Pollution in the Harbour of Ostend (Belgium)
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Helgol~inder wiss. Meeresunters. 17, 302-320 (1968) Pollution in the harbour of Ostend (Belgium). Biological and hydrographical consequences GUIDO PERSOONE and NIELS DE PAUW Laboratorium voor Oecologie, Biogeografie en Algemene Biologie, Rijksuniversiteit, Gent, Belgie, and Laboratoriurn voor Oecologie, Rijksuniversitair Centrum, Antwerpen, BelgiF EXTKAIT: La pollution dans le port d'Ostende (Belgique). Consequences biologiques et hydrographiques. Le port d'Ostende reqoit chaque ~our une quantit~ importante d'eaux pollu&s d'origine terrestre. A mar& basse ies ~gofits de ia ville et des faubourgs se d~versent dans le port Far trois ~missaires. De plus, les canaux Noord-Eede et Ostende-Bruges am~nent les eaux noires de pIusieurs villes situdes k l'Est d'Ostende. Rien que pour les ~gofits de la ville m~me, ceci signifie un apport journalier de 3 k 6 litres d'eaux us&s par m ~ d'eau portualre. Des analyses sur la physico-chlmie, la bact~riologie, le plancton et le biotecton, effectu~es ~t 3 points diff~rents du port (A: ~ l'entr& du chenal; B: dans l'avant-port; C: l'entr~e de l'arri~re-port) nous ont permis de tirer les conclusions suivantes: (a) Le d~ficit en oxyg~ne dissous est tr& grand surtout dans l'avant et l'arri~re-port. (b) I1 y aun hombre ~norme de bact4ries portuaires (,,estuarine~ bacteria) ~t la suite du grand apport de mati~res organiques. (c) Le hombre de bact~ries coliformes est tr~s grand et d&ro~t de l'avant-port vers le chenaI. (d) Les courants marins refoulent une partie des eaux pollu&s vers les plages voisines. (e) La tr~s grande turbidit~ de l'eau dans le port g~ne consid~rablement la production phyto-planctonique. (f) Le benthos est compl&ement anoxique et riche en mati~res organiques et en sulfures. II ne contient plus de protistes ni d'invert4br&. (g) Par suite de l'apport r~gulier de ddchets riches en mati~res organiques le biotecton se d~veloppe consid&ablement. I1 est constitu4 pour la majeure partie d'organismes bacteriophages et d&ritivores. INTRODUCTION The city of Ostend is economically characterized by fishery; during the summer months it is a typical seaside resort with its normal population of 60,000 inhabitants increasing to about 200,000 in July and August. The harbour of Ostend consists of three parts: the channel, the outer-harbour and the inner-harbour (Fig. 1). The draught in the outer-harbour (which is connected with different basins) is 4 m, in the channel 4.7 m at mean low water spring. The tidal range is mostly around 5 m. The inflow of sediments from the North Sea is very important and the thickness of the mud layer increases by almost 1 cm daily, so that at some points it reaches 1'/~ m. To preserve the necessary draught, 106 m s are dredged out yearly. All sewage from Ostend and its suburban districts is drained into the harbour Pollution in the harbour of Ostend 303 without any previous treatment. Sewers enter the harbour at three points: "Vissers- kreek", "De Mey sluis", and "Spuisluisbrug'. At low tide all the wastes accumulated in the sewers are released into the harbour water. Roughly estimated this means an input approximately 8,000 m 8 per day rising to more than 15,000 m 3 during the summer months. In relation to the water volume of the harbour (an estimated 1,3.10 ~ m 3 at low tide), this means an inflow of 3 to 6 litres sewage per m 3 harbour water, twice a day. Moreover, there is the waste of ! i/ I '~ i1 I/ i/ SLUICE - DOCK / J i I .z- / BEACH t o 0 / / / i L Fig. 1: Harbour of Ostend with (a) sampling points: (A) at the Pier, (B) in the outer harbour, (C) at the beginning of the inner harbour; (b) mouths of sewers: (1) "Visserskreek", (2) "De Mey sluis", (3) "Spuisluisbrug" the canal Noord-Eede which ends in the inner-habour and is strongly polluted by sewage from different cities located eastward of Ostend. This water is drained into the harbour once or twice a day during 2 to 8 hours. Unfortunately, the volume of this input is not known. The last source of pollution comes from the canal Ostend- Bruges, which carries waste from as far as Bruges. Due to the low water-mark of this canal, the sluices of the inner-harbour are usually closed during summer; but there is always a certain volume of new water which enters the harbour during, the passing of ships. At all other times of the year, there is a large