The oxygen budget of the western Wadden Sea, The Netherlands

Esruarine,

Coastal

and Shelf

Science

The Oxygen Budget Sea, The Netherlands

(199 1) 32,483-502

of the Western

Wadden

J. M. J. Hoppema The Netherlands Institure Texel, The Netherlands Received

28 February

Keywords:

for

Sea Research,

1990 a?rd in revised

P.0.

form

Box 59,179O

4 December

AB

Den Burg

1990

oxygen balance; transport; seasonal; tidal basin; Dutch Wadden Sea

The annual cycle of oxygen in 1986 was studied in the western Wadden Sea by full tide measurements at representative locations in the tidal channels. In winter a situation close to saturation was encountered, whereas in the period May to October substantial deviations from saturation occurred. In late May in the outer Marsdiep basin oxygen supersaturation up to 148”,, was found, which was caused by a high rate of phytoplankton primary production. During most of the summer and autumn undersaturation occurred, but only to a minor extent. There was a general gradient of decreasing oxygen towards the basin interior during this period. Near the IJsselmeer sluices which discharge freshwater into the area, some strikingly deviant tidal cycles of low oxygen were observed. These were probably caused by the decomposition of large amounts of dying freshwater phytoplankton. On an annual basis the Marsdiep basin was calculated to be a pronounced sink for oxygen, the deficit amounting to 8.1 umol 1 ‘. Budget and transport calculations resulted in two independent estimates of ‘ excess mineralization’ in the Marsdiep basin of 132-138 x lo6 kg 0, year ‘. The supply of oxygen from the atmosphere amounted to more than half this figure, the remainder being supplemented through the tidal inlet.

Introduction A number of chemical and biological processes take place in estuaries that do not occur on the same scale in other aquatic environments. This is because an estuary is a transition zone between the continent and the sea and between fresh- and salt water. Sharp gradients in dissolved and suspended matter are observed. The chemical and biological processes and their variability have been investigated with salinity as an index (e.g. Boyle et al., 1974; Liss, 1976). Most processes are strongly interrelated and thus the observed concentration changes are mostly the result of a combination of them. Dissolved oxygen (DO) concentration is determined by biological (primary production, decomposition) and physical (estuarine mixing, atmospheric exchange) processes. In addition, it is a measure of the extent of human influence (pollution, e.g. Morris et al., 1982; Sharp et al., 1982; de Sousa & Sen Gupta, 1986). The Wadden Sea, in the north of The Netherlands, is a shallow coastal sea with estuarine properties. The two western basins receive freshwater from lake IJsselmeer 0272-77

14/91!050483

+ 20 $03.00/O

@ 1991

Academic

Press

Limited

J. M. 3. Hoppema

484

4045'

5OE

5015'

5030. 1

/'

53030'

NORTH

SEA

53QN

IJSSELMEER

4045'

50E

5015'

5030

Figure 1. Map of the western Wadden Sea with positions tidal channels where the stations are located are named B, Doove Balg; C, Wierbalg; D, Vlieree; E, Blauwe Slenk;

TABLE

1. Properties

of the Marsdiep

Tidal mean depth Tidal mean volume Total surface area Surface area tidal flats Surface area subtidal

basin (from

EON,

of the anchor stations. The as follows: A, Texelstroom; F, Meep.

1988)

3.9 m

3x 742.5 x 143.8 x 470.8 x

109m) 10bm2 lo6 m* 106m2

through sluices in the enclosure dam Afsluitdijk (Figure 1). Other, much smaller freshwater sources are situated near the towns of Den Helder and Harlingen. The exchange with the North Seatakes place through two tidal inlets, Marsdiep and Vlie. The inner area consistsof an intricate system of tidal channels, subtidal and tidal flats; the latter run dry around low water. The hydrography of the area was extensively described by Postma (1954, 1982). The properties of the westernmost Marsdiep basin are given in Table 1. It is generally assumed that the Wadden Sea is a net mineralization area, because organic matter originating from the North Seaand other sourcesis eventually mineralized

Oxygen

budget

of the

western

Wadden

Sea

485

in its inner parts (Postma, 1954, 1981; de Jonge 8z Postma, 1974; Helder, 1974; Cadie, 1980). It should follow that there is a net oxygen consumption. In a previous study (de Groot & Postma, 1968) the oxygen content was found below saturation in summer and autumn as a result of high mineralization activity. Other studies have demonstrated that locally on the tidal flats very low oxygen saturations can occur (Tijssen & van Bennekom, 1976; van der Veer & Bergman, 1986). In this paper an extensive set of oxygen data of the western Wadden Sea is presented. Until now no systematic study of the annual cycle of oxygen has been undertaken in this area. By measuring oxygen during full tidal periods at different locations more information was obtained about small-scale processes and variability in time. In addition, a budget is constructed to gain information about the relative importance of oxygenmodifying processes on an annual scale. Sampling

