APPENDIXES

Remark: The appendixes are published in the name and on the respon­ sibility of the different authors.

4 APPENDIX D

ON THE PRODUCTION AND THE CONDITIONS OF PRODUCTION IN THE SEA

BY

K. BRANDT

Translated from the German by 77. M. K Y LE 1 he practice of agriculture has received great benefits from scientific investigation of the cycle of chemical changes on land, by agricultural chemists, plant physiologists, bacteriologists, animal physiologists and by distinguished advocates of agricultural economy. It is now a question of utilising the experience and the results of investigation gained on land for an economy of the sea. It is self-evident that in this, we must take account of the special conditions which the organisms living in water experience as con­ trasted with the life surrounded by air. As many results of science have already been adopted with advantage in the management of ponds, we may be allowed to expect that similar principles can be introduced into the management of the sea, in such a way that the natural production can be helped out in the most rational manner possible. For all investigations which have as end the ascer­ taining of the productivity of a given area of water, a knowledge of the organic and inorganic changes in the sea and the discernment of the natural relations between pro­ duction and conditions of production are quite indispensable. We have tö thank V. Hen sen1 for the advancement of the aims and for the methodical grounding of all general marine biology. As I myself have been engaged in this field for 17 years, 1 have taken some pains to advance this branch of study as much as possible after the opportunity offered itself, with the organisation of the international investigations of the North-European seas, to make some voyages in the North Sea and Baltic on a German research-steamer, well equipped for scientific work, and to investigate some questions of general importance with the help of colleagues, also to improve the already existing methods so as to obtain results free of error. Just as on land, the true production ,in the sea, that is, the formation of new organic substance, arises exclusively from plant-life. It is only the plant which, owing to the possession of chlorophyll, can form organic materials from inorganic (carbonic acid, water and a number of salts) in the presence of light. The animals on the other hand, are all consumers; they must all take tbe organic materials necessary for the building up of the body and the maintenance of life from the plant-kingdom. On the death of animals .and plants the complex organic substances which composed their bodies, break down again, in consequence of the activity of certain bacteria, into the same simple inorganic materials from whicli the plants construct organic substance. The determination of the true production depends therefore entirely on the plants,

J (Jeher die Bestimmung des Planktons oder des im Meere treibenden Materials an Pflanzen und Tieren. (5. Bericht d. Komm. z. wiss Unters, d. deutsch. Meere. 1887). t* APPENDIX D: BRANDT which occur in the sea chiefly in the form of extremely small chlorophyll-bearing organisms. The ocean in its entirety is comparable to very uniform and thickly populated paslure-land. The microscopic plants (diatoms, Peridineæ and fission-algae), which compose this pasture, are distributed like the finest particles of dust through the upper water-layers, sufficiently permeated by light. It is only where the land rises above the oceanic pasture which covers the greatest part of our Earth, that larger plants, such as sea-weeds, red algae, green algæ and sea grass, are found on the very narrow and also very defective ledge of the coast. On Ihe thickly populated pasture — of the land as of the ocean — the stock of plant-life appears to be very scarce because the newly formed plant-substance is con­ stantly being preyed upon, yet a closer investigation shows immediately that a very important amount of useful organic substance must in truth, be constructed in the course of the year. There is but one method for determining the production of plant-substance in the sea, namely, the plankton-method instituted in 1887 by Y. H en se n of Kiel. This method seeks to determine, as exactly as is possible in anything, the quality and the quantity of the smaller plankton-organims contained in a column of water of known dimensions. To ascertain the production for a region, it is necessary to make such hauls regularly once a week during a year at the same places. The same has to be done at other coast- places. The counting of the plankton, though it takes time, is quite indispensable, as it is only by exact numerical determination of the plants on the one hand and of the animals on the other, that we can estimate the production and consumption. For the commonest forms of plants and animals occurring in the plankton, we must determine by repeated investigation and observation, the rate of reproduction, the duration of the various developmental stages under varying conditions and the total organic changes of the species concerned; also for the commonest animals, the amount of food required and the kind of food. All these must be determined before we can estimate the production from the numerical data, obtained from a series of observations extending over at least one year. This is best done if, for the few dominant plant-species, we consider the quantity taken in the first haul as capital, the reproduction as interest, and then from previous knowledge of the rate of reproduction, calculate the year’s production from the interest. After one more year the capital is again almost as small as at the beginning. The interest has all been consumed. From the quantity and species of the animals, as well as from their ascertained requirements in the way of food, we can prove catch by catch, if the consumption has been as great in reality as the calculation has made it. The production of plant-substance in the sea may then be compared with that on land by arranging the chemical composition, as to quality and quantity, along parallel lines. We may determine how much organic substance in general, and in particular how much albumen, fat and carbohydrates, are formed per unit of surface during the course of the year, on the one hand for land e. g. a pasture, on the other hand, for water. The two more closely studied, main groups of plankton-flora, the Peridineæ and the diatoms, are very nearly related in composition to the best forage, and in autumn the plankton of Kiel Hay has in general almost the same composition as pasture ‘.

