Influence or organic pollution on bacterial, macrobenthic and meiobenthic populations in intertidal flats of the dollard

Netherlands ffournal of Sea Research 14 (3/4) : 288-304 (1980)

INFLUENCE OF ORGANIC POLLUTION ON BACTERIAL, MACROBENTHICAND MEIOBENTHIC POPULATIONS IN INTERTIDAL FLATS OF THEDOLLARD* by F. B. V A N ES Biological Centre, Kerklaan 30, 9751 N N Haren, The .Netherlands M, A, V A N A R K E L Netherlands Institute for Sea Research, P.O. Box 59, 1790 AB Den Burg, Texel, The Netherlands L. A. B O U W M A N and H. G . J . S C H R ( ~ D E R Biological Centre, Kerklaan 30, 9751 N N Haren, The Netherlands CONTENTS I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . II. Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . III. Results and Discussion . . . . . . . . . . . . . . . . . . . . . 1. The waste-water discharges . . . . . . . . . . . . . . . . . . 2. Oxygen in the water over the tidal flats . . . . . . . . . . . . . 3. Sediment characteristics . . . . . . . . . . . . . . . . . . . . 4. Bacterial populations . . . . . . . . . . . . . . . . . . . . . 5. Macro- and meiobenthic fauna . . . . . . . . . . . . . . . . . IV. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . V. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . VI. References . . . . . . . . . . . . . . . . . . . . . . . . . . .

288 290 290 290 292 293 295 298 302 302 303

I. I N T R O D U C T I O N

In a previous report

(SCHRODER& VAN Es, 1980) a microbiological

investigation of the Ems Estuary tidal flat sediments was described. That investigation was done in the framework of a comprehensive b i o l o g i c a l s t u d y o f t h e Eros E s t u a r y w i t h t h e o b j e c t i v e t o d e s c r i b e t h e factors regulating the structure and processes within that ecosystem. I n a p a r t o f t h e e s t u a r y t h e i n f l u e n c e o f o n e o f t h e s e f a c t o r s is v e r y p r o n o u n c e d . I n t h e s o u t h e a s t o f t h e D o l l a r d ( F i g . 1) e n o r m o u s a m o u n t s of waste-water are discharged through the Westerwoldse A, a little r i v e r . T h i s w a s t e , w a t e r o r i g i n a t e s m a i n l y f r o m p o t a t o - f l o u r mills a n d * Publication no. 29 of the project Biological Research in the Ems-Dollard Estuary

(BOEDE).

ORGANIC P O L L U T I O N DOLLARD

289

is rich in readily degradable organic matter. If expressed in terms of the waste-water produced by a civil population, up to ten million inhabitant-equivalents or even more are discharged to the estuary per day in autumn and early winter. During low tides the completely anaerobic waste-water is sluiced into the main tidal channel through a creek. The initial mixing with estuarine water in this channel is poor because of the difference in salinity: the fresh water floats on the heavier estuarine water (salinity c a 10%o). During high tide the wastewater becomes dispersed over the tidal flats. In the present investigation the effect of the organic pollution on the microbial and faunal population in the tidal flat sediments of the Dollard was studied in more detail. As in the previous investigation (ScHR6DER • VAN ES, 1980) the three physiological groups of bacteria studied were aerobic heterotrophic, anaerobic sulphate-reducing, and aerobic thiosulphate-oxidizing bacteria. A gradient can be observed in the sediment of the estuary, showing an increase of organic carbon and mud content towards its inner parts. Concomitantly an increase in the numbers of aerobic heterotrophic and aerobic thiosulphateoxidizing bacteria was observed, whereas the number of sulphatereducing bacteria was much less affected by the abiotic gradients. The numbers of different meiofauna species, nematodes as well as harpacticoid copepods, gradually decreased in the direction of the fresh water sources (BouwMAN, 1981). Macrofauna decreases in the same direction by n u m b e r of species present as well as by total biomass (VAN ARKV.L & MULDER, 1979). The aim of this investigation was to study the effect of the wastewater discharges on these gradients. For the bacterial groups a comparison was made between the situation during the period of highest waste discharge in the a u t u m n of 1974, and the more natural situation in the summer thereafter (Fig. 2). Meiofauna was sampled at a station close to the creek in which the waste-water is discharged, and at a station at a distance of 2000 m to the north in 1977 to 1980. Throughout these years the amounts of waste-water discharged were essentially the same as shown in Fig. 2. As macrofaunal organisms generally have much longer life cycles, and their populations would require several years to recover from the most pronounced effects of the discharges, these effects on macrofauna were deduced mainly from spatial distributions. Most samples were taken together with the first bacterial samples (autumn 1974) but some transects were sampled during 1975. Acknowledgements.--Mrs W. H. Uitman-Knol, Mrs G. F. RomanVisser, Mr A . J . Kop and M r M. Mulder assisted with sampling, analyzing, and processing of the data. Dr K. Essink, Prof. H. Veld-

