7. Ökologie der Characeen

Charophytes mainly occur in aquatic ecosystems with low nutrient concentrations where they can grow down to deepwater. Under eutrophic, turbid conditions, charophytes are severely light-limited because of their small hibernacles andbottom-dwelling growth form and therefore restricted to shallow water. Due to different mechanisms of osmoregulation andturgor regulation, salinity tolerances show major differences among single species. Also mechanisms of carbon assimilationdiffer substantially among species. Nitella spp. mainly occur in soft water, whereas manyspecies within the genus Chara can efficiently assimilate bicarbonate and are thereforesuperior competitors in calcium-rich water. While abiotic conditions such as light, salinity and carbon affect occurrenceand species composition of charophytes, these plants in turn have a major impact on their abiotic and biotic environment,especially in calcium-rich lakes where they can form dense vegetation. They accumulate and immobilize nutrients, reduceresuspension and enhance sedimentation, thereby improving light availability in the water column and causing conditions thatfavour their own occurrence. Allelopathic effect against micro-algae has been shown in the laboratory, but the quantitativeimportance of this mechanism in natural ecosystems is unknown. Several investigations indicate that charophyte vegetationmay have a lower refuge function for zooplankton against fish predation than other submerged macrophytes. Charophytes oftenharbour high macroinvertebrate densities. Their interactions with fish are complex. In charophyte-dominated lakes, waterfowlare favoured by high food availability (plants and macroinvertebrates) combined with high water clarity. Singleinvestigations show contradictory results concerning the grazing pressure of waterfowl on charophytes. Waterfowl have animportant function, however, for long-distant transport of charophyte oospores. Single species of charophytes apply fardifferent strategies of reproduction and dispersal. Annual species with high oospore production are typical pioneer plantsand mainly found in small, often temporary water bodies. Other species such as Nitellopsisobtusa have a high vegetative reproduction, but form oospores relatively rarely. Such species are restricted tolarger, permanent water bodies.