We bevinden ons in een tijd van snelle en ingrijpende milieuveranderingen als gevolg van de door de onszelf veroorzaakte klimaatsverandering. Maar hoe kunnen we de huidige impact van de klimaatsopwarming inschatten? Hoe kunnen we weten in welke mate onze omgeving al veranderd is? Waar metereologische waarnemingen niet beschikbaar zijn, kunnen kiezelalgen een tipje van de sluier oplichten!
Kiezelalgen of diatomeeën zijn eencellige algen die gekenmerkt worden door een geelbruine pigmentatie en celwanden (schaaltjes) opgebouwd uit kiezel (siliciumdioxide). Deze celwand bestaat uit twee helftjes, de valven, die in elkaar passen als een smeerkaasdoosje en waarbinnen de cel huist. Diatomeeën vormen de meest soortenrijke groep van algen op aarde. Men vermoedt dat er tussen de 20 000 en 200 000 verschillende soorten bestaan. Hun gigantische diversiteit en hun prachtige morfologie sprak reeds vele wetenschappers tot de verbeelding. Daarnaast zijn diatomeeën zeer wijdverspreid in zowel terrestrische als aquatische habitats. Hoewel ze niet met het blote oog waarneembaar zijn, behoren ze tot de belangrijkste zuurstofproducenten op aarde. Diatomeeën zijn werkelijk onze longen, maar daarnaast kunnen ze ons ook een unieke inkijk geven op het verleden van onze planeet.
Hoe algen het verleden kunnen ontrafelen
Diatomeeën hebben zeer specifieke ecologische voorkeuren en zullen bijgevolg enkel in een omgeving voorkomen die ook geschikt is voor hen. Daarnaast bewaren hun schaaltjes uitzonderlijk goed in meersedimenten en kunnen ze duizenden jaren later nog teruggevonden en geïdentificeerd worden. Door een boorkern te nemen uit de sedimenten van een meer en de hierin aanwezige diatomeeënschaaltjes te isoleren, kan een tijdssequentie bekomen worden van de diatomeeënpopulaties die in het meer voorkwamen. De omgevingsvoorkeuren van de dominerende soorten in deze boorkern kunnen op die manier gebruikt worden om een reconstructie van de omgeving en het klimaat uit het verleden te maken. Door naar de veranderingen in de dominerende soorten te kijken, kunnen zo belangrijke omgevingsveranderingen in kaart gebracht worden. De wetenschap die dit beoefent is de paleoecologie.
Paleoecologisch onderzoek is vooral interessant in polaire gebieden. Er bestaan immers geen lange termijn klimatologische waarnemingen voor deze regio’s en bovendien zijn ze zeer gevoelig voor klimaatsveranderingen. Echter, het gebruik van diatomeeën in paleoecologie in de poolgebieden wordt bemoeilijkt door onze beperkte kennis van de diversiteit en ecologie van diatomeeën in deze gebieden. Dat is voornamelijk een gevolg van het te beperkt aantal studies dat hier werd uitgevoerd. Daarnaast zijn veel diatomeeën in het noordpoolgebied in het verleden foutief geïdentificeerd. Om onze kennis over de diatomeeëndiversiteit en –ecologie in het noordpoolgebied te verbeteren en zo hun toepassing in de paleoecologie te bevorderen, werden in deze studie de zoetwaterdiatomeeëngemeenschappen in Petuniabukta, een fjord gebied in Spitsbergen (Svalbard Archipel, Arctisch gebied), onderzocht op een ruimtelijke en een temporele schaal.
Diatomeeën & hun omgeving
Tijdens de poolzomer van 2013 werden stalen voor diatomeeën genomen uit 40 meren en poelen in Petuniabukta. De aanwezige diatomeeënflora werd geanalyseerd met behulp van licht- en rasterelektronenmicroscopie. In totaal werden 315 verschillende taxa (d.i. een soort, ondersoort of variëteit) teruggevonden. Vervolgens liet statistische analyse toe om de gevonden diatomeeënflora op te delen in vier verschillende diatomeeëngemeenschappen. Deze gemeenschappen komen voor in meren die duidelijk verschillen in omgevingscondities, zoals de aanwezigheid van stroming, vogelkolonies, mosbegroeiing aan de meerranden, invloeden van gletsjers en spatwater van de zee. Op deze manier geven ze een indicatie van de ecologische voorkeuren van de samenstellende soorten en die kunnen dan weer gebruikt worden in paleoecologisch onderzoek.
Nieuw voor de wetenschap
Verschillende van de diatomeeën die werden teruggevonden in de meerstalen bleken tot nu toe onbekend voor de wetenschap. Dit leidde reeds tot de beschrijving van een nieuwe soort, Gomphonema svalbardense Pinseel, Kopalová & Van de Vijver (zie figuur), en het is duidelijk dat in de toekomst meer nieuwe soorten beschreven zullen worden uit deze studie. Dit toont aan dat de diatomeeëndiversiteit van het Arctisch gebied niet goed gekend is en dat nog veel soorten ontdekt en beschreven moeten worden. Er is dus nog voor jaren werk voor enthousiaste algologen!
6000 jaar terug in de tijd
In het tweede deel van deze studie werd getracht de omgevingsveranderingen van de laatste millennia in Petuniabukta te reconstrueren door gebruik te maken van diatomeeën. Hiervoor werd een boorkern genomen uit de meersedimenten van een meer in het studiegebied. Door middel van koolstof-14-datering, een radiometrische dateringsmethode, werd de ouderdom van de 79 cm lange boorkern bepaald op ~ 6000 jaar. Vervolgens werden de veranderingen in de diatomeeënsamenstelling doorheen de sedimentkern onderzocht. De resultaten gaven aan dat de diatomeeëngemeenschappen van Garmaksla een aantal grote wijzigingen hebben ondergaan in de afgelopen 6000 jaar. De ecologische voorkeuren van de samenstellende taxa suggereren opeenvolgende periodes van opwarming en afkoeling in de afgelopen millennia. Echter, de meest dramatische wijziging trad op in de opperste 11 cm van de sedimentkern waar een verschuiving plaatsvond naar een diatomeeëngemeenschap die een duidelijke voorkeur heeft voor een warmer klimaat met minder lange periodes van ijsbedekking. Hoogstwaarschijnlijk kan deze verandering worden toegeschreven aan de invloeden van de recente klimaatsveranderingen in het Arctisch gebied, in het bijzonder het door natuurlijke omstandigheden veroorzaakte einde van de Kleine IJstijd, zo’n 100 tot 150 jaar geleden, en, meer recent, de door de mens geïnduceerde klimaatsopwarming, wat geleid heeft tot luchttemperaturen die hoger zijn dan wat in de afgelopen eeuwen of zelfs millennia de regel was voor het Arctisch gebied.
‘Take home message’
Naast een unieke inkijk op het klimaat uit het verleden, tonen de resultaten van deze studie aan dat de diatomeeëngemeenschappen in de Arctis recent grote veranderingen hebben ondergaan en dat ze zich momenteel verwijderen van hun ecologisch evenwicht van de afgelopen eeuwen tot zelfs millennia. Indien de klimaatsopwarming doorzet, bestaat het risico dat we de typische Arctische diatomeeëngemeenschappen zullen verliezen.
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