290

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kamp and Dr P. de Wolf made valuable suggestions on earlier drafts of the manuscript. II. METHODS

Measurements on the bacterial populations and the sediment were performed as previously described (SCHR6DER & VAN ES, 1980). Undisturbed sediment samples were taken from a small boat at high tide with a corer essentially identical to that described by COLOCOLOFF & COLOCOLOFF (1972). Most macrofauna samples were taken concomitantly with a flushing sampler (VAN ARKEL & MULDER, 1975), mesh width 1 mm. Each sample consisted of 3 combined cores, with a depth of 3 dm and a total surface of 6 dm 2. The macrofauna biomass was determined as ash-free dry weight. For meiofauna 4 cores, 3 cm depth, total surface 18 cm 2 were analyzed as described elsewhere (BouwMAN, 1981). The water column over the sediment varied from 0.45 to 2.5 m, mainly depending on the height of the station relative to mean sea level. Surface water samples (0 to 0.3 m) were taken at each station during the sediment samplings in 1974 and 1975. In these samples, temperature, salinity and oxygen concentration (using the azide modification of the Winkler method, APHA, 1971) were measured. The oxygen saturation values were calculated using the table given by C A R P E N T E R (1966).

III. RESULTSAND

1.

THE WASTE-WATER

DISCUSSION DISCHARGES

The waste-water discharged through the Westerwoldse A (Fig. 1) contained approximately 250 mg.1-1 (mainly dissolved) organic carbon in November and 50 mg. 1-1 in June. The amount of water sluiced out (regulated by the water authorities) was also much higher in November. The resulting difference in polluting load of the wastewater in the two periods is shown in Fig, 2. In November 1974 12.8 × 106 i.e. (inhabitant equivalents of 35 g BOD~ 0) were discharged per day; in J u n e this amounted to 0.6 x 106 i.e. Daily fluctuations were large: at times no water was sluiced out at all, in the second half of November 1974 up to 24 million i.e. were discharged per day (unpublished data from Rijksinstituut voor de Zuivering van Afvalwater, and from Rijkswaterstaat).

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Fig. I. M a p of the inner part of the Ems Dollard estuary, the transects sampled indicated (1 to 6).

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Fig. 2. Monthly averages of the amount of waste-water discharged in the Ems estuary from the Westerwoldse A, expressed as inhabitant equivalents (i.e.) (computed from data provided by the Rijksinstituut voor Zuivering van Afvalwater and the Rijkswaterstaat).

292

r.B.

v A N E S ET AL.

2. O X Y G E N IN T H E W A T E R O V E R T H E T I D A L F L A T S

With respect to oxygen saturation values in the water at high tide (Fig. 3), it has to be noticed t h a t not all samples were taken at the same phase of the tidal cycle but at high tide :k 2 h. M o r e o v e r , in N o v e m b e r the time difference between the samplings of the transects was several days due to stormy weather. Nevertheless it is obvious that in N o v e m b e r 1974 the area of tidal flats flooded with water that was less t h a n 2 0 % saturated with oxygen (Fig. 3) at least extended to transects 4 and 5 (Fig. 1) and covered a b o u t a q u a r t e r o f the total surface of the Dollard. T h e degree of oxygen saturation increased with the distance from the outfall; at the stations of transect 1 saturation values higher t h a n 70% were measured. T h e i n h o m o g e n e i t y of the water masses sometimes observed m a y be illustrated by the large differences in oxygen content of the water at the stations of transect 6: some stations at a distance of 400 m from a n a e r o b i c water masses had saturation values over 50O/o. No measurements were m a d e on stratification, but in the most severely affected area o f transects 4, 5 and 6, the water c o l u m n over the tidal flats was only 0.5 to 1.0 m, and p r o b a b l y well mixed.

