An Ocean simultaneously rich and poor
Since publication of the first results of the BIOMASS programme, various contradictory aspects concerning the specificity of the Southern Ocean have encouraged researchers to persevere with their work and to adventure still further afield.
Two early observations, seemingly paradoxical, channel today the efforts of the international scientific community.
On the one hand, it has been proven that the marine beds of the Southern Ocean cradle the richest and most vast field of biological-origin sedimentary silica on the planet, and makes up two-thirds of the World Ocean. Siliceous mud piles up, in effect, over dozens of metres, containing in particular phytoplankton remains, the characteristics of which being the same as those taken from the surface.
In an attempt to resolve this puzzle, several specialists have taken an interest in the question. An article published by Paul Tréguer and Guy Jacques (1) in the scientific magazine LA RECHERCHE provides some of the answer (2). Redrawing in their preamble the diagram of the water mass circulation in the Southern Ocean, these two scientists began by recalling that the fertilisation of deep water is due in part to the contribution of the other oceans of the world, which come to mix their waters that are very rich in nutrient salts (silicates, phosphates and nitrates) with those of the Southern Ocean. On the other hand, because of a series of complicated physical phenomena, including the plunging of cold Antarctic waters down to great depths and in particular the wind system, circulation to the south of the Antarctic convergence brings to the surface the shallower and warmer water masses coming from the northern seas. "This deep water is rich in nutrient salts such as nitrates, phosphates and silicates", write Tréguer and Jacques. "It is all the richer because, throughout its journey from its distant origin in the Northern Hemisphere, it has accumulated, even before entering the Antarctic system, these essential components of marine life. In effect, as in all the seas of the world, the organic matter that sediments from the surface water (matter made up of waste, corpses and excrement) is degraded at depth by the bacteria that produce the nitrates, phosphates and silicates. This is what is called mineralisation." This is how the surface layers of the Southern Ocean are fertilised: this contribution from rich regions should naturally provoke high primary production - by primary production, one means the production of the first elements without which no life would be possible, namely the product of photosynthesis, in this case the planktonic algae.
However, this is not the case; the average primary production of the Antarctic ocean is less than 100 grams of carbon per square metre per year, whereas that of the African or American coastal resurgence, for example, can reach an average of 200 to 300 grams.
To explain this paradox, the authors of the article turn to three major factors that influence the activities of photosynthesis: the relative concentrations of nutrient salts, temperature and luminous energy.
"Their richness justifies the fertility of certain privileged oceanic regions", write the two French authors again. "But why in the Antarctic do the same causes not produce the same effects? One of us, Guy Jacques, is offering the beginnings of an answer on this point. In effect, the crops of Antarctic diatoms reveal the very particular affinity of this algae for silicon, an indispensable element for the development of their external covering or frustule. Could not this strong demand for silicates during the summer flowering of the phytoplankton entail a lowering of the silicate concentration in the seawater and, therefore, an imbalance between the relative concentrations of nutrient salts? This hypothesis would seem to be confirmed by the development, on a north-south profile, of concentrations of nutrient salts: in the extreme south, there is an abundance of nitrogen, phosphor and silicon, then, very quickly towards the North, a strong deficiency in silicon appears. At the level of the polar front, the silicate concentrations in the surface waters decrease by a ratio of 10 to 1 whereas the nitrate and phosphate concentrations decrease by only 10%."
Temperature. Here, the authors recall that the severe temperatures of the Southern Ocean are not propitious for phytoplankton development. Within a critical temperature threshold, maximum 4°C in the summer in the Antarctic Ocean, the metabolism of every living creature is in effect slowed down. "So, the time needed for vegetable plankton to reach its flowering stage (at least a million cells per litre of seawater) is, in the most favourable case, of the order of one month in the Antarctic whereas it is only of three days in the regions of tropical coast resurgence, where the cells divide more than twice a day." (3).
Energy dispensed by light. We have seen in the chapter on climate that the illumination of the Southern Continent is a simultaneous function of the season, sunshine and the inclination of the sun's rays on the surface of the earth; the same observation can be made on the subject of the Southern Ocean, adding nevertheless that the huge ice floe that forms during the long winter months and that deprives the marine organisms of light is another factor that discourages phytoplankton development. Two other factors restricting this phytoplankton development (and specific to the waters of the Antarctic) are put forward by the authors of the article; it is on the one hand a question of the short term variability (from a second to an hour) of solar energy, and on the other, the instabilities of the surface layer of the waters where these microscopic algae habitually are produced and live (between 150 and 200 metres). "However, the early estimations that we made about the Southern Ocean", write the authors, "show that the phytoplankton takes one hour to travel through a luminous gradient that drops from 100% to 5% of the light received on the surface. This rapid change of luminous energy does nothing to encourage the production of algae, of which the ability to adapt is already inhibited by the low temperature."
The authors stipulate nevertheless that the Southern Ocean, even though it has its own characteristics, is not a perfectly homogenous whole. So the ice floe's withdrawal zone would seem to be a particularly propitious geographical region for phytoplankton development because of the stabilisation of the water (less turbulence) due to the creation of lighter fusion water (coming from the melting of the ice) that is superimposed on the denser and more saline ocean water. "This system is more stable than the classic oceanic system", they further write, "and therefore more propitious for phytoplankton development. The area influenced by the melting of the ice at the beginning of the southern summer can stretch for up to 200 kilometres from the ice floe and be over 20 metres thick, and the biomass of diatoms in this frontier area is ten times superior to that of the open sea." (4)
What is true for one part of this immense ocean does not necessarily apply to other geographical regions of the same ocean. It remains to be explained why the Antarctic waters become poor in silicates in a northerly direction where the phytoplankton seems to be of an insufficient quantity for consuming them. One would have to call upon a too specialised knowledge of organic chemistry for a clear and complete explanation of the assumptions proffered by the authors, who are endeavouring to provide some elements of explanation for this second paradox. Briefly (and we would invite people who want to know more about this to refer to the article in La Recherche mentioned above) here are nevertheless the few lines that end the article: "To summarise", write Tréguer and Jacques, "the ecosystem of the Antarctic is seen to be poorly productive at the primary nutritional level (food), that is to say in phytoplankton, and on the contrary a major exporter of silica to the deep layers. This second paradox is explained by the slow diatomic consumption of silica and by their rapid sedimentation".
(1) Paul Tréguer was a Senior Lecturer at Bretagne University and directed a marine geochemistry team at NSRC (National Scientific Research Centre), and Guy Jacques was the NSRC Research Director at the Arago laboratory (Banyuls-sur-Mer) and responsible for the co-ordinated "Pelagic Production of the NSRC" research group.
(2) The Southern Ocean, Paul Tréguer and Guy Jacques, La Recherche n° 178, Juin 1986, volume 17, pp 746 to 755.
(3) Antarctica, The Last Frontier, Richard Laws, Boxtree Limited, 1989, p 91. p 750.
(4) id 3 p 751.