Breakdown and production of fluorescent substances in Dutch waters

Netherlands oTournal of Sea Research 10 (4) : 499-516 (1976)

BREAKDOWN AND PRODUCTION OF FLUORESCENT SUBSTANCES IN DUTCH WATERS by H. POSTMA, M. W. MANUELS andJ. W. R O M M E T S (.Netherlands Institute for Sea Research, Texel, The Netherlands) CONTENTS I. Introduction . . . . . . . . . . . . . . . . . . . . II. Methods . . . . . . . . . . . . . . . . . . . . . . III. Distribution of fluorescence . . . . . . . . . . . . . . IV. The behaviour of fluorescence in Rhine water . . . . . . . . . . . V. The North Sea . . . . . . . . . . . . . . . . . . . VI. The Wadden Sea . . . . . . . . . . . . . . . . . . VII. Discussion . . . . . . . . . . . . . . . . . . . . . VIII. Summary . . . . . . . . . . . . . . . . . . . . . . IX. References . . . . . . . . . . . . . . . . . . . . .

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499 500 501 505 508 509 513 515 516

I. I N T R O D U C T I O N The natural fluorescence of sea water has been used to obtain information about the nature of dissolved organic compounds (KALL~., 1962), but also as a tracer for specific water masses and river water in the sea (KALLE, 1937, 1949, 1957; VAN ANDEL &; POSTMA, 1954). For the second application it is necessary that the intensity of fluorescence does not change rapidly by breakdown of old or formation of new fluorescing substances, i.e. that it behaves as a conservative property. OTTO (1967) and VAr~ 'T HOF (1972) demonstrated for the southern North Sea that in specific instances fluorescence can indeed be used to determine the sources of fresh water admixtures. ZIMM~.~tMAN & ROMMETS (1974) successfully used fluorescence to estimate the percentages of Rhine and IJsselmeer water in the Wadden Sea. The Rhine water enters this area via the North Sea through the tidal inlets, mainly the Marsdiep, and has a relatively high fluorescence, whereas the IJsselmeer water with mostly a lower fluorescence enters from the opposite direction through sluices in the Afsluitdijk (Fig. 1). The practical importance of estimating the exact amounts of these two fresh water masses in the Wadden Sea lies in the fact that especially the Rhine water is heavily polluted, so that its influence in the W a d d e n Sea has to be studied in detail. The IJsselmeer water, although also originating chiefly from the river Rhine, is much less polluted since

500

H. POSTMA~ M. W. M A N U E L S & J . w . R O M M E T S

the lake acts as a sink for polluting substances (PosTMA, 1967 ; DUINKER, 1974). The investigation of ZIMM~RMAN & ROMMETS was carried out in J a n u a r y 1973. It covered the western Wadden Sea and the North Sea coastal water down to the mouth of the Rhine. In 1974 it was decided to continue this investigation in order to obtain measurements over a full year.

Fig. 1. M a p of the area investigated.

The results of these measurements will be discussed in this paper. They give several indications that fluorescence does not behave as conservative as was thought previously. T h e general conclusion is that the fluorescence of Rhine water decays slowly during its passage through the larger fresh water bodies as the IJsselmeer and the Haringvliet, and in the North Sea coastal water, whereas in the Wadden Sea interior new fluorescent substances are produced. II. M E T H O D S

The methods of investigation have been discussed by DUURSMA & ROMMETS (1961) and ZIMMERMAN & ROMMETS (1974) to which the reader is referred. The authors gratefully acknowledge the assistance

FLUORESCENCE

IN DUTCH

WATERS

501

of "Rijkswaterstaat" (Directie Noordzee), the "Rijksinstituut voor Zuivering van Afvalwater" and the "Dienst Zuiderzeewerken", which collected m a n y samples. All other samples were taken with the ships "Aurelia" and "Eider" of the Netherlands Institute for Sea Research. Cruises in the North Sea were made in 1974 and 1975 and in the Wadden Sea between 1970 and 1975. In addition, measurements were carried out in the IJsselmeer and in the main branches of the Rhine. III. DISTRIBUTION

OF FLUORESCENCE

An example is given which compares the salinity distribution with the distribution of natural fluorescence (Fig. 2). Since the fluorescence in the river water is always much higher than in the sea, both distributions have a great similarity.