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In J u n e 1975 the spatial distribution of dissolved oxygen was clearly different; due to the lower pollution from the Westerwoldse A, the oxygen saturation values varied between 60 and 8 0 % (Fig. 3). O n l y at the 3 stations closest to the outfall (1.5 kin) anaerobic water was observed.

ORGANIC

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293

DOLLARD

The suspended matter content of the water was high; in November the average content was 83 mg.1 -t. Consequently the photic depth was generally less than 0.5 m. As a result primary production by phytoplankton in the Dollard is low (CAD~E & HEOEMAN, 1974, 1979 ; COLIJN, 1978), and has only little influence on the oxygen content of the water in the Dollard. The oxygen production by benthic algae is high in that part of the Dollard where oxygen concentrations were lowest. This production stops, however, soon after the flat surface gets submerged, again resulting from the high turbidity of the water. The amount of oxygen thus supplied to the water by the benthos is probably small as compared to the withdrawal of oxygen by benthic community respiration during the periods of submergence. 3.

SEDIMENT

CHARACTERISTICS

In November as well as in J u n e the organic carbon content of the sediment (Fig. 4a) showed a gradient towards the innermost part of the estuary (cf. SCHR6DER & VAN Es, 1980: fig. 4). There was also a gradient from the centre towards the borders of the Dollard. Along these borders relatively small particles (clay and detritus) are deposited caused by low tidal water currents. This results in a higher mud and organic carbon content of the sediment. VAN Es (1977) calculated that for the entire Dollard the amount of organic carbon deposited by sedimentation of material from natural sources (North Sea, River Ems), is of the same magnitude as the amount that enters the Dollard by the waste-water discharges. Therefore, the high organic carbon values on transect 6 (Fig. 4a) can only be partly ascribed to organic enrichment by the waste-water discharged. A comparison of the November and June values is presented in Table I. For all areas sampled the mean values were higher in J u n e TABLE I

Comparison of mean values per transect of sediment characteristics and viable counts of bacteria in November and June: organic carbon content (mg'g dry sediment), mud content (relative units) and counts of 3 groups of bacteria (per g wet sediment). Significant differences between bacterial counts in November and in June indicated by* (0.01 < P < 0.05) or** (P < 0.01). Transect

OrganicC (mg/g) Nov Jun

Mud Nov aTun

1+2 3 4+5 6

3.8 4.8 13.6 15.5 16.1 19.0 26.0 28.4

35 6 9 * * 114 132 253 189 382 242**

Heterotr. bact. (lOS'g) Nov yun 23

45**

37 103"* 142 128 165 117

Sulphate reducers (lOS'g) Nov yun 4

76 239 483

25*

13"* 31"* 94**

Thiosulp. oxidizers (106 'g) Nov yun 9

5*

24 70 62

12" 23** 21

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and June 1975. 1975 than in November 1974. For each separate area the difference in organic carbon content was not significant according to Wilcoxon's test (Table I). But when for each area the mean November and J u n e values were compared using the paired t=test (after In transformation to obtain normal distributions) the June values were significantly higher ( 2 a - 0.02). The average increase for the whole Dollard of 2.0 rag.g-1 (dry sediment) in the upper 2 cm is equivalent to about 4 0 g C . m -2. This increase between November and June would account for a 90% fraction of the organic matter that was imported as waste-water in the same time. Net accumulation of organic carbon fixed by benthic algae eventually could contribute to the increase of sediment organic carbon. In the DoUard tidal flats, however, annual

