has given may to a murky gray-green color indicative of a heavy biological population. Chemical and physiral analysis of the mater reveals drastic changes in these properties. Phosphate has increased unt,il it is now ranging from 1 to 1.5 microgram atoms per liter. Oxygen has increased and salinity has decreased. The temperature of the xater is quite variable but generally much colder. There are several factors responsible for this remarkable change iii the ocean water. One is the fact that me have crossed a front between two distinct mater masses. Here the cold subarctie water mass converges with the nrarmer subtropical waters of the central Pacific. In addition we are now in a temperature zone u~herethermocline development weakens a t the end of the summer season and is almost completely broken down by the storms that prevail in the area during the fall and .vinter season (7). If v e proceed due south from Hawaii to the equator we find that the phosphate poor region extends as far south as 5 degrees north latitude. As \ye pass 10 degrees north latitude me enter one of the strange marine desert areas which border the equatorial zone. Here the surface phosphate concentration is very 1011and the water itself appears t o be almost completely devoid of plant or animal life. This paucity of marine life in the surface water is reflected in the chemistry of the bottom sediments. Continuing on toward the equator, a t about 5 degrees north latitude we again enter a positive phosphate gradient. And then in the neighborhood of 3 degrees north we again find all of the visual and analytical evidence of strong upvelling of nutrient rich mater and its highly productive biological population.
This should come as a surprise, tecause in this torrid tropical zone we could reasonably expect to find a welldeveloped thermocline existing throughout the year. However, other factors have entered into the picture. As a result of the combined effects of the trade winds and the rotation of the earth upon its axis, the action and reaction of the current systems from both hemispheres come to a focus along the equatorial belt. The interplay of these hemispheric forces promotes the upwelliog of deep nutrient rich water into the euphotic zone. We should keep in mind that we have been discussing the typical pattern of the distribution of surface phosphate in the Pacific Ocean and that there are frequently temporary and localized deviations from this pattern. For example, ships often pass through patches of discolored water which are called "plankton blooms." These blooms may reflect an unexpected and oftentimes inexplicable enrichment of the surface water. They are often followed by a depletion of the nutrient responsible and a subsequent disappearance of the phenomenon. From these observations we see that the surface concentration of inorganic phosphate is primarily determined by two factors-the rate of biological consumption and the rate of physical replenishment. Note also that phosphate concentration may be a limiting factor regulating the size of the bio-mass. Let us now examine the distribution of surface oxygen in these same waters. Surface Distribution of Oxygen
Normally all surface water is supersaturated with oxygen as a result of wind and wave turbulence a t the air-water interface, often in combination with a diurnal photosynthetic enrichment. The surface oxygen concentration is a t a minimal value in the tropical waters of the equatorial belt. Each change in northerly progress from this boundary is associated with a corresponding increase in the concentration of surface oxygen. Iu the latitudes of Hawaii the concentration value is approximately 5 ml per liter, and we will notice that there is no predictable difference between the concentrations in inshore and extreme offshore waters as we had in the case of phosphate. We now know that the concentration of dissolved oxygen on the surface is affected by many more variables than phosphate. For example, oxygen values are a result of the interaction of biological consumption, biological production, turbulence, temperature, and salinity (14). Surface oxygen is not normally a limiting factor to biological growth as is frequently the case with phosphate. However, there are rough correlations between oxygen and phosphate concentrations on the surface as well as a t specific points down the water column. For example, a heavy phytoplankton bloom resulting from phosphate enrichment will probably be associated with a significant increase in the degree of supersaturation. Verticol Distribution of Nutrients
In Figure 6 we have vertical profiles of the oxygen and phosphate concentration plotted against the depth in meters. We note the general inverse relationship between the concentrations of the two substances.
An increase in phosphate is associated with a decrease in oxygen, and the prominent features of one curve are replicated on the other although not necessarily at exactly the same depth. Observing the oxygen curve in detail, we note that the concentration is a high .5 ml per liter at the surface but increases to a maximum a t about 50 ni. From this point downward a gradual but continuous depletion of oxygen is observed which is markedly accelerated a t a depth of 500 m. Finally, a t 1200 m a minimal value is reached beyond which the concentration gradually increases. Extrapolation of this curve would show the oxygen concentration increasing in the deep abyssal waters until it might equal or exceed the surface value. Phosphate on the other hand is a low 0.2 microgram atoms per liter on the surface and diminishes even further immediately below. Beneath this point the concentratioil increases in direct opposition to that of oxygen u ~ ~ tthe i l abyssal maximum is reached. Extrapolation of the phosphate curve would show relatively small fluctuations about this maximal value. The most interesting portions of these profiles are the significant maxima and minima occurring in both the euphotic zone and the upper abyssal zone. These features are commonly found in many oceanic water masses. At one time these apparent anomalies were a complete mystery. However, we can now definitely explain the euphotic zone anomalies and even reproduce them on a laboratory scale. I t seems that ultraviolet light has a detrimental effect on the photosynthetic process, and in the uppermost layer of the water the intensity of ultraviolet radiation is great enough to
-1;6.