Fig. 2. a. Salinity distribution (%o S), March 1974. b. Fluorescence distribution (mFl), March 1974.

For the determination of the portions of Rhine and IJsselmeer water in the Wadden Sea the data are plotted in a salinity-fluorescence diagram. The November 1974 cruise is taken as an example (Fig. 3).

502

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POSTMA~

M. W.

MANUELS

&

j.

W.

ROMMETS

Assuming t h a t n a t u r a l fluorescence behaves as a conservative property, all d a t a must fall within a triangle, o f w h i c h the corners are p u r e N o r t h Sea water, R h i n e water a n d IJsselmeer water. T h e position of a point in this triangle can be used to calculate the ratio between the two water species. Fig. 4 gives the distribution of the resulting fresh w a t e r percentages, which is quite similar to the distribution tbund by ZIMMERMAN t~; ROMMETS (1974).

mFl IOO-

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. ..,. N..o\

0

t

5

I I0

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*5

210 - - - -

51 2

t 30

r 35

~/ooS

Fig. 3. Relation between fluorescence and salinity for the western Wadden Sea, 4 to 7 November t974. Upper and lower lines indicate the relations for pure Rhine and IJ~selmeer water, respectively. The distance of a point to these lines is a measure for the percentage of these two fresh water sources in the mixture (open symbols indicate values influenced by drainage water sluiced at Den Helder and Harllngen).

Inspection o f the d a t a for the other m o n t h s o f 1974 soon revealed, however, t h a t the m e t h o d can only be applied to a few cruises. An e x a m p l e where it c a n n o t be applied is M a y 1974. M a n y W a d d e n Sea points are above the triangle, so that no Rhine-IJsselmeer ratios can be calculated (Fig. 5). This failure can only be explained b y assuming t h a t fluorescence does not always b e h a v e as a conservative p a r a m e t e r . I n the following

F L U O R E S C E N C E IN DUTCH WATERS

503

c h a p t e r s the b e h a v i o u r o f fluorescence is therefore a n a l y s e d in m o r e detail, b e g i n n i n g w i t h a discussion o f the b e h a v i o u r in the w a t e r o f the R h i n e . /

Fig. 4. Percentage of Rhine water in the western Wadden Sea in November 1974 (the dashed area is influenced by water from the Friesland drainage basin sluiced at Harlingen).

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2'5

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3'~ ~,~s

Fig. 5. Relation between fluorescence and salinity in May 1974. Upper and lower

lines indicate the relations for pure Rhine and IJsselmeer water, respectively.

504

H. POSTMA, M. W. MANUELS & j. w . ROMMETS

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Fig. 6. a. Fluorescence of the river Rhine, measured (©) and extrapolated (@), the northern IJsselmeer ([]), the Haringvliet near the dam (L~), a n d of undiluted Atlantic water (35~o S) off the Dutch coast (V) during 1974 and 1975. b. Rhine discharge during 1974 and 1975.

F L U O R E S C E N C E IN D U T C H W A T E R S IV . T H E B E H A V I O U R

OF FLUORESCENCE

IN RHINE

505 WATER

The water of the river Rhine divides in the Netherlands in a branch (IJssel) towards the Usselmeer and two branches (Waal and Lek) to the Dutch Delta region (Fig. 1). The average flow rate of the Rhine is 2300 m3/sec, but varies within wide limits. In periods of flow below 1700 m3/sec all water of the latter two branches, and also most of the much smaller river Meuse, is led to the North Sea through the Rotterdam Waterway. This is possible, since the only other exit, the Haringvliet, can be closed by sluices. These sluices are only opened when the flow exceeds the above value, and therefore, are mostly closed during long periods in summer. The passage time of water in the Dutch section of the Rhine and its branches is only a day or two, except when weirs in the river Lek are closed. The residence time of the water in the IJsselmeer is 3 to 11 months. The residence time in the Haringvliet area, also an artificial lake, can also be many months, depending on the sluicing intervals. Fig. 6 presents the river flow in monthly values during 1974 and part of 1975 (5 kmS/month equals 1700mS/sec) and fluorescence measurements in the Rhine, near the sluices of the IJsselmeer and near the exit of the Haringvliet. It also gives extrapolated Rhine values which will be discussed later. The Rhine fluorescence values are averages from observations in various branches (Waal, Lek, IJssel and Rotterdam Waterway). The

b

Fig. 7. Distribution of fluorescence (mF1) in the IJsselmeer. a. 4 to 7 March; b. 4 to 7 J u n e 1974.