ORGANIC POLLUTION

DOLLARD

295

mineralization rates are higher than annual primary production (VAN Es, 1977), also for the period November to June. Therefore, the overall contribution of the activity of benthic organisms to the increase of organic carbon concentrations in the sediment will even have been negative. Consequently, deposition of organic material originating from natural sources and the waste-water discharges has taken place to a significant degree. The distribution of the mud content of the sediment (Fig. 4b) is comparable to that of the organic carbon content, with which it is highly correlated (r = 0.90, P < 0.001 in both November and June). However, the gradient in mud content in J u n e was less steep than in November (see also Table I). This indicates that meanwhile considerable amounts of that fraction of the sediment had been redistributed within the DoUard, probably during some storms that passed in the time between the two sampling series. T h a t the gradient in organic carbon concentrations was the same in both months must mean that the distribution had either not been affected, which is unlikely, or had been restored by an as yet unknown mechanism in which the high waste-water discharges, continuing for I or 2 months after the first sampling series (Fig. 2), can have played a significant part, as discussed above. From the distribution of both sediment characteristics it is clear that from the centre of the Dollard towards the borders the sediment changed from rather hard and sandy to very soft and rich in organic matter. The highest values were found in the southeastern part of the Dollard near the outfall. However, the increases caused by the discharges, especially for organic matter cannot be distinguished from the natural gradients.

4. B A C T E R I A L

POPULATIONS

In November on transects 1, 2 and 3 the distribution of aerobic heterotrophic bacteria in the sediment (Fig. 5a) was closely correlated with the organic carbon content (r = 0.80, P < 0.001). This was in agreement with previous findings for the same and other parts of the estuary (SCHR6DER & VAN Es, 1980). O n these transects the increase of aerobic heterotrophs between November and J u n e can be explained to a high degree by the increase of organic carbon and mud content. In J u n e the correlation was lower though still significant (r -----0.70, P < 0.001). Apparently the importance of other factors influencing the numbers of aerobic heterotrophs, such as local differences in benthic primary production and predation had increased.

296

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ORGANIC POLLUTION DOLLARD

297

In both periods the numbers of aerobic heterotrophs on the transects 4, 5 and 6 did not correlate with the organic carbon content, in contrast to the other parts of the estuary. In November the numbers of aerobic heterotrophs were about three times higher than would be expected according to the computed regression line relating their numbers and the organic carbon content of the sediment, for the transects 1, 2 and 3. O f 21 observations on transects 4, 5 and 6, 14 were higher than the upper 95% confidence limit of that regression line. The J u n e values of transects 4, 5 and 6 were almost evenly distributed above and below the regression line for the J u n e values of transects 1, 2 and 3, although the covariance was too high to give a significant correlation at P = 0.01. Moreover, on transects 4, 5 and 6 the numbers of aerobic heterotrophs tended to decrease between November and June, in contrast with the two- to three-fold increase on transects 1, 2 and 3 (Table I). This indicates that in November in the area that was under direct influence of the waste-water discharges (transects 4, 5 and 6), the numbers of aerobic heterotrophs were in average 2 to 3 times too high. But even if this could be ascribed entirely to the discharges of waste-water in November, the effect of the discharges on the development of these organisms was surprisingly small. Natural fluctuations by a factor 2 to 3 were regularly observed on transects in all parts of the estuary (ScHRODER &; VAN Es, 1980). In November the distribution of sulphate-reducing bacteria (Fig. 5b) showed a steep increase towards the outfall. This corresponds with the decrease in the oxygen content in the water above the sediment. On the transects 1 and 2, viable counts were comparable to those in the outer part of the estuary (ScHRSD~.R & VAN ES, 1980). On the transects 3, 4, 5 and 6 the numbers of sulphate-reducers had dropped a five- to eightfold between November and June (Table I). This general decrease is supposed to be the result of at least two factors. The bioturbation by macrobenthic organisms is more intensive in J u n e (water temperature 17 ° C) than it was in November (water temperature 6.5 ° C), especially on the transects 1, 2 and 3. Therefore, in J u n e the top layer of the sediment was more oxygenated, inhibiting the growth of (anaerobic) sulphate-reducing bacteria (SCHR6DER & VAN Es, 1980). Secondly, on the transects 4, 5 and 6 the oxygen content of the water had also considerably improved. In that area the strongest decrease in numbers of sulphate-reducing bacteria was observed. Only on the stations near the outfall, where low oxygen concentrations were also found in J u n e (Fig. 3), the numbers of these strictly anaerobic bacteria were still high. The increasing numbers of sulphate-reducers towards the outfall also indicate that in the south-eastern part of the Dollard the anaerobic

