Abstract:Estuarine ecosystems play an important role in biodiversity due to the close interrelationship between riverine and marine environments. Phytoplankton and zooplankton serve as ecological indicators of water quality in estuarine ecosystems. Therefore, an integrated evaluation of the effects of multiple environmental factors on the phytoplankton and zooplankton communities in estuarine ecosystems is essential. Previous studies have often been limited to the interaction between a single phytoplankton or zooplankton species and environmental factors, whereas studies on the mechanism of the overall planktonic response to environmental factors are lacking. Therefore, we collected water samples from seven stations in the waters of the Liuqinghe Bay in March (spring), August (summer), and October (autumn) 2021. Redundancy analysis and Pearson’s correlation analysis were used to explore the effects of environmental factors on dominant phytoplankton and zooplankton species and communities. The results showed that there were large seasonal differences in water temperature in the study area, with the highest (23.70±0.17 °C) in summer and the lowest (5.91±0.03 °C) in spring. Changes in the mean salinity of offshore estuarine waters—with variations ranging from 30.29 to 31.70—were usually caused by inputs from estuarine runoff, showing obvious seasonal characteristics, and salinity among different stations did not show significant differences. The pH decreased with increasing water temperature. Compared with the first three hydrological parameters, chemical oxygen demand, Chl a, and nutrient salts did not show obvious seasonal patterns. In 2021, 94 phytoplankton species from three phyla were identified during the three cruises, with 80 Bacillariophyta spp. being the most abundant, followed by 13 Pyrrophyta spp. and 1 Chrysophyta sp.—it is worth mentioning that the same Chrysophyta sp. was identified in all the 3 cruises. The spring cruise in 2021 identified 44 species from three phyla, the summer cruise had the richest community composition, with 58 species from three phyla, and the autumn cruise identified 55 species from three phyla. In addition, the mean abundance of phytoplankton in Liuqinghe Bay in 2021 reached its maximum in summer (242.50×103±136.40×103 cells/m3), with the mean abundance in spring (19.38±12.23 cells/m3) at the lowest level. However, the seasonal variation of mean phytoplankton biomass in 2021 in Liuqinghe Bay showed the same abundance trend, with a maximum carbon biomass (946.89±810.66 µg C/m3) in summer and the lowest carbon biomass (31.15±20.96 µg C/m3) in spring. A total of 48 zooplankton species were identified in 10 groups, with copepods being the most numerous (15 species), followed by pelagic larvae and hydroidomedusa (11 species each), tunicates (3 species), chaetognaths and cladocera (2 species each), and one species each of jellyfish, amphipoda, mysidacea, and euphausia. A total of 14 zooplankton species from five taxa were identified during the spring cruise, 28 species from seven taxa during the summer cruise, and 25 species from eight taxa during the autumn cruise. Copepods and pelagic larvae had the highest occurrence frequency in the samples from each seasonal cruise; however, the frequency of Hydroidomedusae gradually increased over time. Moreover, the mean zooplankton abundance in the study area showed a clear seasonal pattern ranging from (55.00±12.52) – (5 665.71±4 576.32) ind./m3, with the maximum and minimum mean abundances in spring and autumn, respectively. Biomass exhibited a seasonal pattern like that of abundance, showing an overall decreasing trend. In 2021, the phytoplankton Shannon–Wiener diversity index (H′) and Pielou’s evenness index (J) in Liuqinghe Bay varied greatly among the three seasons, and there were significant differences among different stations. The average J of phytoplankton did not differ significantly among seasons, especially in spring and summer, but it fluctuated greatly among different stations. In general, phytoplankton biodiversity was the highest in summer and lowest in spring, whereas the evenness index was relatively evenly distributed in spring and summer and more scattered in autumn. The zooplankton H′ and J showed very similar trends, being generally higher in summer and autumn than in spring, and the H′ fluctuation in summer and autumn was also stronger than that in spring. The overall trend was higher in the summer than in the autumn. Overall, there were significant spatial and temporal variations in plankton in Liuqinghe Bay, in which the main influences on plankton-dominant species in spring and summer were temperature and nutrient salt concentration, while the main drivers affecting plankton-dominant species in autumn were temperature, salinity, and nutrient salts, and there was a potential for red tides to occur in spring and autumn. In addition, ocean currents, land runoff, and meteorological hazards are important factors that influence the community composition of dominant zooplankton species. The results of this study will help improve the understanding of plankton communities in estuarine ecosystems. It will also provide a theoretical basis for the scientific management of the ecological environment of Liuqinghe Bay and an in-depth understanding of the mechanisms of plankton community changes in the Bay.