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M. W . M A N U E L S

&

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W. ROMMETS

lowest value measured was 63 mF1, the highest 114 mF1. The variations shown in Fig. 6 do not seem to be correlated with the rate of river flow. The fluorescence values measured in the river are considerably higher than in the artificial lakes near the sluices. A more detailed analysis of the IJsselmeer data shows that this difference is caused by breakdown of fluorescence in this lake. Several cruises have been made to study the fluorescence distributions and of these two are presented. Fig. 7b gives the distribution after two months in which little water passed through the lake (680 × 10e m 3 in April plus May). The fluorescence decreased from about 70 mF1 at the IJssel mouth to about 40 mFl near the sluices. The "oldest" water was found near the eastern sluicing gates (Kornwerderzand) where only one third of the water passed, whereas two third left the lake via the western sluicing gates (Den Oever). Fig. 7a presents the distribution after a period of fast flow (3570 × 106 m s in J a n u a r y plus February) .The fluorescence decrease is now rnFI 70-

o

60-

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oo °°

50. o °o

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o °o

o ° °°

30-

C) 5 ~0 months Fig. 8. Relation between the fluorescence (mFl) near the sluicing gates of the I,Jsselmeer and the passage time of river IJssel water through the lake.

F L U O R E S C E N C E IN D U T C H W A T E R S

507

much smaller, since the passage time through the lake was shorter (4 months). Again, the "oldest" water was found near Kornwerderzand. From the total volume of the lake in 1971-1972 (6500 × 106 m a) and the amounts discharged through the sluices the passage times of water through the lake can be calculated. In Fig. 8 this time, in months, is plotted against the fluorescence of the stations near the sluices. The measurements cover the years 1971 to 1974. It is quite clear that the fluorescence of the lake water decreases with increasing residence time of the water. This effect can only be due to a slow breakdown of fluorescent matter during its stay in the lake. Returning to Fig. 6 it seems reasonable that also the low values with respect to Rhine water near the Haringvlietdam are due to a long residence time in the Haringvliet. During most of 1974 the values near the d a m are low, since hardly any water passed through the sluices. Only in October of that year the fluorescence sharply increased. Because of the faster flow of the Rhine the sluicing gates were opened and new Rhine water entered the lake. rnFI I00

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Fig. 9. Decrease of fluorescence (mF1) along the axis of the Haringvliet (kin from dam), illustrating that the main fluorescence source is not the Meuse (sampled at Drimmelen), but the Rhine (sampled at Gorcum). Measurements in April (O), May (Zk), June ( ~ ) , July ( I ) , August ([2]), September (&) and October (Q) 1974.

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H. P O S T M A , M. W. M A N U E L S & J. w . R O M M E T $

The dccrea.se in fluorescence along the axis of the Haringvlict is shown in Fig. 9. The data near the d a m arc the same as shown in Fig. 6; the Rhine and Meuse data are from stations just upstream from the Haringvlict (Gorinchem and Drimrnclcn). Thc figure demonstratcs that the low fluorcsccnce near thc d a m cannot bc duc to admixture of Meuse watcr, sincc in the eastern part of the lake thc values arc higher than in the Mcusc. In fact the Rhine provided most of the water to the lake (88%), as can be derived from ch!orinity data of the river water (Rhine 207 rag/l, Meuse 53 rag/1 and Haringvlict 188 mg/1; data from RIJKSWATERSTAAT,] 974). V. T H E

NORTH

SEA

It has already been shown in Chapter III that in the coastal water a strong correlation is found between salinity and fluorescence (compare Fig. 2). Therefore, most fluorescence-salinity diagrams for the North Sea water along the Dutch coast are simply straight lines (examples in Fig. 10). Since the mixing process between salt and fresh water is already partly completed within a short stretch of the Rhine estuary before the fresh water reaches the open sea, the salinities of most open North Sea data are above 25~oo S. Hence the salinity-fluorescense lines must

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Fig. 10. Examples of salinity fluorescence relations in the N o r t h S e a , a. 4 to 8 M a r c h ; b. 27 to 29, M a y 1974.