298

F. B. V A N ES ET AL.

mineralization becomes increasingly important, particularly during the time of high waste-water discharges. The distribution of aerobic thiosutphate-oxidizing bacteria (Fig. 5c) was intermediate to that of the aerobic heterotrophs and the anaerobic sulphate-reducing bacteria. In November a steep gradient in their numbers was observed towards the outfall. The increase was in good agreement with that of the aerobic heterotrophs (Fig. 5a). But the numbers of thiosulphate-oxidizing bacteria had fallen drastically between November and June, as had the numbers of sulphate-reducers (Table I). Apparently in the Dollard most of the (aerobic) thiosulphate-oxidizing bacteria, present during the time of waste-water discharge, were strongly dependent on the sulphide, produced by (anaerobic) sulphate-reducers. In the middle and outer part of the estuary a close relationship was found between the distribution of thiosutphate-oxidizing and heterotrophic bacteria (ScHRODER & VAN Es, 1980). Numbers of both aerobic groups of bacteria increased parallel to the main axis of the estuary whilst the numbers of sulphate-reducing bacteria stayed almost constant. It was concluded by SCHRODER ~: VAN Es (1980) that in a large part of the estuary sulphide was oxidized by mixotrophs or heterotrophs gaining energy from sulphide oxidation rather than by autotrophic sulphide-oxidizing bacteria. Because of the increase of sulphate-reducing bacteria in the southeastern part of the Dollard, an increase of the sulphide production is to be expected. The results presented here indicate that in this part of the estuary autotrophs using sulphide as a sole energy source might have become predominant during the period of high waste-water discharges. These specialists, having competitive advantages over mixotrophic and heterotrophic sulphide oxidizers when sulphide is constantly available in significant quantities (Ku~N~N, 1975) might decrease again in numbers when the population of sulphate reducing bacteria declined during the summer months. 5. M A C R O -

AND MEIOBENTHIC

FAUNA

Throughout the Doltard the following 12 macrofaunal species were found: Wereis diversicolor, Heteromastus filiformis, Hydrobia ulvae, Macoma

balthica, Mya arenaria, Corophium volutator, Arenicola marina, Eteone longa, Anaitides maculata, Lanice conchilega, Cerastoderma edule and Carcinus maenas. The first 6 species were found frequently, the last 6 only at a few sampling stations. Mean biomass values of the total macrobenthic fauna (Fig. 6a) and the diversity, expressed as the number of species per sampling station (Fig. 6b), are rather low in comparison to other

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tidal flats in the Wadden Sea (BEuKEMA, 1976). Most of the species in the Dollard are very tolerant to extreme values and large fluctuations in environmental factors (VAN ARX<~.L & MULDER, 1979). As they depend largely on the sediment in which they live, the distribution of the species shows a relation to the distribution of the different types of sediment; e.g. Mya arenaria was not found on the high and most muddy parts of the flats along the border of the Dollard. During high tide the animals also depend on factors associated with