and methods

During 1986 water samples were collected at six stations in the western Wadden Sea, three in the Marsdiep tidal basin (A-C, Figure 1) and another three in the Vlie tidal basin (D-F, Figure 1). Stations A and D with depths of 19 and 22 m, respectively, were close to the tidal inlets. All other stations were situated in smaller tidal channels in the basin interior and were consequently shallower: station B 11 m; station C 8 m; station E 11 m; and station F 11 m. Each station was visited every 2 months. During a complete tidal period subsurface and near-bottom samples (and at the main channel stations A and D mid-depth samples also) were taken approximately every hour with a 5-1 Niskin bottle. Moreover, subsurface samples were collected along traverses in the rest of the basin and in the northern part of lake IJsselmeer. From the Niskin bottle, subsamples were taken in calibrated 11%ml bottles for determination of dissolved oxygen by the Carpenter (1965) modification of the Winkler method. Reagents were added immediately and samples were stored under water at approximately in situ temperature, under repeated shaking. The determinations were conducted on-board during the cruises in the Vlie tidal basin. For the other cruises the analyses were done in the laboratory within 1 week. The precision, determined from replicate analyses, amounted to 0.2”,, . Salinity and temperature were obtained from a Guildline estuarine probe CTD. The CTD salinity was checked frequently by comparing it with discrete water samples taken concurrently, the salinity of which was measured in the laboratory by a Guildline Autosal salinometer, model 8400 A. Oxygen saturations were calculated using the solubility equations of Weiss (1970). Results Table 2 gives the mean monthly data for oxygen, salinity and temperature during 1986. The coefficient of variation (CV), defined as the standard deviation as percentage of the average, gives a measure for the intratidal variation. At stations A and D water from the North Sea exerts its largest influence. Obviously the highest salinities were observed here, whereas the variability of the salinities (and the oxygen values) was the smallest. During ebb flow, stations C and F are expected to be directly influenced by water running off the tidal flats that surround them. In addition, station C is situated in the tidal channel that receives most of the freshwater discharged by the sluices near Den Oever (Postma, 1954). This was reflected in the mean salinity at this station, which was relatively low. An exception occurred in January, when the salinity and also the temperature were much

486

J. M.

3. Hoppenu

TABLE 2. Tidal mean salinity (S), temperature (T), dissolved oxygen (DO) and oxygen saturation with tidal range (in brackets) and coefficient of variation (I’,,) for all cruises in 1986 Cruise

Station

Marsdiep basin 1 21-27 Jan.

A B C

s

T( ‘3

A Mar. B C

A May B C

342+ 1.6”,, (330-350) 332k 1.2”,, (325-338) 329k l.l”,, (323-335) 334+2.3”,,

97.5 +0.9”,, (95.3-98.5) 96.7+0,8”,, (954-97.4) 99.6* l,l”,, (97&101~4) 97,9* 1,5”,,

27.41 k 1.7”,, (26.92-28.32) 25.54* 3.3”,, (24.61-27.25) 23,02* 10,6”,, (1510-26.35)

0.41 + 50.9”,, (0.19-1.21) 1,04+ 12.6”,, (0.84-1.29) 1,19+ 19.0”,, (0.84-1.56)

374&0.7”,, (368-378) 374*0.8”,, (368-383) 386 _+4.4”,, (371-452) 381 *2.8”,,

100~0~0~7”,, (986-101.4) 100.3f0.7”,, (986-103.0) 102.2+3.2”,, (99.7-l 14.7) 100.8 & 2.0”,, 128.5*4.2”,, (123.3-148.0) 118.3k2.4”,, (114G123.7) 102.1*6.9”,, (92.9-I 12.8) 116.3 + 10.6”,,

25.61 f 50”,, (22.99-27.52) 21,12+6.0”,, (19.3tL23.29) 25.85 k 7.8”, (1985-27.47)

12.88+3.8”,, (12.23-14.50) 14.59+2.2”,, (14.17-15.26) 13.58+ 3.9”, (12.88-14.64)

361 + 4.0L’,, (348-408) 329k 2.3”,, (319-345) 282_f6.5”,, (257-310) 330 * 11 .O”,,

26.92*2.9”,, (25.70-28.42) 25.93k2,900 (2489-27.40) 24,66F2.2°,, (23.74-25.53)

17,06+4,7”,, (15.88-18.37) 18.43+2.4”,, (18.03-19.51) 18.86 + 2.9”,, (18.0619.64)

223 ;t 3.8”,, (214-238) 230 + 11.4”,, (203-296) 242 k 18.7”,, (188-317) 230 k 12.5”,,

87.2+ 3.4”,, (84.2-98.4) 91.8*12.1”,, (80.9-120.9) 96.7+ 19.4”,, (74.5128.4) 91.9* 13.2”,,

28.62 k 1.2”,, (28.17-29.76) 24.66+ 8.6[‘,, (21.30-28.21) 1957 k 24.2”,, (11.062538)

13.55&1.1”, (13.24-13.89) 13.19kO.5”,, (13.09-13.37) 12.98*2.0”,, (12.64-13.40)

247 f 1,3”,, (243-256) 254+2.4”,, (246-273) 273 f 6.4”,, (253-318) 256&5.6”,,

90.9+ 1.2”,, (89.5-94.4) 90.6+2.1”,, (874-95.7) 93.9+ 3,9”,, (89.7-103.6) 91.8+2.8”,, (Conrimed)

mean 7 24-26

A Jun. B C mean

9 16-18

A Sep. B C mean

O,“,,

2.99 +4.3”,, (2.79-3.25) 4,21*0.8”,, (4.19424) 4.50* 3,0”,, (4.35-4.75)

mean 5 20-22

‘)

26.93+ 3.1”,, (25.02-2810) 25.80k6,7°,, (23.30-28.14) 30.49 * 1,3”,, (29.08-30.95)

mean 3 11-13

DO(pmol1

higher at station C than at the other stations. This indicates that the average conditions were temporarily reversed, probably due to rapidly changing external conditions (meteorology and discharge regime). The generally lower salinity at station B is a result of its position near the sluices of Kornwerderzand. In Table 2 both dissolved oxygen concentration (DO) and percentage saturation (Ozoo) are given. Large mean deviations from saturation occurred particularly in the Marsdiep basin in May, June and September. Oxygen variations within the basin and/or with the tide were sometimes dramatic, for example at station C in June the Ozoo ranged between 74.3 and 128.41~~. This indicates that on a small spatial scale the oxygen regime may vary considerably. 020/~j in the Vlie basin tended to be closer to equilibrium than in the

Oxygen budget of the western Wadden Sea

TABLE

Cruise 11 l-6 Nov

2.

