Report on the results obtained by a new method of fertilization experimented in the marine bay »Mljetska jezera«
Abstract
The experiments aimed at attaining - in a new and economical way - the eutrophyzation of a well-sheltered Mediterranean bay by artificially adding anorganic fertilizer to the water of the area, and at finding out to what extent some economically important organisms are influenced by the growth of organic production. The principal test was focussed on the Oyster (Ostrea edulis).
The marine bay Mljetska jezera (Mljet »Lakes«), a deeply cut bay in the western part of the Island of Mljet (N 42°47’ E 17°21’), lying off the east coast of the Adriatic (Fig. 1) was picked out to serve as experimental ground. The largest depth of the bay amounts to 46 m. The bay is separated from the offing, - their connection being provided by a narrow passage over a 1 m deep sheft. The average depth of the bay does not surpass 22,5 m, while the entire area extends over 183 hectares. The salinity and temperature data for the water of the bay are given in the Tables 1 and 2. More detailed morphological data on the bay can be found in the paper by V u l e t i ć (1953). The hills around the bay are densely wooded by evergreen Mediterranean vegetation. There was no interruption whatsoever in the free communication of the water of the bay with that the offing neither pending the experiments nor preceding them.
The following items were involved in examinations and measurements performed during the investigations in the area: temperature, salinity, and transparence, pH, contents of O2, P-PO4, total P, and silicate. Qualitative and quantitative investigations of bacteria-, phytoplankton-, and zooplankton-populations in the bay were also carried out, and a culture of oysters was planted and observed. Some less intensive investigations included phytobenthos and fishes. Beside the author, who conceived and planned the experiment, and who took care of the hydrographic part of the examinations, several colleagues were also engaged in the investigations, who separately process their own data and prepare papers which hawe been or shall be published in Vol. VI. of the Acta Adriatica.
The planning of the experiment described in the present paper was in connection with some working hypotheses. One of them was that even a lesser enrichment in nutrient salts, particularly in phosphate - occurring in the oligotrophic Adriatic waters - will favourably influence the organic production, very likely more favourably than e.g. the North Atlantic region would be influenced by the same amount of nutrient salts (rule of the minimum; a quicker metabolic process taking place in warmer water; abundance of light in the Meditterranean region).
In order to improve the fertilizing technique and to avoid the immediate sinking to the bottom of considerable quantities of phosphate (about ¾ as shown by the experiments made in Scotland - G r o s s et al., 1946), the fertilizer was first processed by means of a mineral acid of technical grade (concentrated technical sulfuric acid in our case) with sea water added. In this way the following was obtained:
1) All the secundary Ca-phosphate was eliminated and a mixture of the easily soluble primary Ca-phosphate and free phosporic acid resulted. Thus the solution of the phosphatic component in the added fertilizer was a complete one;
2) This made the use of the old, receded, superphosphate possible.
By applying mineral acid we are enabled to obtain larger quantities of earth extract and to use it as additional fertilizer (for heavy metals, other trace elements, and organic matter) in order to stimulate much quicker growth and reproduction of organisms in the sea water. (H a r v e y 1933, 1939; G r a n 1933; R o s e 1954).
As already stated in a preceding paper, the addition of appropriate quantities of mineral acid or acid matter in general, and the thereby produced changes of pH, did not cause any harm to sea water (or probably fresh-water) organisms. Acid matter, on the contrary, has a favourable effect on the process of organic production in a similar way as the addition or appropriate quantities of matter of reductive character. (B u l j a n, 1953 a, and 1953 b).
The ratio by weight N :P for marine organisms being, on the average, 7,2:1 (S v e r d r u p et a l. 1945), for each part of the added and utilized P 7,2 parts of N are required. The artificial fertilization of marine bays will hardly ever become economically feasible and justifiable if large quantities of nitrogen salts have to be added to their waters as it was case with the well-known experiments made in Scotland (superphosphate: ammonium sulfate applied in quantities in the ratio 1:4) and perhaps with all the attempts of water fertilization made until now. As a matter of fact, large quantities of N are already present in the sea water (and in fresh-water too) although in the unfavourable inert form. It is also a matter of fact now that some kinds of bacteria and cyanophyceae are able to fix N2 from fresh-water [H o f e r ’ s experiments (T h i e n e m a n n, 1953), L a n t c h ’ s experiments (N e e s, 1946)]. The well substained opinion that bacteria of the Clostridium strain, present in marine coves too (F r a y, 1947; V a a s, 1952; S c h u s t e r, 1952), are able to fix N2, has been definitely proved by S i s l e r and Z o B e l l, 1951, while analogous proofs with regard to marine cyanophyceae have been offered by F o g g and W o l f e, 1954.
The presence of organic matter, phosphates, potassium salts, solved N2, Mo (or V, or W) salts in traces, and finally the absence of nitrate are prerequisite for the action of these organisms.
One of the author’s hypotheses was that the conditions as regards the investigated area would be satisfied by the addition of phosphate ions and earth extract only, consequently making the fertilization by far less expensive because of the elimination of adding of N-fertilizer.
