International Journal of Aquaculture, 2025, Vol.15, No.2, 57-66 http://www.aquapublisher.com/index.php/ija 60 decomposed person will also be affected by temperature, nutrients, etc. These factors determine whether they metabolize quickly and affect the speed of circulation throughout the ecosystem (Atkinson et al., 2017). Figure 1 The effects of future climate on absolute flows and transfer efficiency between successive trophic levels of the mesocosm food web (Adopted from Ullah et al., 2018) 5 Food Web Structure in Aquatic Ecosystems 5.1 Examples of simple food chains Simple food chains in aquatic ecosystems typically involve a linear sequence of organisms, where each is a food source for the next. A classic example is the phytoplankton-zooplankton-fish chain, where phytoplankton serve as primary producers, zooplankton as primary consumers, and fish as secondary consumers (Feitosa et al., 2019). In some ecosystems, such as the Amazonian floodplain lakes, the microbial food web (MFW) plays a significant role, with microbial interactions contributing substantially to the total plankton biomass (Huang, 2024). This highlights the importance of both classical and microbial food chains in different aquatic environments. 5.2 Interactions in complex food webs Compared to a simple line, a complex food web is like a large interlaced web. There is not only one kind of relationship between “eating and being eaten”, but there are many kinds of organisms intertwined with each other and the trophic levels often overlap. In this complex network, energy still starts from the lowest phytoplankton. They turn solar energy into organic matter through photosynthesis and are then eaten by various zooplanktons, such as mesozooplanktons and protozooplanktons. But that's not all. In some systems, there is also a “mixplankton”. They are special, both photosynthesis like plants and prey on other plankton like animals. In this way, they break the traditional layer-by-layer food chain structure, making the flow paths of energy more complex and diverse. Because of the addition of these organisms, the connection between different trophic levels is closer, the energy flows more frequently, and the entire food web becomes more stable and is not easily broken (Figure 2) (Glibert and Mitra, 2022). For example, in Chishui River, research has found that the food species varies greatly between fish, indicating that there are several different energy transmission paths here and the ecological structure is relatively stable (Qin et al., 2021). In the Arctic lagoon, the food that organisms eat will also change according to season and location, which may be organic matter from land, phytoplankton, or plants at the bottom of the water. These changes make the structure of food webs more varied (McMahon et al., 2021). The consumer’s “position” and “eating method” will directly affect the flow direction of these energy and nutrients. 5.3 The concept of keystone and dominant species In an ecosystem, although some species are not large in number, they have a particularly large role. These are called “key species”. They can affect the structure and balance of the entire food web. In marine systems, we can judge which key species are based on their living location, range of activities and other characteristics (Endrédi et al., 2021). In freshwater systems, small animals like Daphnia are typical examples. They eat algae while being eaten by fish, connecting the upper and lower trophic levels. This creates a “nutritional cascade” effect, that is,
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