Water‐level fluctuations affect sediment properties, carbon flux and growth of the isoetid Littorella uniflora in oligotrophic lakes

Summary Amphibious isoetids such as Littorella uniflora dominate the littoral zone in north‐temperate oligotrophic, softwater lakes. Accumulation of labile organic matter in the sediments from eutrophication represents a threat to isoetids because of insufficient root oxygenation, reduced anchorage and enhanced growth of taller competitors. These threats might be alleviated by fluctuating water levels potentially increasing sediment mineralisation and plant growth. We hypothesised that L. uniflora would produce higher areal biomass when sediments are drained because air exposure assists oxygenation, mineralisation and consolidation of sediments. If so, changing water level offers a possible restoration measure. We tested this hypothesis by measuring sediment metabolism and plant performance under drained, fluctuating and submerged conditions in the laboratory and along an elevation gradient in the field in natural and organically enriched sandy sediments. Laboratory and field experiments confirmed that L. uniflora grew in biomass under all conditions and maintained high leaf chlorophyll and photosynthesis. Increase in plant biomass was highest under drained conditions, particularly for roots probably in response to higher sediment oxygenation and leaf transpiration. The increase in biomass was lowest under fluctuating conditions as a logical consequence of higher costs of frequent shifts between aerial and aquatic growth forms. In the laboratory experiment, mineralisation of sediment organic matter was 5.5‐fold higher under drained compared to submerged conditions in un‐vegetated sediments measured as release of CO 2 . Fluctuating conditions led to an intermediary release of CO 2 . In vegetated sediments, part of the CO 2 released from mineralisation was incorporated into plant biomass, and taking this into account yielded twofold increase in mineralisation rate from submerged and fluctuating to drained conditions. During the field experiment, a substantial part of the added organic matter was mineralised particularly in the dry part of the gradient. The proportion of reduced Fe 2+ to total Fe as a measure of oxygenation of sediments declined from submerged to fluctuating and permanently drained sediments. The proportion of reduced Fe 2+ was lower in plant covered than bare sediments reflecting root release of O 2 . Drainage and air contact increased sediment density and thus potentially the root anchorage compared to submerged conditions. Periodic drainage and air exposure of isoetid beds is a natural phenomenon in oligotrophic lakes causing beneficial sediment conditions for isoetids and positively affect the growth of L. uniflora . Reinstating water‐level fluctuations in regulated lakes or periodically lowering water levels could be used as a restoration measure.