1 K. B ran d t, Beiträge zur Kenntnis der chemischen Zusammensetzung des Planktons. (Wiss. Meeres- untersuch. III. Kiel 1898i. — 5 — APPENDIX D: HRANDT

The very numerous hauls of plankton made during the German quarterly cruises, have The prelimin­ ary work al­ been subjected to enumeration by Dr. Ap stein, partly with Dr. R a u schen plat, in order ready carried to determine exactly, the horizontal and vertical distribution of the principal plankton- out for the determination organisms in the regions of the North Sea and Baltic, and their relation to the seasons of the actual of the year. The results of the investigations for 1903 will be published this winter, production in the North Sea those for 1904 not before next summer. The opportunity has been utilised also, to make and Baltic special hauls for the investigation of the chemical composition of the plankton. A portion of this new material has already been worked out by Dr. Stiehr. Further investigations will also be set on foot to ascertain the structure, development, reproduction and habits, of the characteristic plant and animal forms of the plankton and their relation to other organisms, as well as to the outer conditions of life. Work is also being done on the plankton-methods, viz. comparison of the different nets, the quantities of the smallest plankton-organisms escaping through the pores of the nets, and the best method of quantitative estimation of the hauls. With respect to the last, Dr. Ap stein has just published an essay in which he maintains the indispensability of counting the plankton and the unserviceableness of mere estimates 1. Whilst these investigations are intended so to perfect Hen sen 's plankton-methods that a great, coherent investigation of the true production (e. g. in the North Sea) may be made in the not distant future, other preparatory work is also being done, which will contribute to deepening and extending the value of the results reached by the plankton- investigations. The aim of these new investigations is to learn more definitely, the general conditions of production in the ocean and to determine the factors on which the produc­ tion depends2. As the existence of animals is dependent on that of plants, so are these again Theconditions of production dependent on the general conditions of production for the amount of production. To gain information concerning the productivity of a region of the sea, it will probably soon be sufficient to make an exact investigation of the general conditions of production, which are-very much simpler and more uniform in the sea than on land. For this, we shall require to take into consideration, the scarce but indispensable food-stuff of plants dissolved in the water, and also the intensity of light — both of which investigations have been but little pursued hitherto — as obviously, the varying amount of the production in the sea depends mainly on these two factors. This will only be possible, however, when we have made more detailed and extended investigations and are better informed than at present, of the natural relations between the conditions of production and the actual production. The plants produce just so much organic substance as is allowed by the general conditions of life in the land- or water-region considered, and in fixed relation to the amount of inorganic food-materials at their disposal. In this, however, they come under the law of the minimum. If one of the indispensable constituents of plant-food is present in very small quantities, the production is also small. The production ceases entirely if one of the indispensable constituents is absent. Almost all marine plants, macroscopic or microscopic, take the nourishing salts not

