Macrophyte-induced thermal stratification in a shallow urban lake promotes conditions suitable for nitrogen-fixing cyanobacteria

© 2017, Springer International Publishing AG. Densely vegetated shallow lakes often experience low water column nutrient levels and reduced phytoplankton growth, but in some cases a high phytoplankton biomass can co-exist with subme rged macrophytes. The conditions that favour phytoplankton blooms within areas colonized by submerged macrophytes remain largely unexplored. We investigated changes in water quality variables and phytoplankton community composition data in relation to macrophyte-induced thermal stratification in a shallow urban lake. The results indicate that submerged macrophytes may favour internal loadings of phosphorus and low DIN:TP ratios, in periods when macrophyte-induced thermal stratification of the water column is common. Blooms of nitrogen-fixing cyanobacteria occurred under the strongly stratified conditions triggered by the high macrophyte biomass and elevated canopy, even though nitrogen limitation was apparent during the whole growing season. These findings suggest that submerged macrophytes can promote blooms of nitrogen-fixing cyanobacteria once they are sufficiently tall and dense to induce stable water column conditions

Charophyceae (charales)

The charophytes, or stoneworts, are a group of green algae with six extant genera in one family, distributed worldwide in freshwater ponds and lakes. They are among the green algal groups most closely related to land plants and exhibit a complex thallus, with multinucleate internodal cells joined at nodes comprising smaller, uninucleate cells giving rise to whorled branchlets. Two genera (Chara, Nitella) contain most of the described species, with a third (Tolypella) containing several dozen taxa. The remaining genera have one or a few species. Reproduction is oogamous, with sperm and eggs produced in separate multicellular structures. The thallus is haploid; the zygote is the only diploid cell in the life cycle, and meiosis is followed by the development of a resistant spore. Thalli and spores are often encrusted with calcium carbonate. Such spores are abundant in the fossil record of the Charales, which extends to the Upper Silurian, and many genera and families have become extinct. These algae provide important ecosystem services, for example, as colonizing species, as biological agents for producing water clarity, or as the base of the food web. Charophytes are important for the study of evolution of embryophyte development, growth meristems, and cell biophysics. As one of the green algal groups most closely related to land plants, the rich charophyte fossil record may reveal clues regarding the earliest algae that invaded the land