487

(Continued) DO(pmol1

‘)

Station

s

A

27.65 If: 2,9”,, (25%-28.90) 22.82* 5,2”, (21.14-24.96) 22.31 i 13,5O,, (17.81-26.28)

9.25 +2%, (8.87-9.77) 8.80*0,6”,, (8.71-8.90) 9.03 kO.9”,, (8.92-9.17)

289*1.4”,, (282-299) 298 f 1.3”,, (291-305) 299+ 1,8”,, (289-309) 294 _+ 2-2”,,

96.1 FO+‘,, (95.5-97.5) 95.2 FOW,, (93.1-96.7 I 95.7&09,, (94.2-96.8 5 95,7 +0+3”,,

28.37_fS.l”,, (25.82-30.11) 29.33 +0.7”,, (28.88-29.70)

4.99_+ 2.0”,, (4.85-5.16) 4.84&8.7”,, (4.38-5.70)

327 & 0.7”,, (32 l-330) 332 *OW,, (325-337) 329 + l-O”,.

99.0 + 1 1 “,, (96.1-100.0) 100.61 l-l”,, (98.7-102.5’1 99.8 +- l-3”.

B C

T

C)

mean

O,“,

Vlie basin 4

E

Apr.

8-9

F

6

D

3-5 Jun. E F

8 19-21

Aug.

10 7-9 Ott

D E F

31.13+2.3”,, (29.64-32.12) 27,78k3.1U,, (26.2S29.56) 28.12+3.1”,, (26.12-29.62)

13.46+2.5”,, (12-94-14.07) 13.77k1.8”,, (13.19-14.10) 11.81_+5.6”,, (10.54-12.80)

244*2.3”,, (236255) 249) 3,4”,, (234-261) 265 f 3.8”,, (246278) 248 + 49’,,

91.112 l”,, (884-94.9) 90.4 & 3.0”,, (866-44.8‘1 93.8 + 2.3” (89.2h7.01’ 01.8 t 2.9”.,

31,95* I.O”,, (31.1 l-32.39) 29.94* 3.6”,, (27.17-31.18) 29.78+ 1.8”,, (28.74-30.50)

17.73F0.5”,, (17.50-17.89) 16.80+0.8”,, (1653-16.97) 17,19* 1.3”,, 116.76-17.411

251 f 1.6”,, (243-261) 246 i 5.0”,, (229-283) 244 k 5.3<‘,, (225-262) 248 f 4.1 j’,,

lW6k 1 7X, (99.LL106.7) 96.7+ 39,, (90.2-104.3) 97.3&4.7”,, (905-103.2) 98.9 & 44”,,

32.47+0.5”,, (32.08-32.77) 29,71+5,7”,, (25.72-31.55) 30.69 k 3.4”,, (27.88-31.61)

14.41+0.3”,, (14.34-14.56) 13.98f0.5”,, (13.84-14.04) 13.99+0.6”,, (13~7Gl4~08)

283 k 1.3”,, (273-288) 262+1.1”,, (255-268) 258 k 2.7”,, (251-271) 271 +4.5”,,

108.41 1 -3”,, (104.9-l lo+! 98.0 f- 2.0”,, (93.1-100.7) 97.2+-26”,, ;92.>102.0‘1 101.2+5.6”,,

320 k 2.2”,, (308-334) 315+2.2”,, (302-327) 317+2.3”,,

99.9,0.7”,, (98.8-101-9) 98.3 & 1 .(I”,, (96.7-101.7! 99.1 * 1.2”,,

mean 12 2-3 Dec.

E F

24.5Ok9.4”” (18.70-27.53) 25~10f%l”, (2 1.20-28.63)

7.35 f 2.2”,, (7.09-7.64) 7.22* 4.2”,, (6.76-7-87)

Marsdiep basin. The high supersaturation in May, which waspresent in nearly the whole area, lasted only a relatively short period (of the order of 1 month). Following this period the seawaterwas generally undersaturated for about 4 months. The variable local environments in the Wadden Sea necessitateconsideration not only of general trends, but also of distributions and variability on a smaller scale. For this purpose a selection of tidal cycles of Ozq;, over the year is presented in Figure 2. No systematic vertical differences within the water column were observed during the tidal cycle for nearly all measurements.This is indeed what we expected to find, becauseof the strong tidal currents and estuarine circulation. For this reason, it is very interesting to find

488

J. M. J. Hoppemu

I

HW

0

I

8

I

I

IO

I

I

HW

10

I

I

I

12

I

12

I

I

LW

I

L

14

I

I

14 LW

I

I

16

I

I6

1

I

t--

I8

,

I

I8

20

Time

Figure 2. Selection of tidal cycles of oxygen saturation station A; (b) 22 May station C; (c) 26 June station II, bottom.

at anchor stations. (a) 20 May C. 0, Surface; x , mid-depth;

a large difference of Oz’-‘/bbetween the surface water and the rest of the water column in May at station A during most of the tidal cycle. Clear maxima or minima in O,Oh nearly always coincided with high water (HW) or low water (LW).

Ox-ygen budget of the western Wadden Seu

389

Time

Figure 2. (c).

