Occurrence of slender naiad (Najas flexilis (Willd.) Rostk. & W. L. E. Schmidt) during the Eemian Interglacial – An example of a palaeolake from the Hieronimowo site, NE Poland

Abstract

Nowadays Najas flexilis is believed to be extinct in Poland, Germany, and Switzerland, while in other European countries it is classified as endangered or vulnerable. During the Early and Middle Holocene the species occurred throughout much of Europe, but in the Late Holocene it underwent a significant decline and reduction in range following, as it is believed, the gradual cooling of the climate and/or the eutrophication and acidification of lakes caused by human activity. The article recognizes the existence and disappearance of slender naiad in the palaeolake at the Hieronimowo site, NE Poland, during the Eemian Interglacial and at the beginning of the last glaciation (Vistulian, Weichselian), when lake transformations were caused solely by natural factors without human impact. Results of palynological and macrofossil analyses reveal that the appearance of slander naiad in the studied water body was probably triggered by the change of its pH and trophic status, resulting from the start of spruce and pine expansion stimulated by climate worsening during the younger part of the hornbeam phase (E5 R PAZ) in the middle Eemian. The appearance of spruce and pine in the area of the lake led to its acidification and oligotrophication, and that caused the strong and rapid development of N. flexilis. The species existed in the palaeolake at the Hieronimowo site through the spruce (E6 R PAZ) and pine (E7 R PAZ) phases, until the end of the Eemian Interglacial. These observations suggest that regressive transformations of the environment typical for the late part of the interglacial cycle, ‘unsupported’ by human activity, do not lead the extinction of N. flexilis. Quite the opposite, these processes promote this species. Its disappearance started only at the beginning of the Early Vistulian (Weichselian), and it was probably caused by a drastic decrease in temperatures and/or by the development of green algae, which limited light availability on the bottom of the water body.