1 C. Ap stein, Die Schätzungsmethode in der Planktonforschung (Wiss. Meeresunters. Kiel). 1 K. Brandt, Ueber den Stoffwechsel im Meere, 1. u. 2. Abhandl. (Wiss. Meeresunters. IV. u. VI. Kiel. 1899 und 1902). APPENDIX D: HK AN DT — 6 -

from the ground, like land-plants and sea-grass — the only marine blossoming plant, but from the surrounding water. This holds good for all algæ, for the small floating plankton- algæ as for the ground diatoms and the larger fixed algæ. The food of the algæ, of by far the great majority of marine plants therefore, must consequently occur in dissolved condition in the water, and the sea-water itself is like a very weak solution of plant-food, which contains all the inorganic materials necessary for the formation of organic substance. If we consider the composition of the sea-water on the one hand and that of marine plants on the other, we find that there are but few isolated inorganic substances; and these are available for the plant-life in such relatively small quantities, that they — having regard to the law of the minimum — command the amount of the production in the sea. The substances of chief importance, according to *the present state of our knowledge, are the inorganic nitrogen compounds (ammonia, nitrites and above all nitrates), salts of phosphoric acid and silicic acid. Very small quantities of these food-stuffs occur in sea-water, of many indeed only traces. For the building-up of organic substance in marine plants, the silicic acid is much more necessary than the nitrogen compounds, and these again are required in greater degree than the phosphates. Exact chemical analyses have therefore to be made with regard to the composition of the principal plankton-plants, in order to learn, how much they require of these food-materials which occur in traces. Further, detailed investigation is necessary concerning the content of sea-water in regard to these rare food-materials, in order to judge how poverty in the one case, richness in the other, may arise. Lastly, simple experiments are also wanted to test the correctness of the conclusions, which have already been won from the chemical analysis of the organisms and of water. According to the methods used in modern agriculture (e. g. by employing a method recommended by Zuntz1 for fresh-water ponds), we can determine the food-stuffs which are especially favourable to the growth of the various forms, and which of the food-stuffs is present in minimum quantities at certain times and in a certain region of water, by adding the separate food-stuffs, which possibly are present in minimum quantities, to the sea-water containing living plankton-plants to be investigated. The results of these three series of investigations must agree with one another. Also, for the further elucidation of the results gained, we require to have bacteriological investigations, investigations of the bottom and also of the composition of the water in general, in addition to the usual hydrographical investigations. Of these manifold investigations, which are quite indispensable for a deeper insight into what is going on in the sea, only a portion could at tirst be carried out. Above all, it is necessary to improve the methods for the quantitative determination of very small quantities of nitrogen compounds, also of silicic acid and phosphoric acid and to such an extent that trustworthy information may be gained regarding the horizontal and vertical distribution of these food-stuffs and their dependence on the seasons of the year. Investigation The greatest difficulties are presented by the nitrogen compounds, ammonia, nitrites of the nitrogen compounds and nitrates, which must be distinctly separated as much as possible. In his work recently published, on the method of the quantitative determination of nitrogen compounds in sea­ water, l)r. Iiaben2 has also given a number of values for the quantities of nitrogen

1 K Knauthe, Die Karpfenzucht. Neudamm 1901, p. 147 et seq. 2 K. Raben, Ueber quantitative Bestimmung von StickstofTverbindungen im Meerwasser, nebst einem Anhang über die quantitative Bestimmung der im Meerwasser gelösten Kieselsäure. (Wissensch. Meeres un ters. VIII. Kiel 1904). APPENDIX D: BRANDT

compounds occurring in the waters of the North Sea and Baltic. 44 samples of surface- water from the North Sea and Baltic contained on an average 0 -08 mgr. of nitrogen pr. liter in the form of ammonia, and 0'189 mgr. of nitrogen in the form of nitrites and nitrates, altogether 0'279 parts of fixed nitrogen in a million parts of water. In the investigations of Raben so far published, a difference according to the seasons of the year is not clearly marked. We should expect, however, from general considerations, that the nitrates especially, which are the most important, would show a regular decrease or increase according to the seasons of the year. In the analyses made by Dr. Raben of the water-samples collected in 1904, such a difference appears very clearly both in the Baltic and in the North Sea, as is shown in the averages below. The number of the various samples analysed is given in brackets.

