International Journal of Aquaculture, 2025, Vol.15, No.2, 57-66 http://www.aquapublisher.com/index.php/ija 57 Research Perspective Open Access From Phytoplankton to Fish: Exploring the Trophic Levels of Aquatic Ecosystems Liting Wang Hainan Institute of Biotechnology, Haikou, 570206, Hainan, China Corresponding email: liting.wang@hitar.org International Journal of Aquaculture, 2025, Vol.15, No.2 doi: 10.5376/ija.2025.15.0007 Received: 15 Feb., 2025 Accepted: 18 Mar., 2025 Published: 23 Mar., 2025 Copyright © 2025 Wang, This is an open access article published under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Preferred citation for this article: Wang L.T., 2025, From phytoplankton to fish: exploring the trophic levels of aquatic ecosystems, International Journal of Aquaculture, 15(2): 57-66 (doi: 10.5376/ija.2025.15.0007) Abstract This study reviews the interactions between different trophic levels in aquatic ecosystems and their impact on ecosystem functioning. Aquatic ecosystems are complex networks of energy and nutrient flows, where interactions between various trophic levels—from primary producers like phytoplankton to apex predators like fish-determine ecosystem stability and productivity. The paper details the critical role of phytoplankton as primary producers in energy transfer, emphasizing the importance of predator-driven trophic cascades in shaping community structures in both freshwater and marine environments. It also explores the influence of factors such as eutrophication, climate change, and predator abundance on these cascades. Case studies further illustrate the trophic relationships between krill and whales in marine ecosystems, the impact of carp populations on zooplankton in freshwater lakes, and the multi-trophic interactions in coral reef ecosystems. This review provides essential theoretical support for the management and conservation of aquatic ecosystems, contributing to a deeper understanding of ecosystem dynamics under global change. Keywords Trophic levels; Aquatic ecosystems; Energy transfer; Predator-prey interaction; Ecosystem stability 1 Introduction Aquatic ecosystems are like a large net, where energy and nutrients are transmitted from lower levels to higher levels of organisms. For example, phytoplankton is the bottom producer, and fish are the top predators. The relationship between them is very complex. To better protect these waters, we must understand how they affect each other, especially when the environment is changing so quickly. In this system, predators can influence the creatures below, and this "top to bottom" effect is called the Predator Cascade. This has a great influence on the species and quantity of flora and fauna in freshwater and oceans (Su et al., 2021; Rakowski and Leibold, 2022). However, sometimes things like water body fat (eutrophication), climate warming, and changing predators’ numbers can also change the strength and direction of this impact (Li et al., 2019). If the upper and lower levels of the food chain are out of balance, the entire ecosystem may become unstable and even affect its output capacity (Bhele et al., 2022). There are about three main types of organisms in the aquatic system: the first layer is phytoplankton, which relies on photosynthesis to produce energy (Stock et al., 2017); the second layer is zooplankton, which eats phytoplankton, and the third layer is fish, which eats zooplankton. A food web is formed between these three layers, which is not fixed, but is constantly changing. Many factors will affect them, such as whether there is enough nutrients in the water, whether there are natural enemies, and the temperature and light of the water, etc. So we need some models to help us see the changes in these relationships and facilitate the formulation of management methods (Mukherjee et al., 2023). Nutritional grade refers to the "position" of organisms in this food web, that is, who eats whom. Each level plays an important role in the transmission of energy and nutrients. This transmission is sometimes fast and sometimes slow, which is called "nutrition transmission efficiency". This efficiency is related to the size of the organism, predation status, environmental changes, etc. This study is to take a closer look at the role of each layer in the
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