Higher Tolerance of Canopy-Forming Potamogeton crispus Than Rosette-Forming Vallisneria natans to High Nitrogen Concentration as Evidenced From Experiments in 10 Ponds With Contrasting Nitrogen Levels

Abstract

Due to excess nutrient loading, loss of submersed macrophytes is a worldwide phenomenon in shallow lakes. Phosphorus is known to contribute significantly to macrophyte recession, but the role of nitrogen has received increasing attention. Our understanding of how high nitrogen concentrations affect the growth of submersed macrophytes, particularly under natural conditions, is still limited. In this study, we conducted experiments with canopy-forming Potamogeton crispus in 10 ponds subjected to substantial differences in nitrogen loading (five targeted total nitrogen concentrations: control, 2, 10, 20, and 100 mg L-1) and compared the results with those of our earlier published experiments with rosette-forming Vallisneria natans performed 1 year before. Canopy-forming P. crispus was more tolerant than rosette-forming V. natans to exposure to high NH4 concentrations. This is probably because canopy-forming species reach the water surface where there is sufficient light for production of carbohydrates, thereby allowing the plants to partly overcome high NH4 stress. Both the canopy-forming P. crispus and the rosette-forming V. natans showed clear declining trends with increasing chlorophyll a in the water. Accordingly, shading by phytoplankton might be of key importance for the decline in submersed macrophytes in this experiment. Both experiments revealed free amino acids (FAA) to be a useful indicator of physiological stress by high ammonium but is not a reliable indicator of macrophyte growth.

Effects of High Ammonium Loading on Two Submersed Macrophytes of Different Growth Form Based on an 18-Month Pond Experiment

“…Therefore, the cover of V. natans in N0–N4 treatments was much lower than that of M. spicatum . These results concurred with experiments demonstrating that the canopy-forming species (i.e., Potamogeton crispus ) is tolerant to NH 4 -N concentration (<7 mg/L) ( Cao et al, 2009b ; Yu et al, 2018 ). Although the cover of V. natans keeps growing in all NH 4 -N addition treatments, the height began to decline after 13 months of N loading.…”

“…However, M. spicatum has another pathway than V. natans , for ammonia detoxification, catalyzed by glutamate dehydrogenase (GDH) under high NH 4 -N conditions ( Gao et al, 2020 ; Xian et al, 2020 ). Moreover, M. spicatum forms canopy vegetation with leaves on the water surface, where there is sufficient light for the production of carbohydrates to assist plants to overcome NH 4 -N stress ( Fu et al, 2012 ; Yu et al, 2018 ; Angove et al, 2020 ). Finally, M. spicatum can reproduce asexually using vegetative fragments (produced naturally or broken mechanically) and disperse to other areas of our experimental ponds, e.g., via waves, invertebrates, and human disturbance ( Li, 2014 ).…”

Interactive effects of benthivorous fish disturbance and ammonium loading on two submersed macrophytes of contrasting growth forms based on a mesocosm study

“…The maximum height and RGR-AGB of V. natans were significantly higher in the N treatment than for M. spicatum, which might reflect that rosetteforming macrophytes 1) usually exhibit a weaker ability to increase plant height in N stress environments, as demonstrated for V. natans by Yu et al (2017;, and 2) mainly grow by making physiological adjustments to tolerate stress conditions, such as adjusting the metabolisms of carbon and nitrogen (Yuan et al, 2016;Chen et al, 2020b), rather than by increasing plant height. Canopy-forming macrophytes, by contrast, usually elongate their shoots towards the water surface to capture light for growing (Yu et al, 2018;Angove et al, 2020). Consistent with previous studies, the high maximum height in our experiment enabled the shoots of M. spicatum to easily reach the water surface where they formed a canopy allowing them to maintain a competitive advantage in the presence of F and N stressors.…”