Abstract
The relative importance of bottom-up versus top-down effects in aquatic ecosystems remains a longstanding and ongoing controversy. To investigate these effects on phytoplankton communities in freshwater lakes, phytoplankton and zooplankton were sampled, and physical-chemical variables were measured during spring and summer in two important freshwater lakes in northern China: Nansi Lake and Dongping Lake. The redundancy analysis results showed that phytoplankton density and biomass were regulated by physical-chemical variables (bottom-up effects) and predation (top-down effects) together, and the former was more prominent in both lakes. However, the correlation analysis indicated that the top-down effects of zooplankton on phytoplankton were not significant in spring and summer in both lakes, while the bottom-up regulation of physical-chemical variables on phytoplankton had different patterns in the two lakes. In Nansi Lake, the bottom-up effects of physical-chemical variables on phytoplankton were weaker in summer than that in spring due to the abundant nutrients in summer. In Dongping Lake, the bottom-up effects of physical-chemical on phytoplankton were significant both in spring and summer, and the dominant bottom-up control factor shifted from total nitrogen in spring to total phosphorus in summer, with an increased ratio of nitrogen to phosphorus due to changes in limiting factors. In the two studied lakes, with fish culture, the bottom-up effects of phytoplankton on zooplankton were more important than the top-down effects of zooplankton on phytoplankton. These results demonstrate the interactions between phytoplankton and zooplankton and highlight the importance of phytoplankton regulation in freshwater lakes, which has implications for the effective management of freshwater lake ecosystems.
Highlights
Phytoplankton and zooplankton play important roles in aquatic ecosystems and serve as key indicators for water quality assessment [1,2,3,4]
Well distributed sampling sites were established across Nansi Lake (n = 10) and Dongping Lake (n = 8) (Fig 1), and samples were collected in spring (April and May) and summer (July and August) from both lakes in 2015
Our study showed that the bottom-up effects of physical-chemical variables on phytoplankton were stronger than the top-down effects of zooplankton in both Nansi and Dongping lakes, which is consistent with previous studies [17,18,26]
Summary
Ethics statementNo specific permissions were required to collect the samples in our study. Nansi Lake (116 ̊ 34’ E– 117 ̊ 21’ E, 34 ̊ 27’ N– 35 ̊ 20’ N) is the largest freshwater lake in northern China, with a total area of 1266 km. Dongping Lake is the catchment for the tributaries of Dahan River in the Yellow River Basin It is one of the storage lakes on the east route of the South-to-North Water Diversion Project, and it has a warm-temperate monsoon climate. Plankton samples were collected at each site with three replications by two different sized plankton nets, with mesh sizes of 112 and 64 μm. Macro plankton samples, which were collected with 40 L water samples and concentrated to 30 mL through a plankton net with a mesh size of 112 μm, were fixed with formaldehyde solution for microscopic enumeration. The biomass calculations, which used the specific gravity and body length-weight regression equation, were converted, taking sample volume into account
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