FLUORESCENCE

IN

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WATERS

509

be extrapolated to zero salinity over a relatively long distance to find the fluorescence of the admixed fresh water. Still, since the correlation between the two properties is mostly quite good, a reasonably accurate extrapolation is possible. Fig. 6 shows that the extrapolated values lie at some distance, roughly 10 mF1, below the values measured in the river. This might be due to a change in the intensity of the fluorescence by the passage of fluorescent material from salt into fresh water. However, VAN 'T HOF (1972), who has made mixtures of the two water species in the laboratory, did not find any effect of this kind. A more probable explanation is that the breakdown of fluorescence observed in the fresh water continues in the sea. O n an average the Rhine water flows northward through the coastal area between the Rotterdam Waterway and Texel in one to two months. In that period a breakdown of fluorescence with 10 to 15 mF1 related to the fresh water component seems reasonable (compare Fig. 8). The extrapolated Rhine values (Fig. 6) confirm the trend of the direct measurements that the fluorescence in the river is higher in a u t u m n and winter than in spring and summer. As already observed there is no correlation with the river discharge. One might assume that in the a u t u m n decaying organic matter gives an extra contribution. The lowest fluorescence off the Dutch coast is found in the Atlantic water which is chiefly entering this part of the North Sea through the English Channel. Fig. 6 gives the values for the salinity of 35~oo which is without any admixture of fresh water. These lie between 2.5 and 5.5 mFl. It should be noted that in the Atlantic Ocean values lie between 0.5 and 1.5 mF1 (KALLE, 1957), SO that the water mass has gained in fluorescence during its passage towards the southern North Sea. There is a slight indication of higher values in the summer, which must be due to a higher biological activity in that time of the year. More pronounced, however, is the difference between 1974 and 1975. To explain this difference more hydrographic information, for example about the rate of flow of the Atlantic water, is needed than at present available. VI. THE WADDEN SEA It has already been observed in Chapter I I I that fluorescence values in the Wadden Sea are often higher than what can be expected by simple mixing of North Sea, Rhine and IJsselmeer water. An example is given in Fig. 5. The distribution of this excess fluorescence is plotted for the same example in Fig. I l a, taking the North S e a - IJsselmeer relation as a baseline.

510

H. P O S T M A , M. W. M A N U E L S & J. w . R O M M E T S

The distribution of excess fluorescence is very consistent with high values near the coast and decreasing to zero in the inlets. A similar distribution, but with somewhat lower values, is obtained by using the North Sea-Rhine relation as a baseline; however, since most of the fresh water in the area originates from the lake, the former relationship is preferred.

Fig. 11. a. Distribution of excess fluorescence (mFl) in the western W~u:lden Sea in M a y 1974, based on the sMinlty-fluorescence line IJsselmeer-North Sea (compare

Fig. 5). b. Same for July 1974. Information is available for the western Wadden Sea to construct similar maps for all months of the year. Essentially, however, all maps show a distribution pattern quite similar to Fig. 11, so that they are not reproduced here. Only the slopes of the gradients from the tidal inlets inward change in the course of the year a n d no inward increase is found in winter. This is demonstrated by Figs 12 and 13. Fig. 12 gives a series of monthly salinity-fluorescence diagrams in 1971 for the Wadden Sea west of the Ameland watershed. The first three months show no excess fluorescence, but the others do. The average excess is plotted in Fig. 13 together with some data of 1972. There is a clear seasonal variation with a m a x i m u m in mid-summer and a m i n i m u m in winter. The production of fluorescent matter in the Wadden Sea interior will be caused by the breakdown of organic matter, which shows a parallel seasonal cycle (DE JONG~. & POSTMA, 1974). This conclusion will be discussed in detail in the next chapter.

