Fangjin Cheng’s research while affiliated with China Institute of Geological Environmental Monitoring and other places

In order to depict the distribution of diatom fossils in surface sediments and to establish a reliable reference data for further paleoenvironmental study in the Changjiang (Yangtze) River estuary and its adjacent waters, the diatom fossils from 34 surface sediment samples and their relationship with environmental variables were analyzed by principal component analysis and redundancy correspondence analysis. The diversity and abundance of diatom fossils were analyzed. Some annual average parameters of the overlying water (salinity, temperature, turbidity, dissolved oxygen, depth, dissolved inorganic nitrogen, dissolved inorganic phosphate and dissolved inorganic silicate) were measured at each sampling site. A total of 113 diatom taxa and one silicoflagellate species were identified in the investigation area. Diatom fossils were better preserved in fine sediments. The absolute abundance of diatom fossils did not significantly differ between inshore and offshore areas, the species diversity decreased from inshore to offshore. This may be because high nutrients and low salinity promoted the growth of more brackish species in coastal waters. The diatom taxa were divided into three groups, on the basis of their response and indication to environmental changes. For example, Actinocyclus ehrenbergii and Cyclotella stylorum were dominant in coastal waters (Group 1 and Group 3) with high nutrients and low salinity; the relative abundances of Paralia sulcata and Podosira stelliger were significantly higher in offshore sites (Group 2, average 39.5%), which were characterized by high salinity and deep water. Four environmental variables (salinity, dissolved inorganic nitrogen, temperature and water depth) explained the composition and distribution of diatom taxa independently ( P< 0.05), this finding can be applied in further paleoenvironmental reconstruction research in this area.

Since 2009, the newly recorded species Aureococcus anophagefferens has successively induced brown tides in the Qinhuangdao coastal waters of the Bohai Sea. Here, we focus on the phytoplankton community structure in this region and try to point out the biological background characteristics of the outbreaks of successive brown tides. We analyzed species composition, cell abundance, diversity indices and size-fractionated chlorophyll a (Chl-a) concentration monthly from March 2013 to January 2014. The phytoplankton community, as observed by microscopy, underwent succession from nano-celled chained diatoms to dinoflagellates in the spring and summer, and then to micro-celled diatoms and nano-celled chained diatoms as the most prominent groups in the subsequent autumn and winter. Canonical correspondence analysis revealed that changes in the abundance of diatoms were related to nutrient availability, especially of nitrogen, while water temperature was the crucial factor influencing the abundance of dinoflagellates and A. anophagefferens. Considering the contribution of nano-celled chained species to the micro-sized Chl-a, small phytoplankton (<20 μm) composed the majority of total Chl-a and were generally abundant during our study. Abundant and diverse small phytoplankton seemed to serve as a background or seedbank for the formation of blooms of A. anophagefferens and other small-celled algae.

Blooms of some pico/nanophytoplankton have occurred frequently along the Qinhuangdao coast since 2009, and it is necessary to identify the critical environmental factors inducing them. In this study, variations in the physical and nutrient characteristics of the seawater were analyzed following the development of local blooms in 2013. The local environmental characteristics were also compared with those of the Changjiang River estuary, China, and the Long Island estuaries in the USA, which are also prone to blooms of special algal species. In Qinhuangdao the local water temperature varied seasonally and rose above 15°C in 2013 early summer, coincident with the water discoloration. The salinity was more than 28 with a variation range of <3 throughout the year. Our results suggest that the physical conditions of the Qinhuangdao coastal area were suitable for the explosive proliferation of certain pico/nanophytoplankton, e.g. Aureococcus anophagefferens. The water supporting the bloom was not in a condition of serious eutrophication, but there were relatively high concentrations of reduced nitrogen (especially ammonium), which acted as an important nitrogen source for the pico/nanophytoplankton bloom. There was also a large gap between total nitrogen (TN) and dissolved inorganic nitrogen (DIN). Although the phosphate concentration was relatively low, there was no evidence of phosphorus limitation to the growth of pico/nanophytoplankton during bloom events.

Brown tides caused by the picophytoplankton Aureococcus anophagefferens have been observed in the Bohai Sea, China on a regular basis, which should be relative with the change in phytoplankton community structure. To better understand this change in the coastal waters of the Bohai Sea, we sequenced 18S rDNA genes extracted from field samples collected from April through October 2013 and evaluated the composition and relative abundance of the eukaryotic phytoplankton community. Most of the operational taxonomy units (OTUs) belonged to Dinophyceae and their sequences were relatively abundant. In late spring and early summer, Mamiellophyceae (chlorophytes), Cryptophyceae (cryptophytes) and Mediophyceae (diatoms) comprised a high proportion of total algal abundance. Small phytoplankton (pico- or nano-sized) were highly abundant in our work, which might dominate the phytoplankton community and provide a ‘background’ for the formation of A. anophagefferens brown tides and other small-celled algal blooms. A total of 71 algal species were identified, among which 34 were new records. Further analysis revealed that half of the identified species were known bloom-forming or toxic algae, including 11 harmful algal species that were previously undetected or unreported in the Bohai Sea. Most of these harmful species are mixotrophs, which provides a competitive advantage during the formation of harmful algal blooms (HABs). In conclusion, our work provides a detailed evaluation of eukaryotic phytoplankton diversity and dynamics in the Bohai Sea, suggesting the need for long-term monitoring to further study and guard against the potential risk of HABs.

Long-term data on diatom assemblages in a sediment core (60 cm) obtained from the Changjiang (Yangtze) River estuary were analyzed in order to assess the environmental changes that took place in the approximately 38 years (as determined by 210Pb measurements), i.e., between 1974 and 2012, of sediment accumulation. From the sediment core, 62 diatom taxa and genera were identified. The diatom biomass in the core generally increased beginning in the mid-1990s (core depth: 35 cm), accompanied by a shift in the dominant species from Podosira stelliger and two species of Cyclotella ( C. stylorum and C. striata) to Paralia sulcata, three species of Thalassiosira ( T. eccentria, T. oestrupii, and T. excentrica), Actinoptychus undulates, and Thalassionema nitzschioides. The changes in both species diversity and abundance suggested that since the 1980s the estuary has undergone extensive eutrophication. This conclusion was supported by the increased proportion of planktonic species, another indicator of high nutrients inputs, in the Changjiang River estuary.