Hydrobiologie van de Polder Westzaan

HYDROBIOLOGIE OF THE “POLDER WESTZAAN”. Investigations into the macrofauna and water quality of a brackish water area in the Dutch province North-Holland. Part I. Water quality of the “Polder Westzaan”. From February till June 1974 macrofauna samples were taken with monthly intervalls at 9 Stations scattered in the “Polder Westzaan”, municipality Zaanstad (for position of the Stations see map). In three of the Stations samples were also taken in July, August, and September. From March till October 1974 the 02-saturations percentage and the water temperature were measured and water samples were taken at monthly intervalls. These water samples were analysed by the Dienst Milieuhygiene Zaanstad for the presence of Cl-, nitrate, nitrite, Ammonium, phoshate, sulphate, and heavy metals as well as for the pH; total hardness and COD values. Conclusions: Stations G5 and G7 “Het Guisveld” gives a highly polluted impression, especially when we consider the high COD,PO3- 4, and NH4+ -values. Likewise Station W6 (situated in the “Westzijderveld”) makes a polluted impression, especially because of its high NO-2 -percentage. – According to the saprobical index of Pantle and Buck (1955) all stations must be considered as β-mesosaprobic. – According to the saprobical index of Zelinka & Marvan (1961) most stations are poly-to α-mesosaprobic. Stations R5 and R7 (“De Reef”) and Station W7 (“Westzijderveld”) have a more βmesosaprobic to α-mesosaprobic character. – Station W6, according to both indices, is the most saprobic Station. – According to the species-diversity- index of Shannon (see Odum, 1971) Stations R5 and R7 (“De Reef”) are the most diverse, Station W 6 the least diverse. – The polder is poor in species. A number of taxa, including Chironomus spec., Tubifex tubifex, Gammarus tigrinus and Neomysis integer appear frequently in very large numbers. Many taxa are only found occasionally. A possible explanation may be found in: a) the oligohaline character of the water (mean Chlorinity above 850 mg/l) with strong fluctuations (from 225 to 1370 mg/l). b) the almost total absence of higher water plants. c) the serious pollution of the polder. Part II. Methodological investigations in two brackish water ditches. In these experiments the relations between the composition of a macrofauna-sample and the actual place of sampling in an uniform ditch (as far as water quality is concerned) as well as the sampling distance were investigated. Macrofauna- (samples were taken in 28 Stations distributed along both banks of a ditch in “De Reef” (september, 1974) and in 16 Stations distributed along both banks of a ditch in the province of Friesland (October, 1974). Conclusions: – The composition of a macrofauna sample is not highly dependant on the place of sampling within the same ditch. – The differences found between samples taken at the opposite banks appear to be of no greater importance than the mutual differences between the samples taken at the same bank. – No significant increase in species number was found when comparing samples taken over a length of 4 or 6 meter with those taken over a length of only 2 meter. – The actual place of sampling (in a uniform ditch) has the smallest influence on the composition of the fauna sample if the sampling methods are standardized. – To obtain the best representative sample it is recommended to sample in several places of the same ditch over relatively short distances. More technical suggestions are made. – Comparing the saprobic valuation, according to the Zelinka & Marvan-index of the individual samples with that of the total catch, one finds out that the first valuation can show a perversion of the reality because of the small number of species involved.