FLUORESCENCE

IN DUTCH

WATERS

511

East of Ameland the properties of the water in the Wadden Sea are no more influenced appreciably by fresh water from the IJsselmeer, but chiefly by water from sluices at Lauwersoog which drain parts of the provinces Friesland and Groningen. This water, as well as that of the Ems estuary further eastward, has a much higher fluorescence than the IJsselmeer water (150 to 350 mFl). As a result the salinity-

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15

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25 2O I°

15I05" O"

,

,

°

,

25~ 2015I05O-

\ 2O

25

3O

35 %~S

20

25

30

3,5'=~S

Fig. 12. Monthly salinity-fluorescence diagrams for the Wadden Sea west of Ameland in 1971, together with the IJsselmeer-North Sea mixing line.

fluorescence relation has a very steep slope (Fig. 14), so that it is almost impossible to use this as a baseline for the determination of fluorescence production. The method presented is, therefore, not well applicable to the eastern Wadden Sea, except in periods that no fresh water is sluiced. This was the case in June to August 1971. In that period concentrations of 8 to 12 mF1 above North Sea water of the same salinity were measured on the Wadden between Lauwersoog and the Ems estuary (average of 7 stations), or about twice the amount measured over the same period in the western Wadden Sea.

512

ri. POSTMA~ M. W. MANUELS & J. W. ROMMETS

mFI

/JJ

2

dJ F t M ~ A J M I j I . j ~ A I S ~ O ' N 1971

JDI j = F = M I A J M J jL j I A I 1972

Fig. 13. Variation in excess fluorescence (mFt) in the western Wadden Sea during 1971. Each point represents the average dbtance from the cluster to the main line in Fig. 12. mFI

L 50-

40-

•

•

30-

20-

10-

0 0

. 5 . . I.0 .

15

20

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5

'5

°/~,S

Fig. 14. E,xamplc of salinity fluorescence relations west (0) and east (©) of the Ameland tidal inlet (Lauwers). The western area is influenced by the IJueimeer (55 mF1) and the eastern by water sluiced from the provinces of Friestand and Groningen (220 mF1). Data of February 1971.

FLUORESCENCE

IN D U T C H W A T E R S

513

VII. DISCUSSION

Little is known about the chemical composition of dissolved fluorescent matter in natural waters and the manner in which this material is formed. Stated in general terms, breakdown processes of dead organic matter and excretion products of living organisms such as phytoplankton may yield a mixture of dissolved compounds named "yellow" or "humic" substances (KALLE, 1937). The fluorescent properties may be due to a chemical structure with iso- and heterocyclic rings as in melanins (DuuRSMA,1965). KALLE (1962) has shown that the fluorescent material in the sea has a chemical structure different from that originating from the land. In the former purely yellow carbohydrate-humic acids (melanoidines) dominate, whereas the latter are phenol-humic acids with a more brown colour. However, besides natural substances a great many industrial products have cyclic structures in their molecules and the Rhine, flowing through a heavily populated and heavily industrialized area, may contain many of such compounds. Since in any case the fluorescence is caused by a mixture, it has up to now been impossible to determine specific compounds. A fluorescence spectrum shows no specific characteristics, and Rhine and North Sea water both show a fluorescence maximum at the same wavelength (460 nm; VAN 'T HOF, 1972). In comparing the degree of fluorescence of various natural waters it is evident that the strongest fluorescence occurs in water bodies which flow through peat formations. DE H,~N (1975) has shown for a lake (Tjeukemeer) in the Friesland drainage basin that the concentration of dissolved humic substances (fulvic acids) in this lake varies between 20 and 50 mg/1, expressed as carbon. Such high concentrations--in the open North Sea in water undiluted by river water dissolved carbon never exceeds 2 rag/1 (DtmRS~A, 1960)--may be correlated with the high fluorescence of the water discharged from the same general area in Lauwersoog. Humic substances, in lower concentrations, will occur in the other water bodies studied, but it is impossible to say to what percentages other natural and man-made products are also present. DE H ~ N (1975) has further shown that fulvic acids are decomposed in the Tjeukemeer by certain bacteria. This observation indicates that these acids are not stable and the decrease of fluorescence of Rhine water during its storage in the IJsselmeer and Haringvliet may be explained by the breakdown of natural humic compounds. However, since it is even unknown what part of the material is natural and what part man-made, a more extensive investigation is needed to support or reject this possibility.

