Abstract
In order to reveal the role of phytoplankton in the spatio-temporal distribution of alkaline phosphatase activity (APA), monthly investigations were conducted in the Xiaojiang River, a tributary of the Three Gorges Reservoir in China. Different APA fractions, environmental parameters, and phytoplankton communities were followed. High spatio-temporal variations of APA were observed, with the highest value in summer and the lowest in winter. The annual average APAT (total alkaline phosphatase activity) ranged from 7.78–14.03 nmol∙L−1∙min−1 with the highest in the midstream and the lowest in the estuary. The dominant phytoplankton phyla in summer and winter were Cyanophyta and Bacillariophyta, respectively. The mean cell density in the midstream and in the estuary was 5.2 × 107 cell∙L−1 and 1.4 × 107 cell∙L−1, respectively. That APA>3.0 μm was significantly higher than APA0.45-3 μm indicating phytoplankton was the main contributor to alkaline phosphatase. Correlation analysis indicated the dominant species and cell density could determine the distribution pattern of APA. Turbidity, total phosphorus, chemical oxygen demand, water temperature (WT), pH and chlorophyll a were proved to be positively correlated with APA; soluble reactive phosphorus, conductivity, transparency and water level(WL) were negatively correlated with APA. It was concluded that spatio-temporal heterogeneity of APA determined by phytoplankton communities was related to WT and WL.
Highlights
Phosphorus is always treated as a limiting nutrient in many freshwater ecosystems because it frequently limits the primary production[1,2]
alkaline phosphatase activity (APA)>3.0 μm contributed in average 28.1% in the APA was determined in unfiltered water (APAT), while bacterial APA accounted for 16.7%, APA in algal fraction (APA>3.0 μm) was higher than that in the picoplankton/bacterial fraction (APA0.45-3 μm)
Though many phytoplankton cells have a host heterotrophic bacteria inhabiting or in close association with cells, and the overlapping size on the filter influenced the final data, which making it difficult to assign the different size fractionation by filtration to individual cells alone, it could be admitted that the coarser fraction (APA>3.0 μm), mainly from algae, was conventionally defined as “algal APA”20 due to phytoplankton was the main contributor according to its size, biomass and physiological activity
Summary
Phosphorus is always treated as a limiting nutrient in many freshwater ecosystems because it frequently limits the primary production[1,2]. Various taxa are exhibiting differences in the presence, localization and labeling pattern of phosphatases Both seasonal and short-term variations have been detected in enzyme activity of phytoplankton[15]. Though many studies have been conducted to screen APase in different water bodies, little knowledge was obtained in the Three Gorges Reservoir (TGR). Due to the complicated relationship between APA and ecological factors, it is necessary to screen the distribution pattern of APA and its influencing factors in the TGR. Based on the related researches focused on the complicated relationship between APA and phytoplankton mentioned above, it was assumed that the phytoplankton community successions might lead to the spatio-temporal heterogeneity of alkaline phosphatase activity. The role of phytoplankton communities in the spatio-temporal heterogeneity of APA and its influence factors in the Three Gorges Reservoir were demonstrated. The results of this study can help to know how APA production changes with phytoplankton communities’ successions in the TGR
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