350______

Rapp. P.-v. Réun. Cons. int. Explor. Mer, 172: 350-357. 1978.

THE NUTRIENT CONTENTS OF WATER: CHANGES IN RECENT YEARS, PARTICULARLY IN THE SOUTHERN BIGHT

H. POSTMA Institute for Sea Research, Texel, The Netherlands

The general distribution of inorganic nutrients in the situation with that of a housewife sweeping a the North Sea is characterized by very low surface room in all directions until the dust settles in the values over most of the area in summer, which - al­ corners. though somewhat more elevated - are maintained Imposed on the rotating tides and the general net during winter in a pronounced minimum in the circulation there are residual currents near the sea Southern Bight near the Dutch coast. Figure 278 floor which preferentially carry bottom water to the presents a simplified picture for phosphate, derived shores (Dietrich, 1955; Ramster, 1965; Fig. 279). from the charts of Johnston and Jones (1965) and Consequently a considerable portion of such material Tijssen (1968, 1969, 1970). may be mineralized near the shores and in the shallow In the minimum the winter values for phosphate coastal seas bordering the Southern Bight. remain below 0-3 fjLgat/1 P 0 4-P, which is very low Of the latter, the Waddensea forms the greatest indeed (Fig. 280). Comparable values are only found part. It has been well documented that this area in such very poor areas as the Mediterranean Sea. catches considerable amounts of organic detritus from A combination of factors is responsible for the per­ the adjacent North Sea, which is subsequently min­ sistence of this minimum. A pronounced thermocline eralized (Postma, 1954; de Jonge and Postma, 1974). develops over most of the North Sea and the western The seasonal cycle of this process is demonstrated in Channel in early spring, which remains present until Figure 280, which presents the annual variation of October at depths between 25 and 40 m (Dietrich, phosphate in the North Sea nutrient minimum, in 1955) except in the shallow Southern Bight. Most of nearshore water and in the western Waddensea. In the water entering the latter area is from above the the North Sea a phosphate maximum is found in thermocline, and this water mass is soon deprived of winter. In spring - in the minimum already in most of its nutrients. The shortage is not fully repaired February (Gieskes and K raay, 1975) — the phyto­ in the winter season, since the residence time of the plankton starts consuming phosphate until in summer water in the North Sea is too long for complete renewal the concentrations are almost down to zero; in the with nutrient-rich Atlantic water during the winter fall the concentrations rise again. In the Waddensea season. the highest phosphate concentrations occur in summer, A second, possibly even more important, reason is due to mineralization of the organic matter brought that the southern North Sea is dominated by strong inward from the North Sea during that season. tidal currents, which reach spring tide values of more Dissolved silica and nitrogen compounds are also than 3 knots over much of the area. This current mineralized in the Waddensea. Regarding silica, van regime causes the North Sea bottom to consist mainly Bennekom et al. (1974) have shown that this element of coarse sand with a median diameter of 200-500 is chiefly liberated from surface sediments in muddy, microns (Pratje, 1949; Eisma, 1973), which in many shallow regions, more than from suspended diatom areas is piled up in sand-banks and dunes. Fine­ frustules. This is due to the fact that silica goes into grained matter, including organic detritus and even solution only very slowly. In the dissolved nitrogen living plankton, is swept away from these desert-like compounds the liberation of ammonia is dominant in areas to quieter deep water or into adjoining estuaries. the Waddensea, since this substance is transported to Therefore, relatively little nutrient is mineralized in the North Sea before it is oxidized to nitrate and the open southern North Sea. One might compare nitrite (Postma, 1966; Helder, 1974). The nutrient contents of North Sea water: Changes in recent years, particularly in the Southern Bight 351

0.9- ■0.5- 60 60° 6060 .0.0- 0.3 NORWAY NORWAY ,0.7' ■0.4-

0.2 ■0.4- SCOTLAND. .0.3' SCOTLAND, 0.6 Q 5 DEN­ DEN­ MARK. MARK. 0.5 .0 .2 - 0.2 55 55° 0.6 0.7, • IRELAND IRELAND

ENGLAND ENGLAND 'NETHERLANDSNETHERLANDS

BELGIUM

50 50° 50 50 po4 - p po4 - p

FRANCE SURFACE SURFACE 0.6 WINTER SUMMER

A B C D

0.5 -0 .6 - Ö.I 0.6 5 0 - 5 0 - 0 7 0.7 ■ . - 0.6 0.2

,0 .9' 1 0 0 - 100 - P04 -P , /jg a t/I P04 - P , /jg a t/I WINTER SUMMER 150-

Figure 278. Simplified distribution of phosphate in summer and in winter, after data from Johnston and Jones (1965), Tijssen (1968, 1969, 1970) and others. The vertical section follows the transect ABGD.

