It was demonstrated in this study that, salinity increase in the recipient freshwater may lead to significant changes in the composition of plankton community, organism density, and biological diversity. In the sample group mixed with high salinity water (24.8‰), an immediate reduction of 96.16% for ≥ 10–50 µm organisms and 100% for ≥ 50 µm organisms were identified on day 1 after water mixing. After 10 days, only when the salinity was 2.1‰, the total organism density decreased by 15.72% for ≥ 10–50 µm organisms and by 72.26% for ≥ 50 µm organisms, while the total organism density decreased by 96.62% for ≥ 10–50 µm organisms and by 100% for ≥ 50 µm organisms in sample group with salinity of 12.2‰, all organisms died in sample group with salinity of 24.8‰. These results aligned with the findings in previous study by Lin et al. (2017) on decline of zooplankton biomass and Lind L. (2018) decline of cladocerans due to salinity increase. Greco et al (2021) and Moffett (2020) also found in their studies that the diversity and richness of planktons decreased with increasing salinity after 10 days. The salinization phenomenon in the recent years has severely affected the planktons. The species composition of the zooplanktons has become much poorer, and the taxonomic diversity decreased 1.4 ~ 1.5 times than before (Afanasyev, 2019). Mo et al. (2021) demonstrated that at low salinities, even small increases in salinity are sufficient to exert selective pressure to reduce the plankton diversity and alter assembly mechanism and network stability of the organism community. In this study, biodiversity in sample groups has surprisingly shown a slight upward development after salinity increase. When the salinity change was less than 6.5‰, after 7 days, the freshwater planktons have demonstrated certain recovery ability, the plankton biomass had positive response to the salinity change after some time. It may be explain as the ecosystem’s adaptability to the ecological disasters with time, and the species diversity, average size and total number of species in the biological community may increase as a result (Pennesi, C. 2017;Nickolai, 2021). It was thus reasonable to assume that, in real ballast water operations, the biological indicators such as species composition and quantity development may tend to be stable shortly after ballast water de-ballasting, given a relatively mild salinity level of the discharged water.
In estuaries and coastal ecosystems which were normally subject to constant changes, salinity was an important factor that may influence plankton community structure (Cruz,2020;Greenwald,1993). Salinity can expose sensitive organisms to osmotic stress and promote the replacement of salinity-sensitive species by salinity-tolerant taxa (Stefanidou, 2018). The responses of phytoplankton and protozoa to chloride increases were group-specific and depend on nutrient levels. The previous survey and identification results showed that Ankistrodesmus falcatus or Scenedesmus in Chlorophyta survived in the high-salinity ballast water, even though they were freshwater species (Wu, 2019). In this study, it was observed that Chlamydomonas sp. in Chlorophyta had a strong tolerance to salinity increase. Song et al. (2020) inferred that changes of water-mass conditions, especially a sharp reduction in salinity to possibly low-brackish conditions (< 10 psu), were the primary causes of concurrent changes in the micro-algal community, reflecting tolerance of low-salinity conditions by green algae. Besides, this study also found that the Navicula sp. in Bacillariophyta and Tribonema sp. in Xanthophyta were very sensitive to salinity, and can not survive beyond 6.5‰. One of the main mechanisms that allow algae to exist in high saline environment was that with the production of exopolysaccharide ( EPS ), a large number of EPS was released into the environment, and cells survive in it (Abdullahi,,2006 Steele, 2014).
Surprisingly, zooplankton was more sensitive to rising salinity than phytoplankton, in terms of species composition or density, which may be attributed to different physiological characteristics of the two organism groups, such as size and volume. Through 10 days of continuous monitoring, it was found that when water salinity was not over 3.6‰, there was no significant difference on ≥ 50 µm organisms quantity and diversity compared with the freshwater control group, and the biota maintained certain recovery ability, while 10–50 um organisms had recovery ability when water salinity was not over 6.5‰.Phytoplanktons responded to salinity change mainly through the indirect top-down effects of zooplankton grazing (Emma, 2020).In the previous studies, large herbivore species ( e.g. Daphnia ) have disappeared, replaced by more salt-tolerant and smaller species ( e.g.rotifers ), and increased salinity may weaken top-down control ( Thompson and Shurin,2012; coldsnow and Relyea,2018 ). Besides, species differences may drive the change of chloride sensitivity. In our experiment, zooplankton groups differed in the relative severity of their chloride response, copepods experienced the greatest declines, followed by cladocerans and then rotifers. In previous studies, cladocerans tended to be the most sensitive then rotifers(Van Meter et al. 2011; Stoler et al. 2017b). Rotifers were generally the most tolerant, only showing declines in one study (Stoler et al. 2017b). While copepods were more variable in their response to chloride, sometimes declining (Petranka and Francis 2013; Lind et al. 2018) or not responding to chloride (Stoler et al. 2017a, Sinclair and Arnott 2018).The driving factors of chloride sensitivity change in the study were not clear. The reason may be include hydrochemical differences and food quantity and quality ( Elphick et al. 2011 ).Variation in responses to salinity by plankton have been linked to predation, disturbance, adaptation, dispersal, and water chemistry (Coldsnow et al. 2017; Hintz and Relyea 2017; Hintz et al. 2019). Therefore, regional context and water chemistry must be considered in the study of salt impact assessment of ballast water discharge.
Changes on the biological composition may result in loss of certain essential functions of the local ecosystem. As the relative abundance shifts from large crustaceans to small rotifers, the total biomass will decrease (Fig. 3 and Fig. 7). Some species of both zooplankton and phytoplankton can be used as indicator organisms on water quality change as they are very sensitive to salinity change. In this study, Platyias sp., Polyarthra sp., Tribonema sp.and Navicula sp. were found to be sensitive to salinity change, and these species were also found to be existing in freshwater ecosystem (like the Yangtze River Basin in China) with large quantities, they are thus considered as suitable indicator organisms for monitoring water quality changes in freshwater ecosystem of inland ports. The inland ports like Yangtze River Basin are important shipping transportation routes with heavy ship traffic, it is very important and essential to monitor its water quality continuously. In the future, scientific management of ballast water discharge in Yangtze River Basin can be designed by measuring biomass of selected indicator organisms or by continuous monitoring salinity change in this water area with salinity meter. These results are important inputs on evaluating effects of ballast water discharge on bio-diversity and ecosystem service losses in this freshwater environment.