Figure 3 shows02”o asa function of the distance from the tidal inlet for three different tidal channels. In the tidal channel Blauwe Slenk [Figure 3(a)] a small but significant decreasetowards the coast near Harlingen was generally observed. It is probably a result of minor dischargesof organic wastesoriginating from the town and the hinterland. The IJsselmeer sluices are situated at the landward end of the tidal channels Wierbalg and Doove Balg in the Marsdiep basin. The freshwater lake was supersaturated in oxygen for most of the year [values near the sluices are included in Figure 3(b)]. Mixing of the freshwater with Wadden Sea water, therefore, mainly enhances the saturation of the mixture compared with the original Wadden Sea water mass. Small-scale local mixing processeswill further modify the spatial distribution in the tidal channel.

Discussion Seasonal

behaviour

The general cycle of DO in the Wadden Sea (Figure 4, Table 2), i.e. high values in winter and spring and lower values in summer and autumn, can partly be explained by changesin temperature. Salinity changesdo not appear to play a role. The curve for 02’),, , however. differs from the DO curve (Figure 4); it comparesbetter with graphs of primary production and chlorophyll (Veldhuis et al., 1988). This indicates that biological processesare probably the principal determining factors in the observed annual cycle. Deviations of DO from equilibrium with the atmosphere were relatively small compared to less well mixed estuaries (Kuo & Neilson, 1987) or heavily polluted estuaries, such asthe Schelde (Wollast et al., 1979).

490

J. M. J. Hoppemu

6110

Z-416

0

5

IO

15

20

25

30

(b)

35

25/6

I IO Distance

I 15 from tidal

I 20 Inlet

I

, 25

3

(km1

Figure 3. Oxygen saturation along traverses from the tidal inlet to the coast or Afsluitdijk for various periods of the year. (a), Vliestroom basin, to coast near Harlingen; (b), Marsdiep basin, to sluice near Den Oever; (c), Marsdiep basin, to sluice near Kornwerderzand.

In autumn, winter and early spring O,?, values were always close to 100°, and variability was small. This indicates that the rates of biological processes were small compared to mixing and atmospheric exchange. This observation is typical for estuaries

Oxygen

budget

of the western

Wadden

Distance

Figure

491

Sea

from

tidal

inlet

(km)

3. (c).

115 360

I IO

z 0”

i

105

_ -

32c

z zx 28C

100

9: j-

9( I-

24(

20(

I

I

I

I

I

I

I

I

/

I

I

JFMAMJJASOND 1986

Figure

4. Annual

cycle of mean DO (13) and mean O,“,,

(CI) for the Marsdiep

basin

492

J. M. 3. Hoppemu

that do not receive substantial amounts of organic pollutants (Morris et al., 1982; Sharp et al., 1982).

In May the high oxygen supersaturation reflected the spring bloom of primary production (Veldhuis et al., 1988). The OZnc,gradient in the Marsdiep basin deduced from the observations at the anchor stations was one of increasing supersaturation from the basin interior towards the tidal inlet [see also Figure 3(c)], suggesting that the highest production rates occurred in the outer basin and the tidal inlet. The OZO,,variability was large during this cruise, with even undersaturation at station C (Figure 2). The explanation for the observation at station C may be that locally large amounts of oxygen are consumed for the degradation of freshwater plankton discharged into the Wadden Seaby the sluice near Den Oever. A similar observation was made at the freshwater-seawater interface in the Tamar estuary (Morris et al., 1978). In summer the OZO(,gradient appeared to be reversed compared with the situation in May. On average, the basins were below saturation, indicating that mineralization was dominant. Possibly only in the tidal flat areas was substantial primary production occurring, asevidenced by the tidal cycles at stations B and C in June (Figure 2). The tidal asymmetry at these stations is most likely caused by the admixture of water originating from the tidal flats, which can exhibit an extremely broad diurnal range of 02”,) values (van der Veer & Bergman, 1986). In October in the Vlie basin (cruise 10) mean supersaturation wasobserved, with the highest values near the tidal inlet. This supersaturation reflected the prolonged influence of the adjacent North Sea,where a phytoplankton bloom wasapparently occurring. Oxygen budget

Using the present data set, an oxygen budget for the Marsdiep tidal basin for the year 1986 can be constructed. The basisfor such a budget is a calculation of the exchange of oxygen with the North Sea and with the neighbouring Vlie tidal basin. Knowledge of exchange processeshasrecently been greatly improved by the construction of a numerical model of the western Wadden Sea (Ridderinkhof, 1988a,b; EON, 1988). The transport of oxygen was calculated with the following general equations adopted from van Raaphorst and van der Veer (1990) MN = C(K(c,

- c,,,J +

Q,,,cJ

QM

= -e,-fQ,-Qv

(2)

c,

= i(CA + CN)

(3)

Equation (1) represents the massoxygen exchange between the Marsdiep basin and the North Seathrough the tidal inlet; the first term on the right-hand side is the tidal mean diffusive masstransport, which is dependent on the concentration difference between the North Sea and Marsdiep basin. K is an effective exchange coefficient for the Marsdiep basin. It was obtained from calculations of the freshwater content in the basin (Postma, 1954), and adapted to the consequencesof the water import from the adjacent Vlie basin. K amounts to 160 x lo6 m3 tide-’ (Ridderinkhof et al., 1990). According to the assumptions of this method the concentration ci has to be taken in the tidal inlet (Ridderinkhof et al., 1990). Here it is defined as in equation (3) where cNis the North Seavalue near the coast along the northern Dutch mainland (taken from Rijkswaterstaat, 1986), and cA the concentration at station A. The concentration clcIis the mean of all samplesat the anchor