1 liter of surface-water from the open Baltic. Nitrogen as ammonia Nitrogen as nitrites and nitrates February (13)...... 0-068 mgr. O'199 mgr. May (13)...... 0'065 » O’170 » August (13)...... 0-057 » 0-095 »

1 liter of surface-water from the open North Sea Nitrogen as ammonia Nitrogen as nitrites and nitrates February (12)...... 0-063 mgr. 0-216 mgr. May (15)...... 0-065 » 0 217 » August (1 3 )...... 0-061 » 0 079 »

It is of great interest, on the one hand, to see the agreement of the values for February and May, and on the other hand, the very small amount of nitrates during August, both for the Baltic and North Sea. The great diminution in the nitrates and nitrites during the warm period of the year 1904, I consider to stand in relation to the great decomposition of the nitrates and nitrites by the so-called denitrifying bacteria, which are more active in warmer than in colder water. Investigations are in progress at present, with the aim of reducing as much as possible the size of the error still occurring, though closely determined, in the quantitative deter­ mination of nitrogen compounds. At the same time, it is also being determined why and from what circumstances, Dr. Feitel and 1 obtaineddifferent results in our direct examination of freshly obtained samples from Stettin harbour and mill-water in August 1904, from those obtained by Dr. Raben, who examined thoroughly, some weeks later, sterilised samples from the same regions. From the determinations of Raben, we may assess the average content of the water of the North Sea and Baltic in organic nitrogen compounds, at 0’2 parts at least in 1,000,000 parts of water. The question is now, if this amount is sufficient to explain the quantity of albuminous nitrogen in marine organisms. Earlier investigations' have shown that 1,000,000 parts of sea-water from Kiel Bay contain on an average only 0’03 parts (0*0097—0‘052) of albuminous nitrogen. Consequently, about 7 times as much inorganic as organic

1 K. Brandt, Beiträge zur Kenntnis der chemischen Zusammensetzung des Planktons (Wiss. Meeresunt. Bd. 3, Kiel 1898). Derselbe, Ueber die Bedeutung der Stickstoffverbindungen für die Produktion im Meere (Beihefte z. Botan. Centralbl. XIV. 1904). APPENDIX D: BRANDT — 8 — nitrogen occurs in the western Baltic. Even in the case of the greatest quantity of albuminous nitrogen yet found, the quantity of fixed nitrogen occurring in inorganic form, is at least four times greater than that in the other form. For land-plants, such an extensive absorption of the food-stuffs, which must be taken from the ground through the roots, is impossible. It is also known, that the absorption- coefficient as regards nitrogen compounds, amounts to only a few per cent for the land- plants. That the small plankton-algæ can use up the food-stuffs to such an extraordinarily greater degree, is readily comprehensible when we consider, that they float in a solution, even though weak, of plant-food, that they take up the food-stuffs over their entire, relatively very large surface and that they are in general distributed in the upper, light- illuminated water-layers in a sufficiently uniform manner so as not to disturb one another too much in the absorption of the food-stuffs of their immediate surroundings. We may believe that the absorption-coefficient e. g. for nitrates, is greater than 50 °/o. Conse­ quently, the nitrogen compounds will be in sufficient quantities in general in the northern seas, even though it is not altogether excluded, that they may occur in minimum quantity at many seasons and under special conditions. It is quite different in the warm seas. It is a very striking fact, that the plankton- organisms occur, not in very much greater, but to all appearance in smaller, quantities in the tropical than in the colder seas in spite of better light and higher temperature, though we might expect and it has many times been asserted that they did. I have endeavoured in an earlier paper to explain this by the following hypothesis1 : “ If the denitrifying bacteria of the sea, like the closely investigated denitrifying bacteria of the land, develop a strongly disturbing activity at higher temperatures, only a relatively small production would take place in the warm seas in spite of much more favourable conditions (according to the law of the minimum!, owing to the great disturbance amongst the indispensable food-substance ; whilst, in the cold seas, more nitrogen compounds would be at the disposal of the producers owing to the retardation or suppression of the disturbing process.” It is necessary, therefore, to seek for the denitrifying bacteria in the sea and determine at what temperature they are most active. The first investigations have been made by Erwin Baur2, who showed that denitrifying bacteria with a high temperature- optimum occurred in Kiel Bay. By means of different culture-methods, Gran3 has also discovered several species of another group of denitrifying bacteria on the Dutch coast. The German quarterly cruises gave us, later, many opportunities to add to and determine the wide distribution and relative abundance of both groups of denitrifying bacteria in the open North Sea and Baltic4. Sterilised culture-solutions according to the methods of Baur and Gran, were mixed during the cruises with I cc. of water or a small sample of the bottom-soil. These rough cultures taken at different seasons of the year showed different results. The Baltic was always found to be richer in these disturbing bacteria than the North Sea. In August

