Nutrient pollution degrades microbialites in Lough Carra, an Irish marl lake

Marl lakes often have microbialite crusts in the littoral zone, containing a community of cyanobacteria and algae. Previous work has shown that these crusts are most prevalent in oligotrophic lakes and that crust metrics, such as chlorophyll content and green algal abundance, are correlated with total phosphorus concentration (TP) in lake waters, with no intact crusts being found when mean TP > 0.02 mg l⁻¹. Two experiments were carried out to examine how microbialite crust communities respond to nutrient pollution. A culture experiment exposed crusts to various concentrations of nutrients and measured responses over a 43 wk period. In a parallel controlled field experiment, crusts were moved between areas of different trophic state within Lough Carra, a marl lake in the west of Ireland, to see how the community responded. In both experiments, increased nutrient conditions caused a change in relative abundances of taxa in the crust community, leading to a dominance of green algae and degradation of crusts. There were significant differences in chlorophyll concentrations between crusts grown in different nutrient environments. It was concluded that these metrics offer the prospect of a useful quality assessment method for marl lakes.