Oxygen

budget

of the western

Wadden

Sea

497

stations in the Marsdiep basin. The second term on the right-hand side of equation (1) parameterizes the tidal mean advective export out of the basin with QM (m3 s’) the transport rate. Q, and Q,are the freshwater discharges from the sluices at Den Oever and Kornwerderzand, respectively. The factor f stands for that part of the freshwater that is actually discharged into the Marsdiep basin from the sluices of Kornwerderzand, the other part flows through the Vlie basin;fis taken to be 0.7 as in van Raaphorst and van der Veer (1990). Q,. represents the tidal mean water transport from the Vlie basin, which was derived from model calculations (EON, 1988) and amounts to 815 mP3 s-l. Table 3 compiles the exchange of oxygen in selected periods of 1986. As a comparison the separate contributions of diffusive and advective transport are shown. The overall transport is highest in winter and lowest in summer. The advective transport is always directed to the North Sea. The diffusive transport is merely a modifier of the more important advective transport. The high total oxygen transport in winter is caused by the high freshwater discharge and the high DO as a consequence of the low water temperatures. The diffusive transport in winter is generally small due to the fact that both the North Sea and the Wadden Sea are close to saturation and the DO difference is small. The diffusive transport in summer is generally directed opposite to the advective transport. This is related to plankton blodms in the North Sea and Wadden Sea not occurring simultaneously. The total annual diffusive transport is directed into the Marsdiep basin and amounts to 27 x 1O6kg 0, year ‘. In a previous study (de Groot & Postma, 1968) the oxygen transport was calculated based on a phosphate concentration difference between the North Sea and the Wadden Sea (Postma, 1954). In the summer and autumn of 1963 the average amount of oxygen per tide transported inward was calculated to be 1 x lo5 kg (de Groot & Postma, 1968). This figure may be compared with the diffusive transports calculated in the present study. Unexpectedly, they compare fairly well (Table 3). From 1950 until 1970 the gradient of phosphate increased significantly (de Jonge & Postma, 1974) and the rising trend continued in recent years (van Raaphorst & van der Veer, 1990). According to the method used by de Groot and Postma (1968), this should be accompanied by an increase of the amount of oxygen transported. Possibly, the increased oxygen demand of the Wadden Sea from 1950 onwards is compensated by an enhanced oxygen production as a result of increased primary production (Cadee, 1986). Oxygen is transported into the Marsdiep basin by advective water flows from the I Jsselmeer and from the Vlie basin (Ridderinkhof, 1988~). Transport from these sources was calculated from the following equations

in which the symbols are the same as in the previous equations. M, is the oxygen import from the I Jsselmeer with crY the DO in the northern part of the lake near Den Oever. M,, is the oxygen import from the Vlie basin with cl..the mean DO at station E (Figure 1). Station E was assumed to be representative of Vlie water entering the Marsdiep basin, because the station is closest to the region through which most water is transported (EON, 1988). This may cause some overestimation of M, since cv at the tidal watershed is probably lower on an annual basis. The DO in the northern IJsselmeer was found to be remarkably constant at 397 + 5 umol I-’ during the measurements. Therefore, the total oxygen import from the

January/mid-February Mid-February/mid-March Mid-March/April May June/mid-July Mid- July/August September October November December

Period

334 381 327” 330 230 246b 256 262” 294 320b

Marsdiep” (cd (urn01 1~ ‘)

DO

328 359 369 453 253 244 262 266 284 297

North (4

OMarsdiep mean, except when stated otherwise. %‘alue from station E.
DO

Sea<

567.5 273.9 377.8 318.9 245.9 110.9 230.7 229.0 307.6 310.0

(Q,“)

401.9 28.1 231.2 174.3 76.6 64.4 163.5 141.2 332.1 253.6

(Q,?

discharge

(m3 s-‘)

Freshwater

TABLE 3. Transport of oxygen between the Marsdiep basin and the North Sea for selected advective and diffusive contributions. A negative sign indicates transport to the North Sea

periods

-8.0 -5.8 -6.7 -7.3 -3.8 -3.4 -4.2 -4.3 -5.6 -5.7

Advective

of 1986. The

f0.1 -0.7 + 1.0 +3.9 +0.4 0 -0.1 +0.1 -0.4 -0.6

(105kgtide~‘)

Diffusive

Transport

total transport

-7.9 -6.5 -5.7 -3.4 -3.4 -3.4 -4.3 -4.2 -6.0 -6.3

Total

is the sum of the

.,/

Oxygen

TABLE

budget

of the western

4. Data used to calculate

Wudden

Sea

oxygen

transport

from

the Vlie basin

DO Vlie(c,.) (pm01 1 ‘)

Period January/February March/April May June/August September/October November/December “Mean

495

Marsdiep

Transport (105kgtide

334” 327 330” 248 262 320

‘1

3.9 3.8 3.8 2.9 3.1 3.7

value.

5. The effect of varying transport coefficients on the closing entry of the transport budget. Budget 5 is considered to be the standard budget with the most probable values according to the literature of K, the effective exchange coefficient, and Qv, the tidal mean water transport from the Vlie basin. Budget units lob kg 0, year ‘. Import entries are added, and export entry subtracted to give the closing entry TABLE

