13 páginas, 3 tablas, 4figuras. ; The increase in alkalinity and SO4 2- in softwater lakes can negatively affect pristine isoetid population because the increase in alkalinity and SO4 2- can stimulate sediment mineralization and consequently cause anoxia. The consequences of increased sediment mineralization depend on the ability of isoetids such as Lobelia dortmanna to oxidize the rhizosphere via radial O2 loss. To study how alkalinity and SO4 2- affect the isoetid L. dortmanna, and if negative effects could be alleviated by neighboring plants, three densities of L. dortmanna (‘‘Low’’ = 64 plants m-2, ‘‘Medium’’ = 256 plants m-2 and ‘‘High’’ = 1,024 plantsm-2) were exposed to elevated alkalinity in the water column, or a combination of both elevated alkalinity and SO4 2-, and compared to a control situation. The combination of SO4 2- and alkalinity significantly increased mortality, lowered areal biomass and reduced actual photosynthetic efficiency. Plant density did not significantly alleviate the negative effects caused by SO4 2- and alkalinity. However, actual photosynthetic efficiency was significantly positively correlated to redox potential in the sediment, indicating a positive relationship between plant performance and sediment oxidation. The negative effects on L. dortmanna were probably caused by long periods of tissue anoxia by itself or in combination with H2S intrusion. Therefore, increase in both SO4 2- and alkalinity surface water can dramatically affect L. dortmanna populations, causing reduction or even disappearance of this icon species. ; The project was funded by The Danish Council for Independent Research Natural Science grant No 645-06-0287 and the Centre for Lake Restoration, a Villum Kann Rasmussen Centre of Excellence. ; Peer reviewed