300

F . B . V A N ES ET AL,

the overlying water. The quality of the water does change irregularly and in relatively short time. Therefore, it is difficult to indicate the factors in the water that will be related to the distribution of the macrobenthos. All the same we believe that e.g. the absence of Arenicola marina and Cerostoderma edule in the southern part of the DoUard is caused by the usually low salinities. Nevertheless the fauna in the area near the outfall seems exceptionally poor. No macrofauna was present at all in its immediate vicinity. The special state of this area was clearly shown by the results of cluster analyses and factor analyses which were used for pattern recognition. In this grouping of the sampling stations according to the total macrofauna composition, the stations near the outfall appeared always to belong to one or two separate groups with a very poor fauna (Fig. 6c). The measured sediment characteristics in this part of the Dollard did not differ so much from those in other parts that they could explain the low macrofaunal biomass and diversity. Consequently, the waste-water discharges must have reduced the macrobenthos. ESSlNK &; KLEEF (1979) studied the distribution of Nereis diversicolor in the area of transects 4, 5 and 6 in 1977 and 1978 in more detail. The amounts of waste-water discharged in these years were comparable to those in 1974 and 1975 (Fig. 2). They found a spatial distribution of Nereis diversicolor which confirms present findings. By means of repeated sampling they could show a change in the distribution coinciding with the strong increase in waste-water discharges. In a u t u m n the worms completely disappeared in an area within 2 to 3 km from the outfall, comparable to transect 6, whereas they remained present in the area of transects 4 and 5. From these results it may be concluded that macrofauna is severely affected by the waste-water discharges in an area of approximately 7 kme around the outfall. The area near the outfall as indicated in Fig. 6c is considered to be the minimum estimate of the area where the macrobenthic fauna is affected by the waste-water discharges. In a study on the distribution of nematodes in the Dollard (BouwMAN, 1981) 52 different species were isolated. Of these species, 17 are widely distributed within the area and the remaining 35 are partly local, partly rare species of which only a few specimens were isolated at all. In the mud flats in the southeastern part of the DoUard, at about 3 km and more from the outfall, the nematode community is dominated by the widely distributed species Ptycholaimellus ponticus, Dichromadora

geophila, Microlaimus globiceps, Leptolaimus papilliger, Sabatieria pulchra, Daptonema procerum and Daptonema setosum. All these species feed on bacteria, diatoms or both and occur in the upper 3 cm of the sediment. Highest densities of Sabatieria pulchra are found below the upper 0.5 cm

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layer while all other species are mainly confined to this upper layer. The community structure is common for brackish mud flats and has been observed in the Exe estuary (WARWICK, 1971) and the estuaries of Etbe and Weser (RI~.MANN, 1966, 1979). At the station close to the creek in which the waste-water is discharged, the nematode fauna is dominated by the species Eudiplogaster pararmatus and Dichromadora geophila, while sometimes Daptonema setosum and Anoplostoma viviparum are numerous as well (BouwMAN, 1978). Often, one of these species is present as a monoculture while almost all other species are absent. In this part of the estuary nematodes are confined to the upper 3 m m of the sediment, below which the sulphide layer is located. Within the oxidized sediment, densities of nematodes are extremely high (up to 5 × 10 e m-~ versus 1 × l0 s in other parts of the Dollard). It is remarkable that the most numerous species, E. pararmatus and D. geophila are both epigrowth-feeders which mainly feed on diatoms (RoMEYN, 1980). Brood-care is probably advantageous in a stress area as 2 species, Eudiplogaster pararmatus and Anaplostoma viviparum, show this rare phenomenon. Oligochaetes are numerous in the southeastern part of the Dollard as well (DE RUYTER VAN STEVENINCK, 1978). Densities and number of species decrease, however, in the vicinity of the outfall. Extreme high densities of the diatom-feeding species Amphichaeta sannio (up to I x l0 s m -z) were found at a distance of 3 km from the outfall. However, close to the creek densities were considerably lower (up to 0.1 x 106m-2). Besides this species, the bacteria-feeding species Tubifex costatus is numerous in this area as well. The observations on benthic animals in the DoUard showed severe effects only in the vicinity of the outfall. T h a t the effects were rather limited as compared to the amounts of waste-water discharged results from two coinciding factors, concerning time of the year that the waste-water is discharged (Fig. 2) and location of the outfall in an area that was already an unsuitable habitat for m a n y organisms. In a u t u m n water temperatures in the Dollard rapidly drop to values below 10 ° C. At these low temperatures macrofauna, and probably also meiofauna species are able to survive by anaerobiosis much longer than at summer temperatures of 2 0 ° C (DRIES & THEEDE, 1974). Furthermore, the tidal flats in the southeastern part of the Dollard are situated high in the tidal zone and are submerged for only a few hours per tide. During periods with prolonged eastern winds which occur several times a year large parts may even stay emerged for a week and the sediment surface dries up, causing severe stress for the organisms inhabiting the sediment. The Westerwoldse A causes large salinity fluctuations. In the water that remains behind after the flats