Import K Budget 1 2 3 4 5 6 7 8 9

Qt. (m’s

2505 3579 4653 2505 3579 4653 2505 3579 4653

‘) 489 489 489 815 815 815 1141 1141 1141

from

+

diffusive North Sea

+

+

19 27 35 19 27 35 19 27 35

+

Vlie

146 146 146 244 244 244 341 341 341

from I Jsselmeer 174 174 174 174 174 174 1’74 174 174

-

Export

--

to North Sea

-

287 287 287 387 387 387 486 486 486

=

=

Closing entry 52 60 68 50 58 66 48 56 64

IJsselmeer was calculated with this value for the whole year and the total discharges through the sluices of Den Oever and Kornwerderzand of 9.5 x lo9 m3 and 6.0 x lo9 m3, respectively. The annual oxygen import from the I Jsselmeer amounted to 174 x 1O6kg. In Table 4 the data used to calculate the oxygen transport from the Vlie basin and the transport rates are listed for selected periods. For the periods January/February and May no data were available for station E; therefore, the mean DO for the Marsdiep basin was taken. The advective oxygen transport from the Vlie to the Marsdiep basin in 1986 was calculated to be 244 x lo6 kg. With the annual oxygen transport terms a budget can be constructed which is entirely based on transport calculations. The closing entry of this budget includes budget errors and the amount of oxygen involved in internal processesin the Marsdiep basin, i.e. primary production, mineralization and atmospheric exchange. Large uncertainties imposed on the transport budget are the transport rate and the exchange coefficient, the freshwater dischargesexcepted. To investigate theseuncertainties, the standard exchange coefficient K and the tidal mean water transport rate from the Vlie basin (Q,,) were varied for 30” 0 and 40” v, respectively. Budgets with all possiblecombinations of variations of k’ and Q,, are shown in Table 5, together with the standard budget, no. 5, for which the

496

J. M. J. Hoppema

TABLE 6. Data used to calculate the mean deviation from conservative mixing of oxygen in the Marsdiep basin for selected periods of 1986. A is the difference between the calculated and observed oxygen concentration; a negative A means a deficit. Subscripts M, N, I J denote Marsdiep, North Sea and I Jsselmeer, respectively

DO,,, Period

Salinity

January/mid-February Mid-February/April May

June/July August/September October/December “Interpolated

2774 25.62 24.66 26.02 24.28 24.26

DO,

DO,,

A

(pm01 1 ‘) 334 381 330 230 256 294

328 367 453 247 259 278

402 400” 399 400” 401 391

0 f9 -113 +38 -31 -6

value.

values of the transport coefficients given in the literature were used. It appeared that the variation around the standard closing entry of 58 x lo6 kg 0, year-’ was only 17”,,, which proves that the closing entry is relatively insensitive to transport coefficients. Another uncertainty in the calculations is the low frequency of the measurementsin the Marsdiep basin. This necessitateda representation of the DO for a period of l-1; months by one single set of measurements.However, the observed trend of DO and 0200 (Figure 4) was consistent with measurementsof primary production or mineralization, and with previous measurementsof oxygen in the samearea (de Groot & Postma, 1968), suggesting that the data were at least qualitatively reliable and consistent with the expected trends. The transport calculations demonstrated convincingly that the import of oxygen into the Marsdiep basin is substantially larger than the export. This is a strong indication that in 1986 annual mineralization in the Marsdiep basin washigher than primary production. With the present data set it is possible to double-check the outcome of these budget calculations. This can be done by calculating the annual non-conservative part of oxygen assumingthat in the Marsdiep basin mixing takes place between coastal North Sea water of salinity 30 and freshwater from the I Jsselmeerwith salinity 0.5. The calculated deficit or excessthen is the result of non-conservative processesfrom which primary production, mineralization and atmospheric exchange are the most important ones. The calculations were performed for selected periods of 1986 (Table 6). The annual mean deviation from conservativeness was - 8.1 umol lP ‘, meaning that the Marsdiep basin wasa pronounced sink for oxygen. As the mean ageof Marsdiep water is about 5.5 days (Zimmerman, 1976) the mean consumption rate of oxygen was 1.5 umol 1-l day-’ over 1986. With the volume of the basin (Table 1) a total oxygen consumption of 52 x lo6 kg over 1986 is calculated. This figure should be compared with the closing entry of the standard budget in Table 5. Considering the errors associatedwith the budget calculations this is a very satisfactory result. The deficit calculated above is not to be confused with the mean saturation state, which is the mean difference between the actual DO and the oxygen solubility. The mean undersaturation was 6.7 umol 1-l. This figure is dependent on the atmospheric exchange rate of oxygen, but its magnitude is another indication for the dominance of mineralization. We may finally arrive at an estimation of the imbalance between primary production and mineralization by taking into account the atmospheric exchange of oxygen. The atmospheric exchange was calculated according to the empirical relation

Oxygen

budget

of the western

Wadden

497

Sea

TABLE 7. Data used to calculate oxygen exchange flux Marsdiep basin for selected periods of 1986. A positive seawater

Period January/mid-February IMid-February/April *May June July .4ugust;September October /December OData from ‘Calculated

with the atmosphere flux means evasion

Mean wind” velocity (ms ‘1

Transfer* velocity (mday ‘:

0,-O,* (mm01 m ‘)

7.7 6.2 6.0 5.0 4.8 6.8

3.0 1.9 1.8 1.1 0.9 24

-7 +3 +47 -20 -23 -13

KNMI (1986). according to Liss and Merlivat

Flux(mmo1 mP2 day-‘)

for from

the the

‘dav

‘1

Flux (mmolm

21.0 1-j-7 + 84.6 -22.0 -20.7 3 1 .2

(1986).

