Responses of five submerged macrophytes to NaCl salinity in a tropical mesocosm study

Abstract

Submerged macrophytes are important primary producers in freshwater ecosystems in tropical zones. Extreme climate events such as strong typhoons in tropical coastal areas are expected to increase in frequency in the near future, which could induce salinity changes and significantly influence aquatic macrophytes. However, few studies have focused on the effects of salinity on submerged macrophytes in tropical zones. Here, we conducted a three-month, tropical-zone mesocosm study to investigate the effects of salinity stress on the growth of five submerged macrophytes (two widely distributed species Stuckenia pectinata and Myriophyllum spicatum and three narrowly distributed species Vallisneria denseserrulata, Ottelia alismoides, and Cabomba caroliniana). Four salinity-level treatments were established, including P0 (control, no addition of NaCl), P2 (salinity of 2 ‰), P5 (salinity of 5 ‰), and PD (dynamic salinity, initial 2 ‰ with monthly salinity increases of 0.5 ‰). We hypothesized that species with broad distribution would better tolerate higher salinity stress compared to species distributed in narrow ranges. Consistently, both S. pectinata and M. spicatum tolerated P5 treatment, and M. spicatum exhibited smaller leaves and a reduced aerenchyma area in the leaf midrib following exposure. Both V. denseserrulata and O. alismoides survived the moderate salinity levels but showed distinct responses to the increase in salinity in regard to growth and reproduction. In conclusion, in tropical Hainan Island, China, we found distinct species-specific responses of the five submerged macrophytes to salinity stress. Those species with broader ecological niches will likely demonstrate better survival under extreme salinity changes in the future.