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have emerged, salinities vary between 7 and 17%o in summer, and between 0 and 8~oo in winter. Apparently, species that are able to survive such extreme conditions are also resistant to other stresses such as occasional anaerobic conditions (at low temperatures), Other species, more susceptible to environmental stresses seem to be absent irrespective of the waste-water discharges. Consequently, as also concluded in relation to the distribution of bacteria in the sediment, it is impossible to discriminate between the effects of gradients created by the waste-water discharge and of natural gradients in the Dollard. IV. CONCLUSIONS

The clearest effects of the organic waste-water discharges in the DoUard were the strong local decrease in oxygen saturation of the water, the increased numbers of sulphur cycle bacteria in the sediment and the strong reduction in faunal diversities in the vicinity of the outfall. But basically, a complete ecosystem, with primary producers, consumers and aerobic and anaerobic decomposers still remained present during the waste-water discharges, though its structure was relatively simple. These investigations on both the bacterial and the faunal populations showed it impossible to quantify the effects of the waste-water discharges with much precision. The reason for this is that these effects could not be separated from the influences o f m a n y natural factors, such as salinity, salinity fluctuations, relative emersion time and sediment composition. These conditions make this part of the estuary an increasingly unfavourable habitat for many organisms, and suitable for others. Oxygen saturation values are easily measured and give direct indication where the waste-water discharges can be expected to affect the natural populations of organisms in the tidal flats. From the results presented it may be clear, however, that more knowledge is required, not only on the distribution and physiology of the organisms, but also on hydrological and other natural processes in the area concerned, to explain the observed effects. Execution of the plans to reduce drastically the waste-water discharges in the Dollard, will offer an opportunity to test many of those explanations. V. S U M M A R Y

Large amounts of organic waste-water are discharged into the EmsDollard estuary, mainly in autumn and early winter. In November t974 and J u n e 1975 a number of samples were taken from the sediment

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o f the tidal flats a n d analysed for bacterial n u m b e r s , organic c a r b o n a n d m u d content. Cores were taken to q u a n t i f y macro- a n d meiofauna. O x y g e n s a t u r a t i o n was m e a s u r e d in the w a t e r c o l u m n over the sediment. I n N o v e m b e r the o x y g e n s a t u r a t i o n was less t h a n 2 0 % in a considerable p a r t of the Dollard. I t is not clear to w h a t extent the i n w a r d increase in organic m a t t e r content of the sediment was influenced b y the waste-water discharges. T h e correlation b e t w e e n the viable counts o f aerobic h e t e r o t r o p h i c bacteria a n d the organic m a t t e r content, as observed in the other parts o f the estuary, was absent in the most polluted p a r t o f the Dollard. H o w e v e r , the differences between the n u m b e r s c o u n t e d in N o v e m b e r a n d J u n e were not significant in t h a t area. N u m b e r s o f sulphate-reducing b a c t e r i a decreased sharply between N o v e m b e r a n d J u n e , indicating a d e p e n d e n c y on the waste-water discharges. N u m b e r s of sulphide-oxidizing bacteria decreased in s u m m e r c o n c o m i t a n t l y with the s u l p h a t e - r e d u c i n g bacteria. These d a t a suggest that, in contrast to the o t h e r parts of the estuary, in the Dollard obligate a u t o t r o p h s play a p r e d o m i n a n t p a r t in the populations o f sulphlde-oxidizing bacteria d u r i n g the time o f high waste discharges. P o p u l a t i o n diversities of meio- a n d m a c r o f a u n a o f the tidal flats were strongly decreased in the vicinity o f the outfall. T o t a l biomass o f m a c r o f a u n a decreased to zero, whereas m e i o f a u n a biomass significantly increased. Some possible explanations for the effects on the benthic f a u n a are given. VI. R E F E R E N C E S APHA, 197 I. Standard methods for the examination of water and wastewater (13th ed.). American Public Health Association, Washington D.C. : 1-874. ARKEL, M. A. VAN & M. MULDI~R, 1975. A device for quantitative sampling of benthic organisms in shallow waters by means of a flushing technique.--Neth. J. Sea Res. 9 (3-4): 364-370. , 1979. Inventarisatie van de macrobenthische fauna van het Eems-Dollard estuarium. Publicaties en Verslagen Biologisch Onderzoek Eems Dollard Estuarium 1979-2: 1-122. BEUKEMA,J. J., 1976. Biomass and species richness of the macro-benthic animals living on the tidal flats of the Dutch Wadden Sea.--Neth. J. Sea Res. 10 (2) : 236-261. BOOW~AN, L. A., 1978. Investigations on nematodes in the Ems-Dollard estuary.-Annls Soc. r. zool. Belg. 108: 103-105. , 1981. A survey of nematodes from the Ems estuary.--Zool. Jb. (Syst.) (in press). CAD~E, G. C. & J. HEOE~N, 1974. Primary production of phytoplankton in the Dutch Wadden Sea.--Neth. J. Sea Res. 8t 240-259. , 1979. Phytoplankton primary production, chlorophyll and composition in an

