Abstract
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. dortm- anna, 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 combina- tion 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 photosyn- thetic 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 poten- tial 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.
Highlights
Softwater lakes have low surface water alkalinity (\1 mM) and are naturally inhabited by slow-growing aquatic macrophytes called isoetids that are adapted to carbon and nutrient-limited systems (Arts 2002; Murphy 2002)
Actual photosynthetic efficiency was significantly positively correlated to redox potential in the sediment, indicating a positive relationship between plant performance and sediment oxidation
The overall aim of this study was to test if the joint action of increased water column concentration of SO42- and alkalinity, above those typical for pristine softwater lakes with healthy populations of isoetids, adversely affected L. dortmanna, and whether potential impacts can be counteracted by high plant density
Summary
Softwater lakes have low surface water alkalinity (\1 mM) and are naturally inhabited by slow-growing aquatic macrophytes called isoetids that are adapted to carbon and nutrient-limited systems (Arts 2002; Murphy 2002). Increasing alkalinity (alkalinization) above the low natural levels is among the main causes of degradation of the vegetation in softwater lakes (Arts 2002). Alkalinization can cause a shift in the vegetation from plant species characteristic of softwater (isoetids) to species characteristic of buffered conditions (elodeids, and other hard-water macrophytes) (Arts 2002). High SO42- concentrations alone do not seem to impact the natural vegetation as concentrations above 1 mM have been reported in pristine isoetid lakes (Roelofs 1983).
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