Catabrosa aquatica; the Whorl-grass is a plant of moist and wet grounds - often inundated - like pond margins. hence the Latin name "aquatica". Next to mires it also grows often on moist sandy soils. It prefers nutrient-rich habitat's (springwater indicator). It spreads horizontally with the inflorescence stems growing somewhat upwards. Characteristic are the (often) inflated leaf-sheaths. Another characteristic feature are the lemmas: when flowering they are much longer than (and thus protruding out of) the glumes. I find the rather short but wide leaves in combination with the inflated leaf-sheaths, quite characteristic. It should also be noted that the spikelet's normally have only one single flower.
The Whorl-grass is a member of the grass family (Poaceae, also known as Gramineae). The Icelandic name of the Whorl-grass is Vatnsnarfagras.