International Journal of Aquaculture, 2025, Vol.15, No.2, 57-66 http://www.aquapublisher.com/index.php/ija 58 system and how they influence each other. This can help us better understand this ecosystem and provide advice on protecting waters and achieving sustainable development. 2 Classification and Characteristics of Aquatic Ecosystems 2.1 Differences between marine and freshwater ecosystems The water in marine ecosystems contains a lot of salt. They cover most of the surface of the earth, and places like the sea and bay belong to the marine system. There are a lot of species in the ocean, and the food web is also very complex. At the bottom are phytoplankton, which provide energy to large animals such as fish and whales (Stock et al., 2017). The freshwater ecosystem is different. It includes lakes, rivers and wetlands, and there is very little salt in the water. These places are often relatively independent and not connected, so the ecological situation is also very special. Freshwater systems are sensitive to external changes, such as temperature rise, water pollution, etc., which may affect the relationship between animals and plants and the circulation of nutrients (Rakowski and Leibold, 2022). 2.2 Characteristics of lakes, rivers, and wetland ecosystems Lakes, rivers, and wetlands each have distinct ecological characteristics. Lakes are typically closed systems with stratified layers that influence nutrient distribution and biological activity. Rivers are dynamic, flowing systems that connect different habitats and facilitate nutrient and organism transport. Wetlands, with their water-saturated soils, support high biodiversity and act as natural filters for pollutants (Li et al., 2019; Bhele et al., 2022). These ecosystems are crucial for maintaining ecological balance and providing habitat for various species. 2.3 Nutrient cycling in different aquatic systems Nutritional circulation is a natural process that is very important in aquatic systems, which mainly includes the movement and changes of nitrogen and phosphorus. In the marine system, the nutrient circulation is mainly driven by ocean currents and seawater upstream. These water flows bring the nutrients of the deep sea to the sea, which is conducive to the growth of phytoplankton. In freshwater systems, nutrient circulation is not only affected by external land inputs, such as nutrients brought in by river water, but also by internal processes, such as the decomposition and deposition of organic matter. However, if the water is too nutritious, it is prone to eutrophication. This can cause a large reproduction of algae (also known as algae blooms), which in severe cases can destroy the ecological balance in the water (Ersoy et al., 2017; Mukherjee et al., 2023). 3 Major Trophic Levels in Aquatic Ecosystems 3.1 Primary producers – the role and function of phytoplankton Phytoplankton is the basis of aquatic systems. They are the lowest “producers”, relying on photosynthesis to create energy and provide food to other organisms. They can also help absorb carbon dioxide and release oxygen, which is also important for the Earth's carbon cycle. Phytoplankton, such as diatoms and dinoflagellates, are rich in omega-3 fatty acids and are a high-quality food source (Winder et al., 2017; Yan et al., 2023). The nutritional value of phytoplankton is related to their chemical composition, which can change due to excessive water (eutrophication) or increased water temperature. In coastal areas, when the nutrients brought by rivers become more, the phytoplankton will grow more and the quality will improve. This can further support the growth of other animals in the water (Su et al., 2021). 3.2 Secondary consumers – small fish and carnivorous invertebrates In the water, small fish and meat-eating invertebrates are secondary consumers. They mainly eat zooplankton or creatures smaller than themselves. These animals play a “transfer” role, transmitting energy from the plant layer to higher predators. The number of small fish will affect the species and amount of zooplankton. For example, if there are too many fish, it may eat a lot of zooplankton, which in turn affects the number of phytoplankton and eventually changes the state of the entire ecosystem (Thomas et al., 2022). Omnivorous fish like crucian carp will eat large zooplankton and may also inhibit the growth of aquatic plants. In this way, water quality will change and the original nutritional structure in the lake will be disrupted (Razlutskij et al., 2021).
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