1 Uebev den Stoffwechsel in Meere 1. u. 2 . Abhandlung (1. c.). Ueber die demnächst beginnenden inter­ nationalen Untersuchungen der nordischen Meere (Verhandl. V. Internat, zoolog. Congress. Berlin 1901). 2 Ueber zwei denitriflzierende Bakterien aus der Ostsee 1901. (Wiss. Meeresunlers. Kiel. VI). 3 Studien über Meeresbakterien I. (Bergens Museums Aarbog 1901. Nr. 10). 4 R. F e ite l, Beiträge zur Kenntnis denitrifizierender Meeresbakterien (Wiss. Meeresunters. Kiel. VII. 1903). K. Brandt, Ueber die Bedeutung der Stickstoirverbindungen für die Produktion im Meere (Beihefte z. Rotan Centralblatt. XVI. 1904). — 9 — APPENDIX D: BRANDT

1902 and (903, they occurred very commonly, but in August 1904, more rarely than at any period of year yet investigated. In general they seem — apart from this exceptional case — to occur in greater abundance in August and February than in May and November. Ail the samples of denitrifying bacteria, even those taken in February, thrive better in the warmth than in the cold. The two groups of denitrifying bacteria act differently physiologi­ cally and their distribution is also different. The one group, reared in Baur’s culture- solution, occurs in general in greater quantities and more uniformly in the North Sea and lîaltic than the other group reared in Gran’s culture-solution. Whilst the latter play their part on and in the bottom-soil chiefly, less on the surface, and are but seldom found in the middle layers, we may meet with Baur’s denitrifying bacteria in almost all layers of water and in the most varied bottom-samples. They occur almost always in the surface-water of the North Sea and Baltic, and they are even more numerous in the bottom-samples from the Baltic than Gran’s bacteria, but decidedly less than these in the bottom-samples from the North Sea. Though it appears from the general distribution of the denitrifying bacteria in the home-waters, that they play an important role in the sea, and though it appears very probable also, from the evidence regarding their better growth in warm water, that the relative richness of the cooler seas is connected with the smaller disturbance of the nitrates and nitrites, yet detailed investigation is wanted concerning the denitrifying bacteria of the warm regions of the sea and the content of the tropical seas in regard to nitrates and other nitrogen compounds. If such an investigation shows, contrary to expectation, that the warm seas have just as great or even a greater content of nitrates than the colder, then my hypothesis regarding the disturbing role of the denitrifying bacteria in the tropical seas cannot be maintained. We should then require to give a new explanatory hypothesis and investigate more closely. The other plant food-stuffs occurring but in traces, e.g. calcium phosphate and silicic acid, would then come into consideration. The termination of this further investigation will be gained most quickly by well-regulated rearing experiments with separate culture-solutions.