inflow (30 to 60 m3/sec). As will be shown, this inflow has an impor- tant influence on the salinity of the harbour water. In addition to the total input of domestic sewage, industrial waste products of 2 factories, situated outside Ostend, partly enter via the canal Ostend-Bruges, and partly through the sewers entering the harbour at the "De Mey sluis". Early biological observations and samplings in the harbour of Ostend were cai'ried out by GILSON (unpublished), SCHULZ (1925), LriouP & VAN M~EL (un- 304 G. PERSOONE and N. DE PAUW published). The first complete study on the hydrography and biology of this estuary was published by LEFEV~RE, LELOUV & VAN MEEL (1956). From 1963 to the present time, we have tried to add to the knowledge of this estuary by investigations on physico-chemistry, bacteriology, plankton and "Auf- wuchs'. In order to compare our results with the work of LEr~VE~E, L~LOUV & VAN %o 34" 30" 26- 22- 181 14- tO[ 6[ 2[ I II ItI Iv v vI vii VIII IX X ×I XII Fig. 2: Salinity in the Ostend harbour during 1965 at sampling points A, B, C MEEL (1956), we chose the same sampling places: points A, B and C. Point A is located at the end of the pier, which lies about 250 m off the coast-line at low tide, and 400 m at high tide. Point B lies approximately at the mouth of the "Visserskreek", point C at the top of the inner-harbour, near the sluices of the canal Ostend-Bruges and the Sluice-dock. RESULTS Physico-chemistry W a t e r t e m p e r a t u r e follows the normal seasonal changes from a minimum of + I o C to a maximum of + 25 ° C; it is usually a few degrees higher at point C than at point A. p H is alkaline, varying between 7 and 8, and about uniform throughout the harbour. Salinity is very low in January and increases considerably until May, remains at this level until October and then decreases again to the winter level (Fig. 2). The influence of the inflow of freshwater coming from the canal Ostend-Bruges is Pollution in the harbour of Ostend 305 6'&'&'6'&'~ O ~,~,~,~,~,o X .~,-~ W m ~ 0 o~ N o , , ,,,, J .. ~'~ f13 C) 306 G. PERSOONE and N. DE PAuw apparent. According to the Venice-system (1958), the water is mixohaline, in the range from b&a-oligohaline (during winter) to polyhaline (during summer). D i s s o 1 v e d o x y g e n data from the literature, as well as our own measure- ments, are illustrated graphically in Figure 3. The results obtained 1949, t952 and early 1953 are from LrrEvrRr, LrI.OUV & VAN MrrL (1956), those of 1954 from VAN MrrL (1964), those of 1960, 1962, 1963 (at point C) from the "CoMMISSiON T.W.O.Z.- GRotJvr DE TRAVAIL OSTt~EICULTURE" and those of 1963 and 1964 (at point B) from LrLouv & PoLK (1967). The most relevant observation is the marked deficit of dissol- ved oxygen throughout the harbour. In the inner-harbour, although the values may fluctuate strongly, it is clear that during most recent years the oxygen deficit is very important (only 20 to 50 % saturation). The outer-harbour is influenced by the daily input of sewage, which contains mostly no oxygen, and by the tide which brings fresh sea-water, rich in oxygen. This is appearent from observations made for several years at high and low tides (Fig. 3). As a whole, the outer-harbour has also an important oxygen deficit; especially during the most recent years, O~ concentration is mostly under 50 % saturation. Even in the channel, at the end of the pier, there is a shortage of dissolved oxygen. At the "Goote Bank" (20 km offshore), water samples showed 100 0/0 and more saturation. O x y g e n d e m a n d was determined this year several times. A~er two days, practically all the oxygen had disappeared in samples obtained at the three points. The oxygen demand of the harbour water is higher than 5 mg/litre within 48 hours, while water from the "Goote Bank" showed an oxygen demand of 0.4 to 0.8 mg/litre, during the same period of time. Transparency, as determined by means of a Secchi-disc, is very low throughout the harbour, i. e., between 30 and 40 cm. At the "Goote Bank", it ranges from 2 to 6 m. The main reason for the low transparency in the harbour is the high concentration of suspended material, mainly due to whirling up of benthos material by tidal currents and the passing of ships. After filtration of different samples of sea- water through a membrane filter, we obtained a dry weight of 75 to 150 mg/l, the plankton content of which amounted to less than 1 °/0.