Ranunculus reptans; The creeping spearwort grows on muddy grounds which are often under water in the spring, drying up as summer progresss. The plant has a roset of blade-shaped narrow leaves and creeping stems from the rosettes. This is not always clearly visible as they can form thick mats. The leaves (mats) are normally green but can also be red. I believe - but I can be wrong - that this is when they just appear from under water. The appearance of the flowers are typically buttercup-like.
It is a member of the buttercup family (Ranunculaceae). The Icelandic name of this species is Flagasóley.