First of all, our endeavours must be directed to make the methods of receiving andInvestigation of the silicic preservation, and of the quantitative determination of the silicic andphosphoric acids,acid asand phos­ free of error as possible, and to determine for the home-waters the amount of these food­ phates, dis­ solved in sea­ stuffs during the different periods of the year. water Since the beginning of the quarterly cruises of the “ Poseidon”, water-samples have been collected suitable for this purpose, and then examined in the laboratory by Or. Raben for their silicic and phosphoric acids. Up to the present, Dr. Raben has only reported on the quantitative determination of the silicic acid dissolved in sea-water, based on values obtained from 25 analyses during the year 1903. If we also take into consideration the silicic acid determinations of water-samples taken in August and November 1902 and in February, May and August 1904, all made by Dr. Raben, we now have 61 quantitative determinations, 27 for the North Sea and 34 for the Baltic. The average of all these determinations is 0 91 mgr. SiOt per liter, for the Baltic alone 0-978, for the North Sea 0-84. I give below, the average of the 34 analyses of the open water of the Baltic, the number of the various samples examined being given in brackets. Appendix D 2 APPENDIX I): BRANDT — 10 —

August 1902 (4) 1-037 November 1902 (3) 1'26 February 1903 (2) I 45 May 1903 (1) 0*65 August 1903 (1) 0'93 November 1903 16) 1-084 February 1904 (6) 1 015 May 1904 (2) 0-655 August 1904 (2) 1-926 mgr. per Liter.

