Strong impact of nitrogen loading on submerged macrophytes and algae: a long‐term mesocosm experiment in a shallow Chinese lake

Abstract

Summary Excess loading of phosphorus (P) and nitrogen (N) triggers a shift in the trophic structure of shallow lakes from a clear‐water, macrophyte‐dominated state to an algal‐dominated turbid state. However, the role of N in the shift is debated, and experimental evidence is, with a few exceptions, based on short‐term studies (days to a few months). We studied the effect of N loading on macrophytes (dominated by Potamogeton lucens and Cabomba caroliniana), periphyton, filamentous algae and phytoplankton in mesocosms over 10 months (starting in October) in subtropical China (Wuhan). There were three N treatments: controls (CN) without nitrogen addition (mean TN = 1.9 mg L−1), low nitrogen (LN) addition (mean TN = 3.5 mg L−1) and high nitrogen (HN) addition (mean TN = 5.5 mg L−1). Total phosphorus (TP) concentration in the water column remained moderate (0.05–0.07 mg L−1) during the experiment in all treatments. Macrophyte abundance declined in the LN and HN treatments in the first 6 months, but not in controls, followed by a partial recovery in the LN treatments. They disappeared completely in the HN treatments the following summer. Periphyton (biofilm on plastic) and phytoplankton biomass remained unaffected during the first 6 months but increased over the summer by two or three times, compared with controls, in low and high nitrogen treatments, respectively. By contrast, the abundance of filamentous algae increased over winter but declined during the summer with no obvious relationship to the N treatments. There was no difference in the TN or nitrate concentrations or soluble protein, soluble sugar and Chl‐a content of P. lucens leaves and stems with increasing N load. Macrophyte populations are partially resilient to abrupt increases in N loading at moderate TP concentrations, but, after prolonged exposure, a complete collapse occurs. Our results further indicate that macrophyte loss is exacerbated by shading by filamentous algae during the winter, and by phytoplankton and periphyton in the summer, while there was no indication of direct N toxicity.

Does the responses of Vallisneria natans (Lour.) Hara to high nitrogen loading differ between the summer high-growth season and the low-growth season?

“…The relatively minor effect of nitrogen may be due to a weak development of periphyton compared with other studies showing a strong positive effect of nitrogen on periphyton growth (Özkan et al, 2010) and an associated decline in macrophyte growth (Olsen et al, 2015;Zhao et al, 2016).…”

“…Active growth may explain why the toxic effect of N is less obvious in longer running experiments Olsen et al, 2015;Zhao et al, 2016) where there is sufficient time for macrophytes to be released from high N stress through growth than in short-term lab experiments.…”

“…Most scientists agree that phytoplankton shading promoted by increasing inputs of phosphorus is an underlying cause of the decreasing abundance, but the role of nitrogen has received increasing attention in recent years (Moss, 2001;Jeppesen et al, 2007;Moss et al, 2013). In previous studies, two main mechanisms have been identified to explain the effects of nitrogen on submersed macrophytes: 1) it promotes the growth and hence shading of phytoplankton (Sayer et al, 2010a(Sayer et al, , 2010b or periphyton (Olsen et al, 2015;Zhao et al, 2016) that uses nitrogen as its nutrient source; 2) it causes physiological stress on the submersed macrophytes and limits the metabolism of carbohydrates (Cao et al, 2009a;Yuan et al, 2015;Gao et al, 2015), inducing oxidative stress (Wang et al, 2010;Zhang et al, 2011) and inhibiting photosynthesis (Wang et al, 2008;Su et al, 2012). Experimental studies have revealed that ammonium (NH 4 )-related physiological stress may be aggravated by low light conditions .…”

“…Previous studies have revealed that the role of different influencing mechanisms might depend on the experiment scale. Toxic effects, for example, have been found to play a key role in acute tests, whereas shading effects from algae have been emphasized in long-term chronic tests Olsen et al, 2015;Zhao et al, 2016). In a long-term whole-ecosystem experiment carried out in 10 ponds in the lowgrowth season, shading by phytoplankton was identified as the main cause of declined plant growth, and toxic stress induced by high N levels may to a certain extent have negative effects (Yu et al, 2015).…”

Exploring the Spatial-Seasonal Dynamics of Water Quality, Submerged Aquatic Plants and Their Influencing Factors in Different Areas of a Lake

“…The pH and water transparency were significant factors positively influencing the growth of Stuckenia pectinata (L.) Börner sago pondweed in dry and wet seasons, respectively. It has been demonstrated in many systems that submerged aquatic plant growth can be seasonally inhibited by periphyton growing on their leaves [53], and water transparency can be negatively affected by algal growth in the water column, again affecting the ability of submerged aquatic plants to grow [54,55]. Therefore, in shallow lakes, increased N and P concentrations could raise the productivity of phytoplankton and periphyton [49], and then influence the growth of submerged aquatic plants in different seasons.…”