A series of distribution charts for various parameters those outside: P:N: Si = 1:14:2, against 1:15:7 in in the nutrient minimum at the end of February shows the margins. The phosphate-nitrate ratio is approx­ more clearly what happens at the very beginning of imately the normal one for the northern North the growing season (Fig. 281a-h). The data are from Sea (Schott and Ehrhardt, 1969), the western Chan­ Tijssen (1969 and unpublished). A very clear patch nel (Cooper, 1933), and in fact for most surface of water (Fig. 281b) is present slightly east of the salt oceanic areas. This means that in the minimum wedge (Fig. 281a), its transparency being due to the NO3-N and especially reactive silica show an extra net removal of suspended matter from the area. In depletion. this patch diatom growth had already started in the It has been said already that silica is mineralized beginning of the m onth (Gieskes and K raay, 1975) with much greater difficulty than phosphate, and we and oxygen concentrations are above saturation (Fig. propose that the extra depletion is due to the fact 281c). Phosphate, nitrate and silicate are low (Fig. that the skeletons of dead diatoms are transported out 28Id, e, and f), but this is only partly due to the of the area before the silica returns into solution. The phytoplankton consumption in this month, since the case for nitrate is less clear, since the missing part may minimum is a persistent phenomenon and already be present as ammonia and nitrite. existed in previous months (Fig. 280). The situation as depicted here for February is not The last two figures (Fig. 281 g and h) show that valid for the rest of the year. After silica has been nutrient ratios in the minimum are different from depleted offshore other phytoplankton species than 352 H. Postma

3561 3552

Cux- haven

Emden 3557

3561 3559

Figure 279. Daily residual currents 2*5 m above the bottom near the Waddensea, 9-25 March 1955. The numbers refer to the day of the measurement; the length of the arrows represents the daily average. (According to Dietrich, 1955.) diatoms succeed and, moreover, growth starts in the and Robinson (1965) and Gushing (1974) have sum­ richer and more turbid areas nearshore. marized the existing information. In the central northern North Sea the behaviour The plankton debris which sink below the thermo­ of nutrients is governed by the establishment of a cline are mineralized and the nutrients return into thermocline in spring and the overturn in autumn. solution. During this cycle the normal P:N: Si ratio The establishment of a thermocline is made possible of 1:15:7 is not disturbed, as can be derived from by the greater water depth and the weaker tides, if the data of Schott and Ehrhardt (1969) ; see Fig. 282. compared with the southern Bight. The resulting In the North Atlantic Ocean the P-Si-ratio gradually distributions in summer and winter are illustrated for increases from an average of 1:7 in the upper 150 m phosphate in Figure 278. to 1:20 below 500 m (Stefansson, 1968). This increase In the central North Sea, phytoplankton production is again due to the fact that silica returns into solution takes place in the period April-May. It stops when more slowly than phosphorus, so that the diatom the nutrients above the thermocline are depleted. A skeletons sink into much deeper water than the organic second production peak occurs in September-October, phosphorus debris before being dissolved. when the decrease of vertical stability brings fresh The Atlantic water entering the North Sea is derived nutrients to the upper water layers. In the northern from the upper layer, so that this area has the lower North Sea, where less light is available in spring and ratio. Since it seems not to change over most of the fall, spring production starts a few weeks later and the North Sea, it might be assumed that the mixed phyto­ autumn peak is absent. This pattern is even more plankton population occurring in this region uses P pronounced in the northeastern North Sea, where and Si in this ratio. Pure diatom populations require, stability is enhanced by Baltic outflow of less saline of course, relatively more silica, the P-Si-ratio being water with low nutrient concentrations. Colebrook 1:16 (Stefansson and Richards, 1963) or even higher; The nutrient contents of North Sea water: Changes in recent years, particularly in the Southern Bight 353

P04-P result is more turbid and offshore the average nutrient level is lower than in the north. Obviously these limiting factors, although quite different, have roughly the same influence. So far, we have only discussed phenomena which involve nutrient cycles not influenced by man. How­ ever, in coastal waters, especially near rivers, increas­ /WADDEN ing amounts of nutrients are brought to the sea from 'SEA the land. Most spectacular is the enrichment of the margins of the Southern Bight. On a large-scale map like Fig. 278 this increase cannot be shown clearly, but extensive sampling programmes of the nearshore zone, such as those carried out by Tijssen (1968, 1969, 1970), reveal phosphate concentrations of more than l-5/igatj\ P and total phosphorus values of more than 2/zgat/l along the Dutch coast. These high values are mainly due to the , 0.5- NORTH SEA,nearshore / and their precise magnitude depends on the percentage of admixed river water. The Rhine itself carries about 18/

23 354 H. Postma

S%o Secci dise, m Febr Febr 1968

02% P04-P Febr 1968 ju.g-at/1 Febr.