514

H. P O S T M A , M. W. M A N U E L S & J . w . R O M M E T S

The present study further gives some indications that the breakdown of fluorescent matter originating from land, continues in the open North Sea. This conclusion also needs further substantiation. For the Wadden Sea it has been shown that in the inner parts considerable amounts of fluorescent matter are produced. There is a striking parallel between the seasonal variations in organic matter supply and decomposition (PosTMA & R O r , UCt~.TS, 1970; DE JONGE & POSTMA, 1974) on the one hand, and the seasonal variations in the production of fluorescent matter on the other. Both reach a maximum in summer and a minimum near zero in winter. Since the outstanding feature of the Wadden Sea is the decomposition of large amounts of organic matter, most probably it is this process which is reponsible for fluorescent matter production. A rough calculation of this production can be made as follows. The average excess fluorescence in the western Wadden Sea is 2.5 mF1 or, the average depth of the area being 4 metres, 10 000 mF1 per m2. On the basis of the water exchange rate (8% per tide) between the area and the North Sea (PosTMA, 1954; ZIMMERMAN,1976) the loss of fluorescent matter can be calculated at 800 mF1 per m 2 per tide or 600 F1 per year. The amount of organic matter mineralized in the area per year is about 800 g per m 2 (DE JONGE & POSTMA, 1974). Therefore, the decomposition of one gram of organic matter roughly would yield 1 mF1. Because of the fast water exchange between Wadden Sea and North Sea after a week or two only 10 % of the water mass present at the beginning of such a period has remained there is no time for significant decomposition of fluorescent matter in the former area itself. Such decomposition will take place in the North Sea, but here the identity of the Wadden Sea water mass is soon lost in a much bigger water volume. It is very speculative to discuss rates of breakdown of fluorescent matter of marine origin in the open North Sea. The following calculation only serves to elucidate the problem. It is assumed that decay of organic matter in the North Sea produces the same amount of fluorescence as in the Wadden Sea; this assumption finds some support in the fact that the organic matter in the Wadden Sea for a large part originates from the North Sea (DEJoNGE• POSTMA,1974), and further that no significant quantities are directly derived from living organisms. Over a sufficiently large area mineralization equals primary production, which for the central North Sea is of the order of 250 g/m 2 year. This would yield somewhat less than 200 F1/m 2 year. The average water depth of the southern North Sea being 25 m, this amounts to 8 F1/m 3 year.

FLUORESCENCE

IN D U T C H

WATERS

515

In the central North Sea the average fluorescence is about 4 Fl/m 3 (Fig. 6), so that the "age" of the fluorescent material in the North Sea would be half a year. Since mineralization mainly takes place in the summer season, fluorescence of the North Sea water at 35~oo salinity should be somewhat higher in that part of the year. Fig. 6 indicates that this may indeed be the case, at least for 1974, whereas the data for 1975 are inconclusive. Also DUURS~A (1960) found no clear seasonal variation in fluorescence at the lightvessel Texel for 1958-1959. As already suggested the variation is often obscured by independent variations in the Atlantic water mass before it enters the North Sea. More information is needed. Summarizing, the principal conclusion of this paper is that the fluorescent matter in the sea as well as in fresh water is not as stable as previously assumed. This conclusion opens an avenue for further investigations. Especially a close look should be taken at variations at specific sites, such as stations on tidal flats, in the central North Sea and adjacent areas, and the Rhine and its tributaries. In some cases controlled laboratory studies m a y give more precise information on breakdown, production and chemical composition. VIII.

SUMMARY

Natural fluorescence in undiluted North Sea water has an average value of 4 mF1 with a variation between 2.5 and 5 mFl, the highest values occurring in the summer season. The fluorescence increases towards the Dutch shore by admixture of Rhine water. The Rhine has a fluorescence between 60 and 100 mF1 with probably the lowest values in summer. During temporary storage of Rhine water in the IJsselmeer and Haringvliet there is a gradual breakdown of fluorescence; the decrease depends on the residence time in these lakes and proceeds with a velocity of approximately 5 mF1 per month. The lowest values in water leaving the lakes are about 30 mFl. The Rhine water discharged directly into the North Sea possibly shows a similar breakdown of fluorescence of about one month. In the inner parts of the Wadden Sea new fluorescent matter is formed, mainly in summer. The regional distribution and the seasonal variation strongly suggest that this material originates from the decay of organic matter in the area. The general conclusion from this study is that fluorescent material is less stable as thought previously.

516

H. POSTMA~ M. W. M A N U E L S & J . w . ROMMETS

IX. R E F E R E N C E S

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