Water Characteristics Control Freshwater Plant Distribution
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Unlike land plants, photosynthesis in many aquatic plants relies on bicarbonate in addition to CO2 to compensate for the low availability of CO2 in water. A study by Iversen and co-authors shows that the abundance of plant species with the ability to use bicarbonate increases in hard water lakes with greater bicarbonate concentrations. In streams, where the CO2 concentration is higher than in air, bicarbonate users are few.
Globally, photosynthetic modes of terrestrial plants are influenced by climatic factors such as adaptations to variation in air temperature and water availability. In water, CO2 often limits photosynthesis because it moves 10,000-times slower than in air and, thus, rapid photosynthesis can deplete CO2 in dense plant stands. In order to meet the requirements of water plants, CO2 concentrations must be 10-20 times higher than in air. This never happens in lakes, where microscopic algae may reduce the CO2 content to 10% of that in air.
The solution among many water plants is to use bicarbonate, which is dissolved in high concentrations in lakes located in calcareous catchments.
"Use of bicarbonate is energy demanding and much less efficient than CO2 use, when concentrations are the same", explains Professor Ole Pedersen. "However, in bicarbonate-rich lakes, the photosynthesis yield is much higher when water plants can use bicarbonate. Globally, this can account for the increasing abundance of bicarbonate users relative to non-users in lakes located in calcareous catchments."
"In order to establish the ability, or lack of ability to use bicarbonate of many species, we first had to diagnose several tropical species and then establish their abundance in relation to water chemistry", explains Professor Kaj Sand-Jensen.
The increase in abundance of bicarbonate users with higher bicarbonate concentrations was consistent among temperate and tropical lakes. In streams, however, where CO2 concentrations are high because of continuous inflow of CO2-rich soil water, CO2 use is the most cost-effective and the abundance of bicarbonate users is low and remains independent of bicarbonate concentrations.
The implications of the study are, that species richness and composition of water plants are expected to change with ongoing and future changes of bicarbonate concentrations in lakes that are caused by anthropogenic changes of acidification, forest cover and use of nitrogen fertilizers. The changes can be dramatic, since CO2 users are generally small individuals compared to the much larger bicarbonate users. Therefore, a change in the balance between the two plant types alters the three-dimensional structure of the underwater meadows and the protection of small animals and juvenile fish against predation.
Iversen et al. (2019) Catchment properties and the photosynthetic trait composition of freshwater plant communities. Science. DOI: https://doi.org/10.1126/science.aay5945
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