Environmental factors responsible for the gyrogonite formation by an endangered macroalga, Lychnothamnus barbatus, a fertility indicator of past and present lacustrine ecosystems

Abstract

Gyrogonites (calcified oospores) of charophytes are commonly used in palaeoecology. Although, the study of morphometry and gyrogonite production by extant charophytes is limited. This concerns the genus Lychnothamnus, frequent in the past but contemporarily represented only by L. barbatus, the rare and endangered species applied in assessment of water quality as a bioindicator of oligo-mesotrophic lakes. In this study two hypotheses were verified: (1) the length and width of L. barbatus gyrogonites increase with increasing depth; (2) at the deeper sites, L. barbatus produces more numerous gyrogonites compared to shallower lake areas.Gyrogonites were picked from L. barbatus thalli (n = 170) collected at three depths (2-3 m, 3-4 m and 4-5 m) and three permanent transects in a meso-eutrophic lake hosting one of few largest populations of the species (Lake Kuźnickie). Subfossil gyrogonites (n = 730) were also analyzed from three sediment cores taken at the same depths. The study was conducted in autumn 2016, spring, summer and autumn 2017. In addition to depth, photosynthetically active radiation (PAR), nutrients and other indices of water chemistry, and interspecific competition were studied.Studied L. barbatus gyrogonites were more circular and wider compared to existing literature data. Gyrogonites production was positively correlated with depth. The smallest, but most numerous gyrogonites occurred at the deepest sites (ANOVA). Subfossil gyrogonites were slightly wider and more circular compared to extant gyrogonites. Reversely to depth, PAR was positively correlated with sizes of gyrogonites and negatively correlated with amount of gyrogonites. Gyrogonites formation was negatively correlated with ammonium ion concentration and Myriophyllum spicatum biomass, but positively with phosphate concentrations.Our results supplement the gaps in the knowledge regarding the ecology of L. barbatus because the species produced numerous gyrogonites in meso-eutrophic waters. We postulate that L. barbatus gyrogonites can be applied in ecology and palaeoecology to interpret water depth and trophic status.