The princess and the pea: seasonal development of the desmid Oocardium stratum along a limestone-precipitating spring stream

The desmid Oocardium stratum is restricted to short sections of active limestone-precipitating springs (LPS) and adjacent headstreams. We studied the succession of photoautotrophic biofilms on artificial substrate at three sites in an LPS stream in spring, summer and autumn. We exposed frosted glass slides between 3 and 12 weeks, and measured calcium carbonate precipitation (CCP), nutrients and environmental parameters. Relative amounts of O. stratum, Bacillariophyceae and Cyanobacteria to overall biomass of the photoautotrophic biofilm were estimated by means of class-specific photosynthetic pigments. Overall, 77% of the photoautotrophic biomass consisted of diatoms, 20% were related to O. stratum and 3% were related to Cyanobacteria. Biomass of O. stratum and diatoms showed a significant correlation with CCP (r = 0.61 and 0.46), while for Cyanobacteria there was no significant correlation. CCP increased exponentially over time and peaked in late summer and late autumn (maximum CCP measured as ash mass was around 10 µg mm−2 week−1). We applied generalised multiple linear regression models (GLM) to evaluate the influence of stream site, weeks of exposure, season and CCP on biofilm development. According to GLM, the biomass of O. stratum was associated with site, exposure time, season and CCP, while the biomass of Cyanobacteria and Bacillariophyceae were not associated with the input variables. The negative standardised beta coefficient of cyanobacterial biomass in the multivariate regression model for the biomass of O. stratum suggest a competition between the two taxa. This likely explains the restricted occurrence of O. stratum within a short section of the active LPS stream.© 2019 The Author(s