Responses Of Submerged Macrophytes To Ammonium Toxicity:A Study Of Functional Traits

Submerged macrophytes are one of the most important components of the aquatic ecosystem such as shallow lakes.These plants have a myriad of uses,e.g.maintaining a clear-state in shallow lakes,and their presence is of the most significance for the survival and interaction of other biota in the aquatic ecosystem.Second to carbon,nitrogen is one of the essential nutrients required by submerged macrophytes for growth and survival.There are various sources of nitrogen in the water such as nitrate,nitrite and ammonium.Reports and studies have shown that most of submerged macrophytes prefer to use ammonium.Recently,ammonium levels have increased in the shallow lakes due to the increased use of nitrogen-rich fertilizers and the dumping of untreated sewage waters.Increased ammonium levels are assumed to have toxic effects in plants.This has been used to explain the disappearance of macrophytes from the shallow lakes.There is a tradeoff between growth and ammonium detoxification for submerged macrophytes.Many studies have been conducted to explain the effect of ammonium to submerged macrophytes,most of which concentrated on the physiological effects of ammonium on aquatic plants or only focused on few species of submerged macrophytes.In this study,we have investigated the effect of ammonium to plants based on the functional traits that indicate the response of macrophytes to environmental changes.Sixteen species of macrophytes were subject to different concentrations of ammonium(5 and 10 mg/L).The plants were grown for two months.The experiment was conducted in a clear water environment with the transparency of bottom view at the depth of 1.5 m.After harvesting,the traits measured included the stem length,the height of the plant,the ramet number and the total dry biomass of the plant species(including the above-ground and below-ground biomass).The results suggest that the response of plants to increased ammonium concentration is species-specific.We observed the tradeoffs from different species as a way to counter the effects of elevated levels of ammonium in the macrophytes.Species like Myriophyllum spicatum,which has previously been reported to be tolerant to ammonium toxicity,increases its stem length but significantly reduces its ramet number when facing the increase in ammonium concentrations.Some species,however,were not affected by the increased ammonium concentrations such as Najas guadalupensis.Hence it is very important to consider the difference of species tolerance to ammonium concentrations so as to use it for restoration and conservation of the aquatic systems.