The importance of ecotype diversity on duckweed growth with and without salt stress

The pollution of freshwater ecosystems is threatening freshwater plant species diversity worldwide. Freshwater plants, such as the common duckweed (Lemna minor), are potentially sensitive to novel stressful environments. To test if ecotype diversity could increase resistance to stressful environments, I used seven L. minor populations and measured their growth rates with and without moderate salt stress across an ecotype diversity gradient. The L. minor populations were grown over 5 months in 92 experimental mesocosms, either in ecotype monocultures or in polyculture with either one or three conspecific ecotypes (23 unique compositions). After growing the duckweed in unperturbed conditions (phase 1), the cultures were subjected to moderate salt stress (50 mmol/L NaCl) for several weeks (phase 2). The experiment was conducted in the presence of the natural epimicrobial community associated with the different ecotypes. In phase 2, a subset of these algae added an unintentional second stressor to the experiment. The ecotypes differed in their growth rates, the fastest growing at twice the rate of others. The diversity context further shaped the ecotype growth rates. Ecotype polycultures showed higher abundances towards the end of the experiment, thus over time, as the environment deteriorated, ecotype diversity gained in importance. These findings show that within-species variation in growth rates can translate to a positive effect of ecotype diversity on population abundance. Exposure of L. minor to moderate salt levels did not significantly impact growth rates, although the effect may have been masked by reduced algal stress in the saline environments.