Another of the author’s hypotheses was that, owing to the artificially caused eutrophization, a considerable impoverishment in O2 of the bottom layers of the marine bay would follow. This may also result in the reduction of the otherwise normally oxidized contact film of the mud which does not allow the diffusion of PO ’ ’ ’ and Fe- - to take place from the silt into the water. (B u l j a n, opera cit). The occurence of O2 disappearance, even on a smaller part of the sea bed in the bay area, would mean an extra contribution of phosphate to the circulation of matter enriching the water of the investigated basin.
It is not necessary to emphasize that the author does not consider it disadvantageous for marine bays if, in a s u f f i c i e n t d e e p basin, owing to the artificially increased production, H2S makes its temporary appearance at the bottom of the basin. The appearance of sulfuretted hydrogen intensifies the freeing of nutrient salts from the sediment and their joining the solution to enrich the reserve supply of phosphate in the bay.
This mobilization in the water of a marine bay of the existing reserve supply of nitrogen and the reserve supply of phosphate present in the sediment is, in the author’s opinion, an element of essential importance in his method of artificial fertilization.
The fertilization technique as applied in the experiment was the following:
On deck of a sufficient large motor boat, to some 70-80 l of sea water contained in an iron vessel of a volume of about 200 l are added 4,75 l of concentrated technical sulfuric acid (specific gravity about 1,68), 100 kg of super-phosphate, and 2 spadefuls of garden or forest soil, and then all the ingredients are well stirred and left standing for a quarter of an hour until the desired effect is obtained, i.e. the solution of salts and extraction of matter from the fertilizer and soil in such an acid medium. At this stage additional sea water is poured into the vessel until a total of 200 l is reached, and the suspension, well stirred once more, is then let in a gentle flow onto the sea surface from the boat in motion. Two iron vessels were alternatively employed to make the refilling not interfere with the flowing. The characteristic feature of this method consists in the obvious possibility of direct application of finely crushed inexpensive phosphorous raw material such as natural phosphorite, degreased bone meal, guano, etc., for the purpose of fertilization of water bodies. This will undoubtedly render the method easier in some places.
The investigations carried out in the area of Mljetska Jezera in order to make a survey of hydrographic and biological properties of the bay, actually started in March 1951 and took place in time intervals of about a month or a month and a half, lasting all through 1954 and, at longer intervals, also in 1955 and 1956. The fertilizing by means of phosphate was begun in January 1954, and a total of 21,5 t of superphosphate was added to the water of the bay until the end of 1954. Earlier, in 1953, a quantitaty of 2,27 t of Ca-cyanamide was added to the same water.
Here is a short review of the obtained results.
The aim of the experiment, i.e. an inexpensive way of sea water fertilization by applying practically phosphate fertilizer only, has been attained, since a manifold increase of organic production in a marine bay was achieved.
A new and advantageous method of dispersion made it possible for small quantities of mineral acid to help the superphosphate-P to reach a good solution in the sea water. Until the end of 1954, i.e. during the period covered by the present report on the results of survey of the Mljetska Jezera area, 36,7 mg of P-PO4 per ton of the bay’s water, or about 117 kg of superphosphate per hectare of the bay’s area were applied.
The experiment has confirmed the correctness of the scheme of circulation of nutrient matter in water basins, according to which, under certain circumstances, by adding PO4 even in smaller quantities, the oxygen supply in the deeper layers of the basin water will grow poorer, as it was noticed in the summer 1954. Given such conditions, the PO4’” will detach itself from the bottom mud to enter the sea water provided that the oxygen supply has reached a certain degree of deficiency. The activation of the PO4 present in the basin sediments and the shifting of a part of that phosphate into the solution was then achieved in the Veliko Jezero area (Pospile) in the summer 1954, owing to the fact that - under otherwise normal hydrographic circumstances in the bay - a production of H2S made its appearance in the bottom layers of the basin.
At the same time the phenomenon of »red water« - a consequence of structural changes in bacteria populations in that part of the bay - was observed in the area for the first time.
A calculation is also given showing that most probably we have succeeded, by adding phosphate salts and acid earth extract to the water of this marine bay in the course of the experiment, to mobilize atmospheric nitrogen, the most expensive ingredient in the process of fertilization. A considerable increase of productions was practically obtained, so to say, without adding any nitrogen fertilizer. Or, to put it more correctly, some N-salt was added, but only a ⅟45 part of the quantity applied in the Kyle Scotnish experiments (G r o s s et al. op. cit.) in relation to the amount ad the added phosphate.
The evidence has been obtained that a part of the added phosphate remains in the fertilized area where it is shared by organisms and detritus, to be regenerated during the next year and to appear in the solution again.
The experiment resulted in favourable changes among the populations of various groups of organisms, such as plankton, phytobenthos, fishes, and shellfishes.
