Elevated alkalinity and sulfate adversely affect the aquatic macrophyte Lobelia dortmanna

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 plants m^?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.