Response of Vallisneria natans to Increasing Nitrogen Loading Depends on Sediment Nutrient Characteristics

Jiao Gu,Hu He,Xiaoyu Ning,Kuanyi Li,Hui Jin,Erik Jeppesen,Jinlei Yu,Zenghong Xu

doi:10.3390/w8120563

Abstract

High nitrogen (N) loading may contribute to recession of submerged macrophytes in shallow lakes; yet, its influences vary depending on environmental conditions. In August 2013, we conducted a 28-day factorial-designed field mesocosm experiment in Lake Taihu at the Taihu Laboratory for Lake Ecosystem Research (TLLER) to examine the effects of high N loading on the growth of Vallisneria natans in systems with contrasting sediment types. We ran the experiments with two levels of nutrient loading—present-day external nutrient loading (average P: 5 μg·L−1·day−1, N: 130 μg·L−1·day−1) and P: 5 μg·L−1·day−1, and with three times higher N loading (N: 390 μg·L−1·day−1) and used sediment with two contrasting nutrient levels. V. natans growth decreased significantly with increasing N loading, the effect being dependent, however, on the nutrient status of the sediment. In low nutrient sediment, relative growth rates, leaf biomass and root biomass decreased by 11.9%, 18.2% and 23.3%, respectively, at high rather than low N loading, while the decline was larger (44.0%, 32.7% and 41.8%, respectively) when using high nutrient sediment. The larger effect in the nutrient-rich sediment may reflect an observed higher shading of phytoplankton and excess nutrient accumulation in plant tissue, though potential toxic effects of the high-nutrient sediment may also have contributed. Our study confirms the occurrence of a negative effect of increasing N loading on submerged plant growth in shallow nutrient-enriched lakes and further shows that this effect is augmented when the plants grow in nutrient-rich sediment. External N control may, therefore, help to protect or restore submerged macrophytes, especially when the sediment is enriched with nutrients and organic matter.

Highlights

Submerged macrophytes have strong effects on the trophic dynamics and water clarity of shallow lakes [1,2]
We found that high N loading inhibited V. natans growth, not least in nutrient-rich sediment
The two-way ANOVA analysis suggests that the relative growth rate of V. natans and root biomass were affected by differences in the sediment nutrient characteristics and leaf biomass by the differences in the water nutrient (Table 2)

Summary

Introduction

Submerged macrophytes have strong effects on the trophic dynamics and water clarity of shallow lakes [1,2]. Worldwide, many shallow lakes are affected by accelerating eutrophication [3], and loss of submerged macrophytes and increasing turbidity have occurred . Lower species diversity of submerged macrophytes has been observed with high nitrogen loading in natural eutrophic shallow lakes [9] and in experiments [10]. Reduced coverage of submerged plants has been recorded [11,12], but the underlying mechanisms are subject to debate. An alternative explanation is that ammonia toxicity triggers a macrophyte decline in eutrophic lakes [14,15]

Methods

Results

Conclusion