Zhibing Jiang

Responses of summer phytoplankton community to drastic environmental changes in the Changjiang (Yangtze River) estuary during the past 50 years

The phytoplankton community in the Changjiang Estuary is subject to intensive physical and chemical stresses because of human- and climate-driven changes. We obtained historical data on summer phytoplankton communities from 1959 to 2009 to explore responses to long-term environmental changes. The nitrogen (N) and phosphorus (P) concentrations and ratios (N/P and N/Si) increased because of persistent riverine loading, but silicon (Si) levels remained constant. Climatic changes and extensive water diversions and withdrawals (sediment retention) resulted in a series of physical alterations, including increased temperature, turbidity reduction, and river plume shrinkage. These changes induced a dramatic increase in microalgal biomass (cell density and chlorophyll a) with a decreasing diatom-dinoflagellate ratio, and exacerbated harmful algal blooms. In the past dominant net-collected species were usually chain-forming diatoms; however, more recently, large dinoflagellates and filamentous cyanobacteria dominate. This was not consistent with information from water samples (co-dominated by small dinoflagellates), because of the loss of solitary species collected using a 76-μm net. The dominant species shifted from temperate-subtropical/eurythermal to subtropical-tropical/eurythermal taxa in the warmer water caused by global warming and hydrographic changes. There was also an increased dominance of euryhaline/high-salinity species due to increase in Kuroshio transport and the northward Taiwan Warm Current and reduction in Changjiang Diluted Water. All these changes in phytoplankton communities appear to be closely related to an increase in anthropogenic activities and climatic changes.

Effects of fish farming on phytoplankton community under the thermal stress caused by a power plant in a eutrophic, semi-enclosed bay: induce toxic dinoflagellate (Prorocentrum minimum) blooms in cold seasons.

Six cruises were conducted in a fish farm adjacent to the Ninghai Power Plant in Xiangshan Bay, East China Sea. Fish farming significantly increased NH4(+), DIP, and TOC concentrations, while it significantly decreased the DO level. These increase/decrease trends were more pronounced in warmer seasons. Although culture practices did not significantly increase phytoplankton density, it drastically enhanced dinoflagellate abundance and domination. Significant differences in species diversity and community composition between the cages and the control area were also observed. Temperature elevation caused by thermal discharge associated with eutrophication resulted in a dominant species shift from diatoms alone to dinoflagellates and diatoms. This is the first report of stress-induced toxic dinoflagellate (Prorocentrum minimum) blooms in winter and the winter-spring transition in this bay. Therefore, the effects of aquaculture activity and power plant construction in such a eutrophic, semi-enclosed bay require further attention.

Seasonal variations of net-phytoplankton community structure in the southern Yellow Sea

Based on the field survey data of four cruises in 2011, all phytoplankton communities in the southern Yellow Sea (SYS) were investigated for the species composition, dominant species, abundance and diversity indices. A total of 379 species belonging to 9 phyla were identified, of which the most abundant group was Bacillariophyta (60.9%), followed by Pyrrophyta (23.7%) and Haptophyta (6.9%). The seasonal distribution of abundance was: summer (4137.1×103 ind m−3) > spring (3940.4×103 ind m−3) > winter (3010.6×103 ind m−3) > autumn (340.8 ×103 ind m−3), while the horizontal distribution showed a decreasing tendency from inshore to offshore regions. The dominant species of phytoplankton varied in different seasons. The dominant species were Thalassiosira pacifica, Skeletoema spp. and Chaetoceros cinctus in spring, Chaetoceros debbilis, Chaetoceros pseudocurvisetus and Chaetoceros curvisetus in summer, Thalassiosira curviseriata, Alexandrium catenella and Ceratium fusus in autumn, Paralia sulcata, Phaeocystis sp. and Bacillaria paradoxa in winter, respectively. In SYS, the group of temperate coastal species was the major ecotype, and the groups of the central SYS species and oceanic species were also important constituents. The average values of Shannon-Weaver diversity index (H’) and Pielou evenness index (J) were 2.37 and 0.65 respectively. The indices H’ and J in the open sea were higher than those in coastal waters. Obvious co-variation tendencies between H’ and J were observed in all but the summer cruise of this survey.

The ecological distributions of N, P utilizing bacteria and heterotrophic bacteria in the moderate hypoxia zone of the Changjiang Estuary

The distributions of N utilizing bacteria (denitrifying bacteria and ammonifying bacteria), P utilizing bacteria (organic phosphobacteria and inorganic phosphobacteria) and heterotrophic bacteria in the Changjiang Estuary, and the roles of main environmental factors in distributing bacteria, are explored with observations from two cruises in June and August 2006. Comparisons between the two important periods of initial hypoxia phase (June) and developed hypoxia phase (August) show differences in both bacterial distributions and the associated main environmental factors. First, the primary group of ammonifying bacteria has larger magnitude with spatial maximum value in the hypoxic stations related to sediment in August. The phosphobacterial abundance and detection rates in August are much lower than those in June, but the denitrifying bacterial abundance becomes greater in August. However, the difference of heterotrophic bacterial abundance between June and August is not obvious. Second, main environmental factors influencing bacteria vary from initial hypoxia phase to developed hypoxia phase. Two parameters (salinity and NO3−) in surface water and five environmental parameters (pH, salinity, PO43−, NO3− and temperature) in bottom water and surface sediment play major roles in the bacterial abundance in June, while different parameter combinations (salinity and PO4−) in surface water and different parameter combinations (DO, DOC, NO3−, PO43− and pH) in bottom water and surface sediment play major roles in August. Moreover, the bottom bacteria distributions in area south of 31°N are related to the position of the Taiwan Warm Current in June. The bacterial abundance and distribution may respond to the environmental change in the hypoxia processes of initial phase and developed phase. During the hypoxia processes, the whole structure of bacterial functional groups probably turns to different states, causing the recycling of nutrient regeneration and aggravating hypoxia regionally.

Seasonal distribution of macrobenthos and its relationship with environmental factors in Yellow Sea and East China Sea

Macrobenthos samples were collected from the Yellow and East China Seas in four seasons during 2011 to 2012. The seasonal distribution of macrobenthos and its relationship with environmental factors were analyzed. A total of 562 macrobenthic species were identified, with polychaetes and mollusks accounting for 67% of the total number of species. A similarity percentage (SIMPER) analysis showed that the dominant species were bivalve mollusks in the Yellow Sea and small-sized polychaetes in the East China Sea. A two-factor analysis of variance showed significant seasonal variations in species number, density and diversity index, and significant regional differences of biomass and density. Two-factor community similarity analysis also showed significant seasonal and regional differences in macrobenthic communities. Canonical correspondence analysis indicated that the main environmental factors affecting the macrobenthic communities were water depth, temperature, dissolved oxygen, and inorganic nitrogen. The results demonstrate significant regional differences and seasonal variations in macrobenthos in the two seas. Sediment properties and water mass characteristics are speculated to be the causes of regional differences.