During the summer 1954 the deeper water layers of the bay were improverished in O2 owing to a more intensive production taking place in the fertilized area. The process led to total disappearance of O2 in some cases. At the same time, oxygen kept amassing in considerable quantities in the vicinity of the productive layer, leading to oversaturation with oxygen (198,3% and even 216,2%) as another consequence of the increased production. These two phenomena were unknown in the investigated area before. It follows from the data illustrating the amassment of oxygen in the deeper water layers and its consumption respectively that the fertilization of the area results in a more abundant production of the marine flora in various parts of the bay, amounting to at least six times more than before the experiment took place. (See Tables 3 and 4).
Although fluctuating, the effect of fertilization on the increase of density of the phytoplankton population was a favourable one, particularly in summertime. Compared with the maximum values of the numbers of phytoplankton cells observed in the Veliko Jezero area in 1951, 1952, and 1953, the 1954 ones, found during the summer stagnation (from July through November) reached up to thirty times as much owing to fertilization. (See Table 6).
A considerably higher and conspicuous degree of phytobenthos density, particularly as regards the blue-green algae (cyanophyceae) and benthonic diatomacae has been observed to follow the fertilization.
No increase of the zooplankton p o p u l a t i o n as a consequence of the experiment was noticed.
A considerable and continuous decrease of transparence of the once very transparent water in the bay was the result of eutrophisation of the area when the fertilization was started (See Table 5).
It has also been established that the experimental area proved an excellent feeding ground for oysters (Ostrea edulis) as soon as the fertilization took place. Judging from the results obtained from 2500 individuals planted at four localities in the area, the increase of the species by growth in 1954 was 4,2 times larger than it was the case with individuals of the same age during the preceding two years, i.e. before the fertilization of the bay was begun. (Fig. 2 and 3, Tab. 8).
In the fertilized part of the bay the weight of oysters showed an average growth of 26,0 g per individual in a year’s time. The rate of the weight increase was five times quicker than in the unfertilized part of the bay. The value was obtained by subtracting the extent of oyster growth observed under normal conditions during the preceding two years from the gross growth in 1954. The most suitable position for oyster feeding in the bay was found at the very entrance (Vrata od Jezera) where the net accretion, resulting from the fertilization of the area, amounted to 48 g per specimen in a years’s time. (See Fig. 4 and 5, Tab. 9 and 10).
It has further been established that fewer oysters perished on the feeding grounds of the fertilized part of the bay in 1954, and the decreasing mortality may be considered as another sing of a general improvement of conditions.
There are indications that the fertilization of the bay has also resulted in a considerably increased occurrence of fry. The fry of Aterina sp. and Mullus sp. were particularly noticed. Round fishes, are obviously influenced by fertilization, so among others, the species g r a y m u l l e t (Mugil cephalus), Pagellus erithrinus and r e d m u l l e t (Mullus barbatus) which added very much to their roundnes owing to accumulation of fat in their muscle tissues and arround their intestines.
The fertilized part of the bay used probably to attract also adult fish from the outside, such as Boops boops or anchovy (Engraulis engrasicholus) in larger numbers than before. It has been observed that considerable quantities of sardine too (Clupea pilchardus) entered the area of Mljetska Jezera after it had been fertilized.
The conclusion is drawn from the observed facts that the fertilization of well sheltered marine bays like the area of Mljetska Jezera is feasible, provided the described method is applied, and that it has a favourable effect on the multiplication, growth, and survival of shell-fishes, fishes, and other organisms in the fertilized area.
It is expected that the method of fertilization, described in the present paper, can successfully be applied to fresh-water basins too. Not all marine basins are suitable for experiments of this kind, the author maintains. Certain hydrographic, morphometric, and geological properties of the basin, however, are required.
The average depth of the chosen basin must not be a large one - not over 10-15 m for the Adriatic (Mediterranean) waters, on order to obtain a very thin water layer between the illuminated part of the water body and the bottom, the latter being a potential supplier of nutrient matter including also phosphates. The thicker and the more deeply situated this »phosphate bridge«, the larger part of phosphate remains outside the direct utilization in the producion. On the contrary, the thinner the »bridge«, the more complete the participation of those nutrient salts in the production of the basin and the more economical the experiment. It is possible that the application of this element can to some extent provide the basis for he explanation of fertility of continental shelves and banks in comparison with the fertility of the open ocean.
Sporadic appearances of H2S in an artificially fertilized area, are not considered disadvantageous provided the basin is large enough, and they can be looked upon as evidences of a positive response of the basin to the artificial fertilization.
The fertilized basin should be carefully wached and the addition of nutrient matter kept under control. The optimum condition in the basin, which is to be aimed at, is marked by a very low O2 value in the deeper water layers of the basin during the summer season (0,5-2 ml O2 1-2 m over the bottom). Such conditions favour the mobilization of phosphate from the sediment and its entering the basin water and taking part in the production.
Winter seems to be the suitable season for the beginning of artificial fertilization of water basins.
Owing to favourable results obtained by the described method of fertilization the opinins has been expressed that - since practically no nitrogen fertilizer is needed - it represents for the present the most economical method of fertilization of water bodies. The method will most likely find easy application wherever phosphate rock and mineral acid happen to be inexpensive and where human food growing in the sea has good market price, e.g. in the industrially developed countries.