+
METERS
-
i
500 METERS
\/A1 PO4-P
(L::s-
C
'PO
Cm'ENTRATION
2P
jpo
POCP
4
m w r a m 0rnlllllltnr pr lit"
pr I*
5P
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Figure 6. Vertical distribution of dissolved oxygen and inorganic phosphate in a typical Pacific Oceon xoter column.
Volume 40, Number 5, May 1963
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inhibit this process. After traversing just a few meters of the water column this radiation is reduced to the poiut that photosynthesis proceeds at its maximum rate, thus accounting for the sub-surface phosphate minimum and oxygen maximum. The abyssal phenomenon is not as readily explained. It is indirectly related to biological activity, but may also be associated with a physical factor. Application of the principles of physics indicates that whenever a water mass is flowing over and in opposition to another mass, vertical exchange between the two water masses is facilitated and if the oxygen renewal exceeds the consumption rate in one mass (and it does) then minimum oxygen values could be expected to coincide mith the current boundary. The mathematicians have come to the assistance of the oceanographer and evolved a system of equations to support this theory (It). We should now consider the phenomenon of oxygen enrichment in abyssal waters. Oxygen concentration in surface waters is highest in the frigid polar regions of the earth's hydrosphere, reaching values in excess of 10 ml per liter. Some of this polar water will inevitably converge on warmer water masses from lower latitudes. As a result of the greater density of this extremely cold water it will tend to sink down through the warmer layers into the abyssal depths, carrying along its rich supply of dissolved oxygen. Since biological activity is limited at these extreme depths the oxygen concentratiou remains high. Now there is no doubt but that this process is continuously taking place and extracting oxygen from the polar atmosphere and transporting it down into the depths of the sea. The obvious conclusion resulting from this knowledge is that the hydrosphere and the atmosphere are in a state of gaseous equilibrium, the primary source of oxygen being the atmosphere. However, Dr. A. C. Redfield of Woods Hole Oceanographic Institute has in recent years (IS) introduced a rather startling new theory which considers the above described phenomenon to be only one small phase of a tremendous oxygen cycle in which the sea itself is the primary generator of our atmospheric oxygen supply. The point of origin of this cycle is at the bottom of the sea where chemosynthetic sulfate bacteria, under anaerobic conditions are extracting nascent oxygen to be utilized in their metabolism and returning soluble combined oxygen to the water in the form of inorganic carbonates. These carbonates provide an immense oxygen reservoir, a portion of which is transported to the photosynthetic zone through the processes of upwelling and diffusion. The carhonate material is then utilized by phytoplankton and elemental oxygen released via the photosynthetic process.
276 / Journal of Chemical Education
Diffusion of the oxygen into the atmosphere and back again now proceeds in accordance mith the principles of chemical and physical equilibria. Another phenomenon may affect the oxygen COIIcentration in the abyssal regions. Whenever abyssal water masses are trapped within deep oceanic basins, as in the Black Sea, deep circulation may be completely stopped. Under these conditions the dissolved Significant concentrations of dissolved hydrogen sulfide will then appear in the water column in lieu of the normal oxygen supply. Summary
We have examined some of the most importaut mechanisms involved in the vertical transport of water. We have studied geographical and vertical distributions of some representative nutrient materials in one of our principal ocean areas, and finally v e hare considered the relationship between the concentrations of these substances and the biological productivity of any given uater mass. All of these items are dynamic variables in the gross aspect of the earth's hydrosphere, and their quantitative values fluctuate in response to both random aud periodic changes in the total environment. Acknowledgments
The photographs from which the illustrations were made were produced by members of the Thai Royal Navy Hydrographic Office in cooperation mith Dr. E. C. La Fond. Copies were made available for this paper through the courtesy of Mr. James L. Faughn, Project Leader of Expedition Naga. Literature Cited (1) RUSSEL,F. S., A N D YONGE,C. i\I., "The Seas." 2nd ed., Frederick Warne and Co., Ltd., S e w York, 1958, p. 227. (2) Ibid., D. 127. (3) Ibid., p. 90. ( 4 ) HARVEY,H . W., "The Chemistry and Fertility of Sea Waten." Cambridee Universitv Press. 1955. u. 23. (5) Ibid,, p. 16. (6) Ibid., p. 12. (7) Ibid., p. 17. (8) DEFANT,A., "Phgsical Oceanagraplphy," Press, New York, 1961, p. 120. ( 9 ) Ibid., 0.100. ( i 0 ) Ibid., p. 200. H . U.,JOHNSON, M.W., FLEMIXG. R. H.,"The (11) SVERDRUP, Oceans: Their Physics, Chemistry and General Biology." Prentice-Hall, Inc.. New Yark, 1942, p. 836. (12) Ibid., p. 161. A. C., Anwican Scientist, 46, 205-21 (1058). (13) REDFIELD, (14) THOMPSON, T. G., J. CHEM.EDUC.,35, 10&12 (1958). (15) GOLDBERG, E. D., J . CHEM.EOTC.. 35,110-8 (1938)~
