IJA_2025v15n2

International Journal of Aquaculture, 2025, Vol.15, No.2, 57-66 http://www.aquapublisher.com/index.php/ija 59 3.3 Tertiary and higher-level consumers - large fish and apex predators In the water, the largest fish and animals like whales are the top predators. They belong to the third trophic grade or higher. These animals will control populations by eating the animals below and keep the food web balanced. Generally, the larger the animal, the higher it stands on the food chain, especially in the ocean (Potapov et al., 2019). These large predators can have a "top-down" impact on the animals in the lower layer, and the stability of the ecosystem is in the left and right. But if an alien species enters, such as fish or other animals that do not belong here, this balance may be broken. This may cause some animal populations to skyrocket or decrease, ultimately destroying the entire food chain (Gallardo et al., 2016). 4 Energy Flow and Material Cycling 4.1 The “10% rule” of energy transfer and its impact In the ecosystem, there is a rule called the “10% rule”. It means: When energy is transferred from one trophic level to the next, only about 10% of the energy can be used, and most of the others are consumed. This process is not very efficient. If the environment changes, such as the temperature rises, the energy transfer efficiency may become worse. Studies have found that in freshwater ecosystems, when the temperature rises by 4 °C, the energy transfer efficiency decreases by 56% (Barneche et al., 2021). This affects the entire food web, especially for animals in the upper layer of the food chain (Barneche et al., 2021; Mooney, 2024). 4.2 Biomass changes between trophic levels 4.2.1 Construction and significance of the biomass pyramid The biomass pyramid is a graph that shows how the number (or weight) of organisms at different trophic levels are distributed. Generally speaking, the higher the trophic level, the less biomass there is. This structure tells us how energy flows through an ecosystem. Because energy is lost a lot every time it increases, there can be too many creatures in the upper layer (Schramski et al., 2015). 4.2.2 Biomass differences between primary producers and consumers Primary producers like phytoplankton generally have more than consumers. This is because they absorb sunlight, make organic matter, and are the energy source of the entire system. However, this relationship is not fixed. Some factors, such as excessive nutrition in water (eutrophication), can change the feeding position of invertebrates, thus breaking this balance (Van et al., 2020). In the ocean, if temperatures rise or water becomes acidic, phytoplankton may become more numerous, but the number of higher-level animals will decrease (Ullah et al., 2018). 4.2.3 Biomass and population dynamics of apex predators Top predators, such as large fish or marine mammals, have their biomass and population changes that are important to the entire ecosystem. These changes are related to energy flow efficiency. Because the energy will be transferred upwards less and less, the top predators can get little energy. In some experiments, different environmental treatments (such as heating or acidification) have different effects on energy flow. At the bottom layer, that is, the phytoplankton layer, the energy changes little. This shows that they are relatively stable to environmental changes. When it reaches the second layer, that is, the zooplankton layer, heating or heating and acidification will increase their energy utilization, perhaps because their metabolism is accelerated. But to the third level, which is the top predator, the impact of different treatments on energy is obvious. For example, under the “OAT processing” conditions, they obtain less energy and are also inefficient. This may be because they do not eat enough food, or they have too little prey, or they may be because they lose too much energy during the transfer process. Together these factors lead to their reduction in numbers and decreased biomass (Figure 1) (Ullah et al., 2018; Mooney, 2024). 4.3 The role of decomposers in aquatic ecosystems In aquatic ecosystems, decomposition is important. They can break down dead plants and animals into simple nutrients. These nutrients are then used by phytoplankton and other plants to create new energy. In this way, energy and matter can continue to circulate and the entire system can remain active. The efficiency of the

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