The Biomass and Physiological Responses of Vallisneria natans (Lour.) Hara to Epiphytic Algae and Different Nitrate-N Concentrations in the Water Column

Fenli Min,Qingwei Lin,Jian Sun,Zhenbin Wu,Feng He,Lei Zeng,Jincheng Zuo,Yi Zhang,Biyun Liu

doi:10.3390/w9110863

Abstract

Increasing N concentration and the high density of epiphytic algae are both key factors leading to the decline of submerged macrophytes in many eutrophic lakes. In order to investigate the impacts of increased nitrate-N concentration and the growth of epiphytic algae on the decline of submerged vegetation, we conducted a 2 × 4 factorial experiment with the submerged macrophyte Vallisneria natans (Lour.) Hara by measuring the biomass of plants and some physiological indexes in leaves of V. natans under four nitrate-N concentrations in the water column (0.5, 2.5, 5, and 10 mg/L) and two epiphytic groups (epiphytic algae group and no epiphytic algae group). The results suggested that epiphytic algae could impose adverse effects on the biomass accumulation of V. natans, while the increasing nitrate-N concentration (0.5–10 mg/L) could oppositely promote this process and counteract the adverse effect of epiphytic algae. When nitrate-N concentration was 5 mg/L, the total chlorophyll content in leaves of V. natans in the epiphytic algae group was prominently lower compared with the no epiphytic algae group, while MDA, free proline, and anti-oxidant enzyme (SOD, POD, CAT) activities were significantly higher. Overhigh nitrate-N concentration in the water column also directly imposed adverse effects on the physiology of V. natans. When nitrate-N concentration was over 5 mg/L, the total chlorophyll content and free proline decreased in the no epiphytic algae group, while soluble carbohydrates and soluble proteins decreased when nitrate-N was over 2.5 mg/L. Meanwhile, epiphytic algae and nitrate-N content imposed a synergetic effect on the anti-oxidant enzyme activities of V. natans. When nitrate-N concentration was over 5 mg/L, SOD, POD, and CAT activities kept constant or decreased, which indicated that the oxidation resistance of V. natans was inhibited by stress. Our results indicate that epiphytic algae and increasing nitrate-N concentration in the water column could severally or synergistically impose adverse effects on the physiology of submerged macrophytes and are both key factors leading to the decline of submerged macrophytes.

Highlights

Aquatic plants, especially submerged macrophytes, play a crucial role in the recovery process of polluted shallow lake ecosystems by providing habitats for a diverse faunal community [1,2], sequestering carbon and nutrients through absorption by the root and stem [3], and reducing phytoplankton biomass by either direct or indirect pathways [4,5]
This study aimed to explore the effects of epiphytic algae and different nitrate-N concentrations on the biomass accumulation and physiology of V. natans and provides a theoretical foundation for the restoration of eutrophic lakes
As the nitrate-N concentration increased from 2.5 to 10 mg/L, the biomass of V. natans in the epiphytic algae group gradually increased, suggesting that increasing the nitrate-N concentration could neutralize the adverse effect of epiphytic algae on biomass accumulation of V. natans, probably because the growth of epiphytic algae was inhibited by a high nitrate-N concentration

Summary

Introduction

Especially submerged macrophytes, play a crucial role in the recovery process of polluted shallow lake ecosystems by providing habitats for a diverse faunal community [1,2], sequestering carbon and nutrients through absorption by the root and stem [3], and reducing phytoplankton biomass by either direct or indirect pathways [4,5]. The decline of submerged macrophytes continually happens in many eutrophic lakes throughout the world as a result of eutrophication with increasing nutrients [6,7]. It is well known that phosphorus (P) is often considered to be the most crucial growth limiting factor for plants in lakes [8,9], but the role of nitrogen (N). The elevating nitrate-N concentration was considered to be the reason for plant species richness declines according to studies conducted in Polish and British lakes [13]

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