= &0,-O,*)

(6)

where k is the transfer velocity (m s-l), and 0, and O,* are the bulk and equilibrium concentration (mmol m-‘) of oxygen in the water column. k is calculated from the wind velocity using the relationship given by Liss and Merlivat (1986). For very shallow systems like the Wadden Sea the relationship is probably formally incorrect, but it serves at least for the calculation of the order of magnitude. For selected periods, ranging from 1 to 3 months, mean transfer velocities were obtained from the wind velocity measured near Den Helder (KNMI, 1986). Data are listed in Table 7. Clearly, oxygen escaped to the atmosphere only in spring. The total annual supply of oxygen from the atmosphere was estimated to be 80 x lo6 kg. This figure is somewhat larger than the oxygen deficit. The atmospheric supply has to be added to the deficit to arrive at the amount of oxygen that was consumed due to the dominance of mineralization over primary production in 1986. This amount will henceforth be called excess mineralization and it is then 132-138 x 10’ kg 0,. The excess mineralization calculated above may be compared with actual measured primary production and mineralization values for the basin. The annual pelagic primary production in 1986 was estimated to be 303 g C me2 year- i (Veldhuis et al., 1988) with an estimated error of 300,, . The primary production by benthic microflora on the tidal flats and subtidals is a very uncertain factor. Data are scarce,the most recent originating in the 1970s(Cadee & Hegeman, 1974; CadCe,1980). Production on the tidal flats wasestimated to be 130f40gCm-2year-’ and on the subtidal areas c. 10gCm-2yearP’. These figures may be substantially underestimated (EON, 1988), but unfortunately, better data are not available. Van Duyl and Kop (1988) cameto an estimateof oxygen consumption in the water column of 683 g 0, mm2year-’ (Ifr 30”,,) for 1986. They (in EON, 1988) concurrently measured the in situ benthic oxygen uptake. The benthic mean mineralization amounted to 135+ 25 g C mP2 year-‘. These literature figures were converted to 0, and are brought together in Table 8. Although the data presented in Table 8 suggest that total primary production and mineralization were balanced, the large errors leave enough room for a possible excess mineralization (but also excess production) as large as that calculated in the present investigation. In all (carbon) budgets of the western Wadden Sea, excessmineralization

498

3. M. 3. Hoppmu

TABLE 8. Literature data on the annual primary production and mineralization Marsdiep tidal basin. Carbon data were converted into oxygen using molar ratio C:O, = 106:138, units 10” kg 0, year ‘. See text for origin of data Primary

production

in the Redfield

Mineralization

Benthic Pelagic

81+25 781 i 234

348+ 507+

104 152

Total

862 k 235

855)

184

TABLE 9. Oxygen budget of the Marsdiep basin in 1986, units 10” kg 0, year ‘. The entry of ‘ excess mineralization ’ comprises large primary production and mineralization contributions, which amount to c. 800 x lob kg 0, year ’ each and those are formally the largest budget entries Import (1) (2) (3) (4)

from from from from

Export I Jsselmeer We North Sea atmosphere

174 244 27 80 525

(5) to North Sea (6) excess mineralization

387 138

525

was not unequivocally established (de Wilde & Beukema, 1984; Vosjan, 1987; EON, 1988), although the indications for excessmineralization are abundant (e.g. de Jonge & Postma, 1974; Helder, 1974). Here it is stated that detailed oxygen measurementsof water samplesare better suited for this purpose. In Table 9 the oxygen budget for the Marsdiep basin in 1986 is given. The budget is dominated by advective transport terms. However, it should be realized that the excess mineralization term embraceslarge primary production and mineralization terms of the order of magnitude of 800 x IO6kg 0,. The merit of the budget presented here is that this excessmineralization was obtained independently of production or mineralization data. Excess mineralization accounts for 26:j0 of the budget total. If the advective transport entries are not considered, the net excessoxygen consumption is more than 50°j0counterbalanced by the input of oxygen from the atmosphere. Another substantial part of the net consumed oxygen is replaced by diffusive exchange with the North Sea. This meansthat on average there will be a gradient of decreasing DO and O,% between the tidal inlet and the Marsdiep interior. This gradient was observed particularly in the summer and autumn. Similar observations were made in the early 1960s(de Groot & Postma, 1968), which suggeststhat the oxygen cycle has not changed much, at least qualitatively, since that time. IJsselmeer

influence

In the original concept of the biogeochemical functioning of the Wadden Sea it was thought that the Wadden Seatraps excessorganic matter from the North Sea, mineralizes

