The Holocene decline of slender naiad (Najas fl exilis (Willd.) Rostk. & W.L.E. Schmidt) in NE Poland in the light of new palaeobotanical data

The article presents a potential explanation of disappearance of Najas fl exilis in two lakes during the Atlantic and Subboreal chronozones in Romincka Forest and Lake Linowek in northeastern Poland. Palaeo- botanical (palynological and macrofossil) analyses revealed the disappearence of N. fl exilis that was possibly con- nected with the trophic state change. Those changes of water trophic state in the lake in Romincka Forest were probably triggered by Picea abies expansion stimulated by climate change at the beginning of the Subboreal. The appearance of spruce in the area of the lake led to acidifi cation of the water body where N. fl exilis grew, which caused the decline of its population. The Lake Linowek site revealed a possible connection of the disappearence of N. fl exilis (as well as other submerged plants) with Botryococcus blooms (probably caused by increasing tro- phy) that led to depletion of light availability on the bottom of the water body. Slender naiad, in both cases, grew on non-calcareous sediments, which reveals its broader pH ecological amplitude, despite the opinion that this species is an indicator of calcareous ground. The simultaneous occurrence of N. fl exilis and Potamogeton pusillus suggests that also in the past, these species built together submerged aquatic plant vegetation.