| 27-08-2020 |
De laatste jaren is er steeds meer te doen over het natuurlijk gedrag van dit meer, terwijl het toch als zodanig onder Natura 2000 is gebracht. Het natuurlijk fenomeen van de jaarlijkse opbouw en afbraak van een zuurstofloze onderlaag in een beperkt deel van het meer wordt als een waterkwaliteitsprobleem ervaren. Doordat de Brouwerssluis het hele jaar geopend is, blijkt dat de maximale zuurstofloze bodemoppervlakte aangroeit tot ca. 15%. Als ‘waterkwaliteitsdoelstelling’ is een maximale bodemoppervlakte van 5% gesteld. Daarnaast zijn er extra wensen van biologen om veel meer getij op het meer te realiseren. Er vindt een ware propagandaslag in de media plaats om een 10x zo grote sluis in de Brouwersdam te kunnen realiseren. Dit op basis van beweringen dat de Grevelingen een dood en stilstaand meer zou zijn en deze sluis de oplossing van het probleem. Het zou ook nog klimaatbestendig zijn. Niet dus. De gebezigde begrippen, zijn onwaar, dan wel grotendeels onwaar.
In het begin van de zeventiger jaren werd studie gedaan naar het zoete Grevelingenmeer. Niet alleen naar ontzilting, maar naar bepaalde problemen, die zich in het meer kunnen voordoen. Het voorkomen in diepere meren van een zuurstofloze onderlaag was bekend. Uit de praktijk bleek dat met de wind, zoals die in Nederland voor kwam, het water tot zo’n 15 m diep kon doormengen. In de zomer ontstond dan een lichtere warmere bovenlaag en een zwaardere koudere onderlaag. De bovenlaag werd van zuurstof voorzien, de onderlaag niet. Daar werd de zuurstof verbruikt. Ook toen was de vraag, wat doen we er mee. Als oplossing werd toen bedacht om met mechanische beluchting op enkele diepe punten tijdelijk een verticale stroming te veroorzaken. (Deze eeuw is deze methode door Stigebrandt in Zweden in een (zoute) baai met succes toegepast.) Echter de discussie werd toen principiëler gevoerd. Ook al was die bacterienatuur in de onderlaag niet aantrekkelijk, het had zijn eigen functie. De conclusie was toen, dat elke natuurvorm evenveel recht had om te bestaan. De situatie van een tijdelijke, beperkte zuurstofloze laag werd geaccepteerd, zoals ook in de andere meren daar geen actie op ondernomen werd.
In een zout meer ligt het gecompliceerder. Daar komen ook verschillen in zoutgehalten door de neerslag en het afvloeien van zoet en brak water van de eilanden. Aan de oppervlakte is het zoutgehalte wat lager en het water lichter van gewicht dan dieper in het meer. Het versterkt het effect en de zuurstofloze laag aan de bodem kan hoger komen.
Er is geen sprake van een dood meer. Ook de zuurstofloze onderlaag is levende natuur. Alleen geen ‘aaibare’, waarmee men wil scoren. Het is ook geen getijloos en stilstaand meer. Via de Brouwerssluis is uitwisseling met de Voordelta onder invloed van het getij. Er is in de bovenlagen altijd stroming als gevolg van de wind. Onderin zijn er waterbewegingen onder invloed van verschillen in dichtheid (gewicht). Daar bouwt o.a. de zuurstofloze onderlaag zich op en wordt ook jaarlijks weer afgebroken.
Het Grevelingenmeer zal door die 10x zo grote sluis als zodanig niet reanimeren. Het water dat vanuit de Voordelta het meer binnenkomt zal in een beperkt gebied direct achter de sluis vertragen. Daar vindt ook de menging plaats. Het is in oppervlakte en diepte beperkt. De opbouw en afbraak van de zuurstofloze onderlaag heeft plaats in de diepe zuidelijke geul. De extra menging door de nieuwe sluis vindt in de bovenlagen van de noordwestelijke hoek van het meer plaats. Een positieve invloed op het waterkwaliteitsaspect van de optredende zuurstofloze onderlaag mag niet verwacht worden. De nieuwe sluis zal alleen kunnen voorzien in de aanvullende wensen van sommige biologen, niet in de waterkwaliteitsdoelstelling.
Ir. W.B.P.M. Lases.
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