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. 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.
“For the first time, in this study we were able to prove the importance of bicarbonate for the occurrence of aquatic plants on a global scale. As a lake researcher, I am particularly excited about the results”, says university researcher Janne Alahuhta from the Geography Research Unit of the University of Oulu, one of the co-authors of the research.
Alahuhta has created a global network of lake aquatic plants scientists covering more than 20 areas on different continents. The network contributed to the study. In addition, Alahuhta provided lake research material from Finland and participated in writing and commenting on the article.
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 alter the three-dimensional structure of the underwater meadows and the protection of small animals and juvenile fish against predation.
Iversen LL, Winkel A, Båstrup-Spohr L, Hinke AB, Alahuhta J, Baattrup-Pedersen A, Birk S, Brodersen P, Chambers PA, Ecke F, Feldmann T, Gebler D, Heino J, Jespersen TS, Moe SJ, Riis T, Sass L, Vestergaard O, Maberly SC, Sand-Jensen K, and Pedersen O (2019) Catchment properties and the photosynthetic trait composition of freshwater plant communities. Science DOI 10.1126/science.aay5945
Last updated: 22.11.2019