Figure 281a, b, c, d. Distribution of several parameters in surface water of the southern North Sea. February 1968. a) salinity, b) transparency, c) oxygen saturation, d) phosphate (PO4-P). are used, assuming that most organic P and N are 281g and h) the nearshore ratio is about 1:30:20 at quickly mineralized in the sea. This may not always a salinity of 29 °/00 (17 % river water). This corresponds be the case, especially in winter. For that time of the best with the calculated ratio on the basis of dissolved year the ratio of the dissolved species is P:N: Si = species, assuming that ammonia is partly oxidized to 1:4-5:19. Dissolved nitrogen is for about half present nitrate: P 04-P: (N03+NH4)-N: Si =1:39:19, and as nitrate and half as ammonia. P 0 4-P :N 0 3-N : Si = 1:23:19. Taking again the example of February 1968 (Fig. Obviously, at that time of year, P and N from the The nutrient contents of North Sea water: Changes in recent years, particularly in the Southern Bight 355

3° 4° 5° t

- 54°- -54°

53°1 - 53°-

30 50 - 52°- -52" 20 40/

2° 4° 5° 2° 3° 4° 5° I- —I— _ i_ N/P Si/P Febr I968 Febr I968 54“ - ■ 54°-

>30,

Figure 28le, f, g, h. Distribution of several parameters in surfac e water of the southern North Sea. February 1968. e) nitrate (NO3-N), f) silicate (H4S i0 4-Si), g) nitrate-phosphate ratio, h) silicate-phosphate ratio. organic matter of the river have not yet been min­ The reverse situation develops in summer : the relative eralized. supply of silica from the river decreases to the marine These calculations, even if not very accurate, dem­ ratio and in warmer water P and N will go into solu­ onstrate that in late winter silica in the nearshore tion more rapidly. Since, moreover, the silica consumed zone is still present in excess quantities because of the by diatoms returns into solution much more slowly, a high winter load of the river and also because not all silica shortage develops soon after the spring bloom organic P and N of the river have been mineralized. (van Bennekom et al., 1975; Gieskes and Kraay, 1975).

23* 356 H . Postma

P0,-P [pgat/l]

0.8

0.7

0.6

0.5 0.5

0.4 0.4

0.2

01 f .

Figure 282. Relation between phosphate and nitrate, and phosphate and silicate in the northern North Sea (according to Schott and Ehrhardt, 1969).