These figures are of interest in several ways. From August 1902 on to November and then to February, the amount of the dissolved silicic acid increased. In May 1903, the amount was very small; it was greater in August and still greater in November. In February 1904, the amount was not so high as in the preceding year; but in May 1904, there was again a very small quantity which again increased towards August. If we exclude for the moment the abnormal value of February 1904, we obtain a curve which agrees with the annual changes in a group of plankton-plants, viz. the diatoms. This parallel would be still clearer, if investigations had been made in the intervals between the seasonal cruises. As shown first of all for Kiel Bay and then for other North-European waters also, the minimum of the plankton occurs in our waters in February and March (according to the year). Immediately afterwards follows the spring-maximum, which is caused by the luxuriant growth of the diatoms (chiefly Chætoeeros). In the period from May to July or August, the total quantity of plankton organisms is relatively small. In late summer and autumn a second maximum occurs, which is smaller in volume than the spring-maximum and comes in part from the copious development of certain diatoms (especially Rhizosolenia), but chiefly from the fact that the large Peridineae (Ceratium) have the main period of their development in October. As the number of the Ceratium gradually decreases towards February, the quantity of the plankton-organisms diminishes to a minimum. The spring maximum is to be explained from the circumstance, that the plant food­ stuffs are greatly stored up in the winter — dissolved silicic acid amongst others — and then the rise of temperature and increased intensity of the light, render an immense production possible of quite definite diatoms (Chætoeeros). These diatoms require a very large amount of silicic acid; the dried substance of Chætoeeros consists of silicic acid to about one half, that of Rhizosolenia to about one third. Before the period of propagation in 1903, the water was richer in dissolved silicic acid than at any time previously. In conse­ quence of the very great demand for this food-stuff — occurring still only in small quantity — the conditions of nourishment became so bad, that the propagation ceased and the resting-stages were formed. In fact, after the diatom-maximum in May of both years, the amount of silicic acid was very greatly reduced. That the silicic acid occurs at a minimum at this time, and that the diatom-propagation is thus suddenly brought to an end, appears very probable from the fact., that about 1 part of diatom silicic acid occurs in a million parts of water at the period of the strongest increase of the diatoms. At this period the relation between dissolved and diatom silicic acid was about 1 : 1 ; it is thus many times more unfavourable that the relation between Ihe inorganic and organic forms of the nitrogen compounds. In consequence of the small demand during the summer months, a similar enrichment of the water as regards silicic acid, again takes place in the period from May to August or September, and this in most years brings on a late summer maximum of diatoms, which soon disappears and is almost always much smaller than the spring maximum. It consists — 11 — APPENDIX D: BRANDT mostly of Rliizosolenia, which require less silicic acid, so that a further increase in the dissolved silicic acid may go on in to November. It is not yet explained how the cessation of this second diatom maximum occurs, nor how it is replaced by the luxurious growth of Ceratium. The spring maximum of the diatoms is very different in different years, as I have shown from volume-curves in an earlier publication1. In 1891, the quite unusual case occurred, that the spring maximum of the diatoms was smaller than that of the autumn. The relatively small amount of dissolved silicic acid detected in February 1904, as shown above, leads to the conclusion that the spring maximum will have been unusually small in this year also. Such a conclusion stands in agreement with the great abnormality, that the Ceratium forms were already occurring in great quantities in the middle of June. If the diatoms have not increased so greatly in the spring of 1904, the silicic acid must have been used up so much, that only the minimum is present for Chaetoceros, conse­ quently, the other food-stuffs have been used in less quantities than usual and the Ceratium forms (constantly occurring in Kiel Bay) have been able to propagate very strongly about two months earlier, in consequence of the greater nourishment. Though I have hitherto considered it almost impossible, that phosphoric acid could occur reduced to a minimum, basing this conclusion on the older information in literature concerning the amount of this important food-stuff in sea-water, yet, from the new analyses of Raben, which have given very much smaller values than the earlier investigations, I must recognise the probability that, just as the silicic acid is at a minimum at the end of the spring maximum, so the Ceratium propagation in autumn is dependent on the amount of dissolved phosphates. The fact that the autumn maximum is caused by the propagation of plant-species different from those of the spring maximum, shows that plants, with different food-requirements, find the most favourable conditions of life and above all of nourishment at the different periods of the year. According to the as yet unpublished investigations of Dr. Raben, concerning the best method of exactly determining the amount of phosphoric acid in sea-water, this amount seems to vary a great deal. In February and May, only a small quantity of phosphoric acid occurred (O' 14—0’25mgr. P2 Ob in the liter), but in autumn, a great deal usually (to 1 ’46 mgr. in a liter of Baltic water), so that much phosphoric acid is present at the beginning of the propagation-period of Ceratium. Rearing experiments on the commonest plankton-plants under the influence of different food-stuffs, offer a means of determining their food-requirements, and the most favourable composition of sea-water as a nourishment-solution for plants. From such experiments, with which I am at present engaged, it appears that the addition of calcium phosphate alone, in autumn, causes a very distinct increase of microscopic plankton-plants, whilst without this the propagation is extremely small. From similar experiments, Knauthe (I.e.) has already shown it to be in high degree probable, that the poverty of plankton, which has been remarked in ponds and freshwater lakes during the summer, is due to the lack of phosphates. As the method of quantitative determination of phosphoric acid in seawater has already been so far tested by the above-mentioned preliminary investigations, that accurate results may be expected, a large number of investigations on water-samples taken from the North Sea and Baltic at different times should now be made. These will show whether the phosphates occur at a minimum according to the period, e. g. in 1 Die Fauna der Ostsee, insbesondere die der Kieler Bucht (Verhdt. Deutsch. Zool. Gesellsch. 1897). r APPENDIX D: BRANDT — 12 — summer. From the analyses of the chemical composition of plankton-organisms, which Dr. Stiehr has carried out at Kiel during the past year, it has been determined, just as for land-plants, that Ceratium and Rhizosolenia require about 2'5 times as much nitrogen compounds as phosphates. Less phosphoric acid is thus absorbed than nitrogen. At the time when it is at a minimum therefore, the phosphoric acid must be presenl in the sea in still smaller quantities than the nitrates. We must remember, however, that owing to the high molecular weight of the phosphates, the absorption-coefficient for these, according to the law of osmosis, will be smaller than for the nitrates.