Clever reproduction strategy makes dwarf eelgrass resilient
Two types of eelgrass occur in the Wadden Sea, common eelgrass (Zosterea marina) and dwarf eelgrass (Zosterea noltii). Common eelgrass seems to be disappearing in large areas of the Wadden Sea and is therefore getting a lot of attention from the scientific world. Dwarf eelgrass, however, is doing well, and has been paid much less notice, so Zipperle researched it. Dwarf eelgrass has the ability to recover well, even after being grazed by geese. However, new beds rarely form in the Wadden Sea. Zipperle studied which dwarf eelgrass reproduction strategies are responsible for these characteristics. He did not focus just on the Dutch Wadden Sea for this, but on the northern Wadden Sea as a whole.
Eelgrass reproduces by means of pollen grains. A male flower produces the pollen grains, which, via water, have to reach a female flower, where the seeds are subsequently formed. It is, however, difficult for those pollen grains and seeds to disperse in water. For this reason, it was feared for a long time that inbreeding occurred primarily within a genetically related colony, whereby little genetic variation is generated within an eelgrass bed. This can be disadvantageous when circumstances in the environment change. If eelgrass is not prepared genetically for that change, whole eelgrass beds could die out at the same time.
Zipperle draws the conclusion from his research that the resilience of the Z. noltii grass beds is mainly determined by the survival of seeds and by cross-pollination. He discovered two important things using genetic analysis. Firstly, inbreeding was a lot less common than previously thought, so the genetic variation within the grass beds is high, and possible changes in the environment will therefore not immediately pose a threat to the entire bed.
In addition, he discovered that there are seed banks surrounding the grass beds, where the seeds of Z. noltii can lie dormant for at least three years. This strategy is a completely new discovery and explains to a large extent the power of dwarf eelgrass to repair a grass bed after damage has been done. If too many plants disappear, the dormant seeds will germinate and fill up the empty spaces.
Zipperle then wondered whether common eelgrass could also be restored using a similar strategy. He concludes that establishing new beds in the Dutch Wadden Sea using transplantation would be difficult to achieve, but that it might be possible in some areas using seeds. Zipperle makes several recommendations about transplanting plants from other colonies to existing beds to increase the genetic variation. Environmental factors will continue to hamper the recovery and maintenance of eelgrass colonies in the Wadden Sea. Construction of the Afsluitdijk in particular, as well as other human interventions, have had a major negative impact, certainly in the Dutch part of the Wadden Sea.
Earlier this year, researchers from the University of Groningen, Radboud University Nijmegen and the Royal Netherlands Institute for Sea Research discovered that common eelgrass works together with small shellfish and bacteria and is therefore able to survive in the Wadden Sea. Scientists suspect that other eelgrass species are also using this strategy. They conclude from this that when attempts to plant new beds are made, these shellfish and bacteria must also be transplanted.
The Dutch Department of Waterways and Public Works (Rijkswaterstaat) and Radboud University Nijmegen have recently started a pilot study into the use of eelgrass seeds.