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outlet of the western W a d d e n Sea ( M a r s d i e p ) . - - N e t h . J. Sea Res. 13 (2): 224-241. CARPENTER, J . H., 1966. New m e a s u r e m e n t s of oxygen solubility m pure a n d n a t u r a l w a t e r . - - L i m n o l . Oceanogr. 11: 264-277. COLUN , F., 1978. P r i m a r y p r o d u c t i o n m e a s u r e m e n t s in the E m s - D o l l a r d estuary d u r i n g 1975 a n d 1976. Publicaties en Verslagen Biologisch O n d e r z o e k Eems Dollard E s t u a r i u m 1978-1 : 1-14. COLOCOLOFF, M. & C. COLOCOLOFF, 1972. Recherches sur la p r o d u c t i o n p r i m a i r e d ' u n fond sableux. 2. M6thodes. Tethys 4" 779-800. DRIES, R. R. & H. TnEEDE, 1974. Sauerstoffmangelresistenz m a r i n e r Bodeneverteb r a t e n aus der westlichen Ostsee. M a r . Biol. 25: 327-333. Es. F. B. VAN, 1977. A p r e l i m i n a r y c a r b o n b u d g e t for a part of the Eros estuary: T h e Dollard. Helgol/inder wiss. Meeresunters. 30: 283-294. ESSmK, K. & H. L. KLEEF. 1979. O v e r de verspreiding, p o p u l a t i e o p b o u w , voortp l a n t i n g en groei v a n de w o r m Nereis diversicolor in de zuidoostelijkeDollard. I. Rijksinstituut voor Z u i v e r i n g van Afvalwater. r a p p o r t B I M - 79.01 : 1-9. KUENEN, J . G., 1975. Colourless sulfur b a c t e r i a a n d their role in the sulfur cycle. PI. Soil 43: 49-76. RIEMANN, F.. 1966. Die interstitielle F a u n a lm Elbe-Aestuar. V e r b r e i t u n g u n d S y s t e m a t i k . A r c h . Hydrobiol. ( S u p p l . ) 3 1 : 1-279. • 1979. N e m a t o d e n aus d e m Brackwasser des Weser-Aestuars u n d Beschreibung von drei Monhysteroidea. -Ver6ff. Inst. Meeresforsch. Bremerh. 1 7 : 2 1 3 - 2 2 3 . ROMEVN, K., 1980. Een inventarisatie van de n e m a t o d e n f a u n a v a n de Dollard en de rol v a n diatomee6n als voedsel voor enkele n a m a t o d e m o o r t e n . Publicaties en Verslagen Biologisch O n d e r z o e k Eems Dollard E s t u a r i u m 1980-7: 1 4 0 . RUYTER VAN STEVENINCK, E. DE, 1978. Begrazing v a n b e n t h i s c h e diatomee~n door' oligochaeten in het Eems-Dollard estuarium. Publicaties en Verslagen Biologisch O n d e r z o e k Eems Dollard E s t u a r i u m 1978-5: 1-28. SCHR6DER, H. G. J. ~¢ F. B. VAN Es, 1980. Distribution of bacteria in intertidal sediments of the E m s - D o l l a r d e s t u a r y . - - N e t h . J. Sea Res. 14 (3/4) : 268-287. WARWICK, R. M., 1971. N e m a t o d e associations in the Exe estuary.---], mar. biol. Ass. U . K . 51: 439-454.