Long-term moderate wind induced sediment resuspension meeting phosphorus demand of phytoplankton in the large shallow eutrophic Lake Taihu.

Jian-Ying Chao,Ming Kong,Yi-Min Zhang,Guang Gao,Wei Zhuang,Long-Mian Wang,Ke-Qiang Shao,Hideyuki Doi

doi:10.1371/journal.pone.0173477

Abstract

The objective of this study was to investigate the impact of sediment resuspension and phosphorus (P) release on phytoplankton growth under different kinds of wind-wave disturbance conditions in the large and shallow eutrophic Lake Taihu in China. Short-term strong wind (STSW) conditions, long-term moderate wind (LTMW) conditions, and static/calm conditions were investigated. To address this objective, we (1) monitored changes in surface water P composition during field-based sediment resuspension caused by STSW conditions in Lake Taihu, and also conducted (2) a series of laboratory-based sediment resuspension experiments to simulate LTMW and calm conditions. The results showed that under both strong and moderate wind-wave conditions, suspended solids (SS) and total phosphorus (TP) in the water column increased significantly, but total dissolved phosphorus (TDP) and soluble reactive phosphorus (SRP) remained low throughout the experiments, indicating that the P released from sediments mainly existed in particulate forms. In STSW conditions, alkaline phosphatase activity (APA) and enzymatically hydrolysable phosphorus (EHP) increased rapidly, with the peak value occurring following the peak value of wind speed for 1–2 days, and then rapidly decreased after the wind stopped. Under LTMW conditions, APA and EHP increased steadily, and by the end of the laboratory experiments, APA increased by 11 times and EHP increased by 5 times. Chlorophyll a (Chl-a) in LTMW conditions increased significantly, but remained low under STSW conditions, demonstrating that the former type of sediment P release promoted phytoplankton growth more effectively, and the latter type did not. Despite the fact that STSW conditions resulted in the release of more TP, TP settled to the bottom rapidly with SS after the wind stopped, and did not promote algal growth. Under LTMW conditions, suspended particulate P was hydrolyzed to SRP by phosphatase and promoted algae growth. Algal growth in turn secreted more phosphatase and accelerated particulate P regeneration, which may be the main mechanism of sediment bio-available P release that promotes phytoplankton growth in shallow lakes.

Highlights

Phosphorus (P) is generally considered the main limiting nutrient for growth of phytoplankton in freshwater systems, and excessive imports of P cause eutrophication [1]
The water samples collected both in the field observation and laboratory experiments were analyzed for SS, particulate organic matter (POM), total phosphorus (TP), total dissolved phosphorus (TDP), soluble reactive phosphorus (SRP), alkaline phosphatase activity (APA), enzymatically hydrolysable phosphorus (EHP), and chlorophyll a (Chl-a)
The above observation results showed that wind-wave disturbance was the main factor that caused intensive resuspension of sediment in the water column, and suspended particulates that increase due to strong wind-wave disturbance will quickly decrease with wind speed

Summary

Introduction

Phosphorus (P) is generally considered the main limiting nutrient for growth of phytoplankton in freshwater systems, and excessive imports of P cause eutrophication [1]. The release of P from sediments is an important nutrient source that can induce continuous eutrophication in lakes even if external inputs are reduced [2, 3]. Research shows that the flux of total P annually released by sediments in Lake Taihu is 21,000 tons, which is approximately 2–6 times the P input from external sources [5]. Research showed that 88% of total P internal release in Lake Taihu occurs under dynamic conditions [4], and lake sediment resuspension accounts for 85% of the vertical flux of resuspended particulates [12]

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Discussion

Conclusion