Overwintering and gyrogonite formation by the rare and endangered indicative macroalga Lychnothamnus barbatus (Meyen) Leonh. in eutrophic conditions

The occurrence of L. barbatus in Lake Kuźnickie was investigated at 1 m depth intervals at nine permanent transects in November 2015 and in April 2016. Spring thalli were observed microscopically to determine whether L. barbatus overwintered (with new outgrowths on old thalli) or grew anew in spring (from oospores). The species occurred at almost 50% of the surveyed sites in both sampling periods. At most sites L. barbatus formed compact carpets in which it was the only species or co-occurred with the charophyte Nitellopsis obtusa or the angiosperm Ceratophyllum demersum. L. barbatus occurred along a wide depth gradient: 1.5-6.5 m deep in autumn and 2-6.5 m after the winter period. The species coverages in April 2016 were comparable to those observed in November 2015 (Mann-Whitney U-test, P > 0.05). The thalli collected in April were well preserved after the winter, with numerous spring outgrowths. In addition, fully developed gyrogonites were found in April 2016. This study indicates wider fertility tolerance of L. barbatus and its ability to overwinter under eutrophic conditions.