The bay is one of the most important areas that hosts natural changes and human activities. Its environmental deterioration causes ecosystem degradation and resource decline, creating negative feedback to the social economy. As a typical subtropical bay ecosystem in China, Xiangshan Port in Zhejiang Province has unique topography, hydrodynamics and biological communities. Its ecosystem's response to human activities and natural changes is an important feedback to the bay spatial unit development pattern.
Recently, the offshore ecological research team of the Key Laboratory of Marine Ecosystem Dynamics of the Ministry of Natural Resources’ Second Institute of Oceanography used the Xiangshan Port, a narrow semi-enclosed bay in the East China Sea, as the research area to systematically reveal the effects of eutrophication and seawater warming under the combined effects of Evolving patterns of plankton in the bay. Relevant results have been published in mainstream international oceanography and planktonic ecology journals such as Marine Pollution Bulletin, Estuaries and Coasts, Estuaries, Coasts and Continental Shelves, and Plankton Research.
Schematic diagram of the historical evolution of the floating ecosystem in Xiangshan Port (drawn by Dong Han)
The study found that large amounts of phytoplankton biomass in eutrophic bays cannot be effectively transferred to higher trophic level organisms. Biomass refers to the total weight of a certain type of organisms per unit area (volume). Phytoplankton are primary producers that perform photosynthesis, and higher trophic level organisms are zooplankton and fish that prey on phytoplankton in the food chain.
According to researcher Jiang Zhibing, one of the main authors of the study, Xiangshan Port is a subtropical sea area with high productivity (that is, high phytoplankton biomass), and the proportion of larger individuals (micro, 2-20 microns; small, >20 microns) phytoplankton accounts for more than 80%, but the biomass and abundance of smaller individuals (picota, 505 microns) showed a downward trend, and fishery resources also declined significantly.
Through the simultaneous multi-parameter high-density sampling method and the random forest model, the research team found that the zooplankton biomass in Xiangshan Port will not increase as the phytoplankton biomass increases, and the ratio between the two decreases as the phytoplankton biomass increases, indicating that A large amount of phytoplankton energy cannot be efficiently transferred to higher trophic level organisms through the food chain. The study also found that zooplankton are most sensitive to warming. The increase in temperature promotes the increase of small individuals (small and medium-sized, 200~505 microns), while the number of larger individuals (>505 microns) decreases, that is, the overall trend of zooplankton is Miniaturization results in the inability of larger individual phytoplankton biomass to be transferred to higher trophic level organisms through zooplankton feeding.
At the same time, the study found that the long-term evolution of plankton in subtropical narrow bays shows spatiotemporal heterogeneity. Spatiotemporal heterogeneity refers to the changes in biotic and abiotic environments over time and the uneven spatial distribution. Xiangshan Port is a subtropical narrow and semi-enclosed bay, with significant seasonal changes and spatial heterogeneity in the physical and chemical environment and biological communities. Based on this, the research team speculated that long-term changes in nutrients, water temperature, and plankton may also have spatial and temporal heterogeneity, and verified this hypothesis using historical data from 1980 to 2019 for nearly 40 years.
According to Associate Researcher Du Ping, one of the main authors of the study, the biomass of large and medium-sized zooplankton dropped rapidly after the operation of the power plant in 2005, mainly at the bottom and middle of the harbor in summer and autumn. This is the synchronization of inorganic nitrogen and water temperature. Significant increases in seasons and regions indicate that the combined effect of eutrophication and high temperature has significantly suppressed the biomass of large and medium-sized zooplankton. One of the important reasons is that the temperature at the outlet of the power plant in summer exceeds the lethal temperature of the dominant species (medium-sized copepods). , while the proportion of smaller brachyuran larvae and sword water fleas increased. The research team also found that in the port area, which is tens of kilometers away from the warm water drainage at the top and middle of the port, the changing trend of zooplankton is significantly different from that at the bottom and middle of the port, and there is no decline in the number; the port area is mainly affected by the seawater outside the port, and is affected by During the period when the Yangtze River diluted water was affected, the number of zooplankton even showed a significant upward trend, which was highly consistent with the change trend of zooplankton in the Yangtze River Estuary.
The research team combined physical and chemical factors and long-term changes and heterogeneous characteristics of organisms at each trophic level to deduce the evolution process of the bottom plankton ecosystem in Xiangshan Port: eutrophication and warming increased phytoplankton biomass, especially promoting the growth of larger individuals. Diatoms and dinoflagellates grow, but seawater warming is not conducive to the reproduction of larger zooplankton, resulting in a lack of effective feeding control of large-sized phytoplankton. At the same time, seawater warming in winter accelerates the growth of phytoplankton, and the feeding effect of zooplankton is weak during the same period. , causing algal blooms to intensify and advance the bloom time from spring to winter. The primary productivity generated by the flourishing of phytoplankton cannot be effectively transferred to zooplankton and fish at higher trophic levels, resulting in a reduction in the efficiency of the grazing food chain. Therefore, in addition to overfishing, changes in plankton particle size structure and phenological processes caused by eutrophication and intensified warming are also important reasons for the significant decline in fishery resources in Xiangshan Port compared with the 1980s. In addition, the relative importance of the microfood loop as a supplementary pathway for primary productivity transmission may be enhanced, and the impact of its changes on energy transfer efficiency remains to be studied.
The results of this study enrich the scientific understanding of bay ecology and suggest that bays with poor water exchange capabilities in inner bays need to strictly control the input of land-based nutrients and reduce cage culture and shrimp pond culture; at the same time, they must strictly control the temperature rise range of warm water discharged from power plants. , especially in the high temperature season. The concept of ecological civilization requires ecological restoration work to aim at improving the stability of the ecosystem and improving blue carbon sink efficiency and other ecological service functions. This study also provides scientific support for the ecological restoration of the bay and supports the cultivation of large economic algae (kelp, etc.) in the inner bay. and filter-feeding shellfish (oysters, etc.), bottom-sown clams, Philippine clams and other indigenous shellfish to reduce the nutrient concentration in the water body and reduce the occurrence of phytoplankton algae blooms.
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