Eleocharis

Jäger, E.J. 2011: Rothmaler, Exkursionsflora von Deutschland, Gefäßpflanzen: Grundband, ed. 20Stace, C. 2010: New flora of the British Isles, ed. 3
but modified
1Spikelets usually with 20-40 flowers; glumes from middle of spikelet 3.5-4.5 mm long; nut (1.3-)1.5-2 mm long
Eleocharis vulgaris
1'Spikelets usually with 40-70 flowers; glumes from middle of spikelet 2.5-3.5 mm long; nut 1.2-1.4(-1.5) mm long
Eleocharis palustris