Oxygen budget of the western Wadden Sea

499

it in its inner parts and subsequently exports dissolved nutrients back to the North Sea (Postma, 1954). The input of organic matter from the IJsselmeer was thought to be negligible in this concept and a residual transport from the Vlie basin was not taken into account. Cadee (1980), however, found that the I Jsselmeercontributes significantly to the particulate organic carbon (POC) of the Marsdiep basin. Recently, it wasclaimed that the North Seaplayed only a minor role in the organic matter budget of the Marsdiep basin and that probably in the last 15 years the Wadden Seaeven exported particulate organic matter (van Raaphorst & van der Veer, 1990). Moreover, the Vlie basin was thought to be a considerable contributor of organic matter to the Marsdiep basin. Based on the excessmineralization the relative oxygen consumption due to organic matter import from the IJsselmeer is estimated as follows: the amount of particulate organic carbon (POC) discharged in 1986 through the sluices of Den Oever and Kornwerderzand was 22 x lo6 kg C and 23 x lo6 kg C, respectively (EON, 1988). Only 70”,, of the discharge of Kornwerderzand eventually showsup in the Marsdiep basin (van Raaphorst & van der Veer, 1990). Not all of this POC will be decomposedin the basin. The POC retained may be estimated from the tracing of a typical I Jsselmeerphytoplankton speciesin the Marsdiep basin (Cadee, 1980). If the concept of fractional loss (Officer, 1979) is applied to the plot of fihytoplankton cells against salinity in Cadee (1980), it is concluded that 80” o of the freshwater phytoplankton isretained in the basin. Hence, it will be assumedthat from all POC 80°, will be completely mineralized within the Marsdiep basin. A large part of IJsselmeer POC consists of living phytoplankton (Cadte, 1980), which renders it very prone to mineralization. The discharge of I JsselmeerDOC waseven greater than of IJsselmeer POC. However, most of the IJsselmeer DOC is refractory, mixes conservatively in the Marsdiep basin and will move unchanged to the North Sea (Laane, 1980). The oxygen consumed by the mineralization of 800,, of the IJsselmeer POC in the Marsdiep basin is 106 x lo6 kg (conversion with Redfield ratio), which is 77”,, of the excessmineralization. This percentage is higher when the lower limit of the excess mineralization is applied. This result is close to the outcome of budget calculations on phosphorus in the Marsdiep basin, in which the freshwater contribution to the particulate phosphorus input in the 1980swas estimated to be 5%60”,, of the total. In the present paper it has already been demonstrated that at station C in May the influence on the oxygen regime of the Marsdiep basin exerted by the I Jsselmeercould be observed directly. A sharp decreasein 0200 after the discharge of freshwater was also observed during other cruises [Figure 3(b)]. A decreaseof oxygen in the initial mixing zone of the Wadden Sea, but also in other estuaries(e.g. Morris et al., 1982; Sharp et al.. 1982), is a rather general phenomenon. However, the causeof such an oxygen depression may differ from one time to another. When the temperature is sufficiently high, mineral-ization may be the major cause, which was proved by coincident changesin total carbon dioxide (Hoppema, 1991). A decrease outside the vegetative seasonwill probably be causedby the rapid evasion of oxygen from the mainly supersaturated freshwater to the atmosphere. Mineralization of POC from the I Jsselmeerin the Wadden Sea will be low particularly in winter. Thus, the calculated IJsselmeer contribution to the excess mineralization in the Marsdiep basin might be somewhat overestimated. It is interesting to view the excessmineralization in the light of the ongoing eutrophication of the Wadden Sea by the inflow of eutrophic source waters. As a consequenceof this eutrophication, the primary production in the Marsdiep basin showed an increase (Cadee, 1986). The resemblance of the annual cycle of oxygen to the annual cycle of primary production suggeststhat these two properties should also co-vary on a longer

500

time scale. No from the past 1960s seem to very intensive

J. M. J. Hoppemu

sufficiently detailed oxygen data set of the western Wadden Sea is available to permit a firm conclusion, but qualitatively the changes since the early be only minor (de Groot & Postma, 1968). This might be ascribed to the exchange of oxygen with the atmosphere. Conclusions

The oxygen cycle ofthe western Wadden Sea resembles that in both relatively undisturbed estuaries and organically loaded estuaries. The oxygen saturations observed were generally not far from the atmospheric equilibrium value throughout the year. The load ofanthropogenie organic wastes is small, and hardly observable in terms of anomalous oxygen distributions. On the other hand, an excess mineralization, i.e. excess oxygen consumption, was established for the Marsdiep basin on an annual basis. The excess mineralization might be the result of the same sequence of accumulation processes as those that originally brought the Wadden Sea into existence, and thus it is a fundamental property of the system. It should be realized that the present study is concerned with the dynamics of oxygen in the tidal channels of the area. The largest gradients and variability are expected to occur on the subtidal and tidal flats, due to the fact that the actual formation process of the Wadden Sea takes place in these areas. Also, an important part of the Wadden Sea ecosystem has found a habitat in the subtidal and tidal flats. This investigation has demonstrated that in the channels the oxygen content throughout the year is probably sufficient to support life. However, this is not certain on the tidal flats, and more study is needed. Another issue that has received much attention lately is the role of the discharge of organic matter originating from the IJsselmeer on the organic matter budget of the western Wadden Sea (EON, 1988; van Raaphorst & van der Veer, 1990). The present study investigated what part of the excess mineralization was caused by I Jsselmeer discharges. Some direct indications were found for the influence of organic matter of freshwater origin on the oxygen level of the Marsdiep basin at a station close to the sluice. Budget calculations showed that the oxygen demand arising from the freshwater inflow might account for more than half of the excess mineralization. The present investigation gives no decisive answer to the role of the North Sea on the organic matter budget of the Marsdiep basin. The diffusive oxygen import due to concentration differences between the coastal North Sea and the Marsdiep basin is significant and comparable to the atmospheric contribution. Acknowledgements I wish to thank J. W. Rommets for doing part of the experimental work, M. W. Manuels for the salinity and temperature data and the EON group for supplying the samples of the transects. The hospitality of the Ministry of Transport and Public Works on its ships Breesem and Blauwe Slenk was appreciated. Professor H. Postma, Dr W. van Raaphorst and Dr S. B. Tijssen gave valuable comments on an earlier version of the manuscript. References Boyle,

E., Collier, R., Dengler, A. T., Edmond, J. M., Ng, A. C. & Stallard, R. F. 1974 On the chemical massbalance in estuaries. Geochimica ef Cosmochimica Acta 38,1719-1728. CadPe, G. C. 1980 Reappraisal of the production and import of organic carbon in the western Wadden Sea. The NetherlandsJournal of Sea Research 14,305-322.

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of the western

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SO1

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W. 1990 The phosphorus budget of the Marsdiep tidal basin (Dutch Wadden Sea) in the period 1950-1985: importance of the exchange with the North Sea. Hydrobiologia 195,21-38. van der Veer, H. W. & Bergman, M. J. N. 1986 Development of tidally related behaviour of a newly settled O-group plaice (Pleuronectes platessa) population in the western Wadden Sea. Marine Ecology Pro.gress StTics 31, 121-129.

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