Such a shortage is a rare phenomenon in nearshore Sea and the adjacent Channel over the last thirty waters, since most rivers carry dissolved silica in excess years. This indicates that an increase of primary pro­ amounts. A few decades ago this was also still the case ductivity may have taken place over the whole area. with the Rhine, but since then the amounts of N and For the regions outside direct river influence there are P have risen rapidly, whereas the silica concentration no firm indications of a rise in nutrient concentrations has remained approximately constant. in the same period. However, one would expect that Van Bennekom et al. (1975) and Gieskes and Kraay these regions benefit in some way or other from the (1975) have shown that this relative shortage of increased nearshore productivity. silicate limits diatom growth, so that primary pro­ Much more evident is the influence of this increased duction by this species may not have increased. After productivity on the Waddensea which, as discussed the spring bloom, however, species not using silica, earlier, catches particulate organic matter produced such as Phaeocystis, take over, and phosphate, even in the adjacent North Sea. De Jonge and Postma near the coast, may occasionally almost be depleted, (1974) have shown that in the western Waddensea so that addition of the activities of all species may give this amount has increased from 80 g/m2/year in 1950 a greater primary production than in the past. to 240 g/m2/year in 1970, expressed as carbon. If Direct measurements of primary production before these numbers would hold for the whole Waddensea 1965 are lacking, so that only indirect estimates are (6000 km2), the increase would be from 0-5 x 106 tonnes possible. The author has made such an estimate, in 1950 to l-5xl06 tonnes in 1970. It is not amazing taking into account that potential productivity (meas­ that the percentual increase (65%) is higher than for urement of 14C uptake under constant illumination) the North Sea as a whole (30%) since the Waddensea in the vegetative season increases sharply towards the receives mainly Dutch North Sea coastal water in coast in direct correlation with phosphate and nitrate which most of the eutrophication has occurred. concentrations (Postma, 1973). Assuming that this In addition, it is interesting to note that of the increase is mainly caused by the eutrophying effect of roughly 3xl06 tonnes increase in the North Sea the Rhine, and supposing further that the basic pro­ between the early fifties and 1970, about 10® tonnes, duction of the Southern Bight without river enrich­ or one third, would be carried into the Waddensea. ment would be 100 g C/m2/year, the primary pro­ One might speculate that another third moves into duction would have increased from 10 x 106 to 14 x 106 other estuaries around the Southern Bight and that tonnes/year, expressed as carbon. This means that the remaining third is mineralized in nearshore waters. 30% of present production is due to river influence. The estuaries would then be the principal beneficiaries Since 1955 the increase would have been about 25%. from the increase in North Sea productivity. In other Continuous Plankton Recorder data show an in­ words, the nursery grounds for fish would be the main crease of chlorophyll for the whole southern North receivers of extra food. Whether the fish stocks them­ The nutrient contents of North Sea water: Changes in recent years, particularly in the Southern Bight 357 selves benefit from this state of affairs depends, of Gieskes, W. W. C. & Kraay, G. W. 1975. The phytoplankton course, on whether food is a limiting factor here. The spring bloom in Dutch coastal waters of the North Sea. Neth. J. Sea Res., 9: 166-196. answer must be given by the marine biologists. Helder, W. 1974. The cycle of dissolved inorganic nitrogen com­ Possibly, the mussel culture in the Dutch Waddensea pounds in the Dutch Wadden Sea. Neth. J. Sea Res., 8: 154— has gained from the increase of particulate organic 173. m atter. Johnston, R. & Jones, P. G. W. 1965. Inorganic nutrients in the North Sea. Serial Atlas of the Mar. En v., 11. Am. Geogr. Soc. REFERENCES Jonge, V. N. de & Postma, H. 1974. Phosphorus compounds Bennekom, A. J. van, Krijgsman-van Hartingsveld, E., Veer, in the Dutch Wadden Sea. Neth. J. Sea Res., 8: 139-153. G. C. M. van der, and Voorst, H. F. J. van. 1974. The seasonal Postma, H. 1954. Hydrography of the Dutch Wadden Sea. cycles of reactive silicate and suspended diatoms in the Dutch Arch, néerl. Zoöl., 10: 405-511. Wadden Sea. Neth. J. Sea Res., 8: 174—207. Postma, H. 1966. The cycle of nitrogen in the Wadden Sea and Bennekom, A. J. van, Gieskes, W. W. G. & Tijssen, S. B. 1975. adjacent areas. Neth. J. Sea Res., 3: 186-221. Eutrophication of Dutch coastal waters. Proc. R. Soc. Lond. B., Postma, H. 1973. Transport and budget of organic matter in 189: 359-374. the North Sea. North Sea Science, MIT Press, Cambridge, Cartwright, D. E. 1961. A study of currents in the Strait of Dover. Mass., 326-333. J. Inst. Navigation, 14: 130-151. Pratje, O. 1949. Die Bodenbedeckung der nordeuropäischen Colebrook, J. M. & Robinson, G. A. 1961. The seasonal cycle of Meere. Handb. der Seefischerei Nordeuropas, 1 (3). the plankton in the North Sea and the northeastern Atlantic. Ramster, J. W. 1965. Studies with the Woodhead sea-bed drifter J. Cons. perm. int. Explor. Mer, 26: 156-165. in the southern North Sea. Lab. Leaft. Fish. Lab. Lowestoft, Cooper, L. H. N. 1933. Chemical constituents of biological im­ 6: 1-4. portance in the English Channel, November, 1930 to January, Schott, F. & Ehrhardt, M. 1969. On fluctuations and mean 1932. J. mar. biol. Ass. U.K., 18: 677-753. relations of chemical parameters in the northwestern North Cushing, D. H. 1973. Productivity of the North Sea. North Sea Sea. Kieler Meeresforsch., 25: 272-278. Science, M IT Press, Cambridge, Mass., 249-266. Steele, J. H. 1956. Plant production on the Fladen Ground. Dietrich, G. 1955. Ergebnisse synoptischer ozeanographischer J. mar. biol. Ass. U.K ., 35: 1-33. Arbeiten in der Nordsee. Tagungsber. Deutsch. Geographentag Steemann Nielsen, E. 1960. Productivity of the oceans. Am. Rev. Hamburg, 376-383. Plant Physiol., 11: 341-362. Eisma, D. 1973. Sediment distribution in the North Sea in relation Tijssen, S. B. 1968, 1969, 1970. Hydrographical and chemical to marine pollution. North Sea Science, M IT Press, Cambridge, observations in the Southern Bight. Annls biol., Copenh., 24: Mass., 131-150. 52-56, 25: 51-59, 26: 73-81.