Charophyte stoichiometry in temperate waters

In recent years, little scrutiny has been devoted to charophytes with regard to carbon, nitrogen and phosphorus contents, their ratios and the factors controlling them. Data from published literature, and our own unpublished data, will be used along with environmental data to depict overall statistical distributions and controlling factors of charophyte stoichiometry, along with two hypotheses already suggested for plankton. Data on whole and decarbonated plants will be reviewed separately, but the role of the phosphorus-bound, external crust in the charophyte metabolism is still unclear. These algae have an intermediate carbon content, and lower nitrogen and phosphorus contents, among other aquatic plants. Hence their C:N and C:P ratios are usually higher than those of other submerged plants. Phosphorus is the outstanding key factor for charophyte growth and stoichiometry. Charophyte P content appears to be related to C, N and P sedimentary contents; a higher P content will result in increased growth rates. The maximal growth rates recorded occur at charophyte C:N:P ratios of 343-759:18-44:1, higher than the well-known Redfield values for optimal growth in other algae. While PAR irradiance partly controlled charophyte nitrogen content and the C:N ratio, UV-B radiation increased C and N content, albeit not affecting nutrient ratios. We conclude that the light:nutrient and the growth rate hypotheses hold true for charophytes. Increasing temperatures by global warming could decrease charophyte P content, thereby augmenting the C:P ratio. Further studies on charophyte stoichiometry are encouraged, specifically to disentangle the relationship between plant and carbonate crust and the role of stonewort stoichiometry in aquatic system.