The coastal zone surrounding the Bohai Bay is one of the important economically developing areas around the Bohai Sea, with high density of populations and rapidly developing agriculture and industry. High-intensity anthropogenic activities brought intense pressures on eco-environments of the Bohai Bay, resulting in heavy eutrophication and frequent red tides. According to the 13th Five-Year Plan, seawater pollution control of the Bohai Bay was an important task of the “Blue Bay Regulations”. It is important to study the progress of eutrophication, the historical changes of phytoplankton assemblages, and their correlations, which would provide scientific supports for the scenarios of terrestrial pollution control to improve seawater quality and reduce red tide risks. Based on the three cruises conducted in the Bohai Bay and previous investigations during 1950s~2010s, this study analyzed the temporal and spatial distributions of nutrients and phytoplankton assemblages and their correlations. The main results in this study are summarized as follows:
(1) Temporal and spatial variations of nutrients
The investigations showed significant temporal and spatial variations of dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP) and dissolved silicate (DSi). The DIN concentration in spring, summer and autumn was 15.32 μmol/L, 7.37 μmol/L, and 10.42 μmol/L, respectively. Spatial distributions of DIN in the three seasons showed similarities with high values ordinarily observed in the western inshore waters, which was largely related to terrestrial inputs. The DIP concentration was 0.16 μmol/L in spring, 0.19 μmol/L in summer, and 0.13 μmol/L in autumn, respectively. DIP exhibited high values in the inshore waters in spring, near Caofeidian in summer and autumn, mainly associated with terrestrial inputs, sediment release, etc. The DSi concentration was 3.06 μmol/L in spring, 3.34 μmol/L in summer, and 4.69 μmol/L in autumn, respectively. The distribution of DSi was largely related to sediment release and circulation with high values in spring observed near the Haihe Estuary and the southern sea areas, those in summer distributed in the north and the mouth of the Bohai Bay, and those in autumn distributed from the west of Caofeidian to the mouth of the Bohai Bay. Based on the cluster analysis of nutrients, the Bohai Bay could be partitioned into western and eastern parts with 15 m depth as the separation. The western sea areas were primarily influenced by terrestrial factors, and showed consistent high values of DIN and DIN/DIP in the three seasons, while the eastern sea areas were mainly affected by marine factors, and showed seasonal high values of DIP, DSi, DSi/DIN and DSi/DIP.
(2) Temporal and spatial variations of phytoplankton assemblages
A total of 109 species were identified, in which 77 species were Bacillariophyta, 29 species were Dinophyta, 2 species were Chrysophyta and 1 species was Chlorophyta. The phytoplankton abundance was 0.74×104 cells/L in spring, 16.31×104 cells/L in summer, and 0.89×104 cells/L in autumn, respectively. Spring and autumn were dominated by diatoms, and dominant species included Pseudo-nitzschia delicatissima, Thalassiosira nordenskioeldii, Paralia sulcata, etc., while in summer, dinoflagellates dominated, and dominant species included Karenia mikimotoi, Ceratium furca, etc. Spatially, phytoplankton abundance showed significant seasonal variations with high values in spring distributed near the northern and Cangzhou coastal waters, those in summer observed from the northwest sea areas to the southern inshore waters, and those in autumn distributed near Tianjin, the southern inshore waters and the northern mouth of the Bohai Bay. Statistical analysis indicated that temperature, salinity, and nutrients were important factors responsible for the seasonal and spatial variations of phytoplankton assemblages. According to the cluster analysis of phytoplankton, the Bohai Bay could be partitioned into western and eastern sea areas with 20 m depth as the separation. The western inshore waters showed higher proportions of dinoflagellates, closely related to relatively high values of DIN and DIN/DIP, while the eastern inshore waters were dominated by diatoms, associated with relatively high values of DIP and DSi.
(3) Correlations between nutrients and phytoplankton assemblages
Through the analysis with historical investigations during 1950s~2010s, the changes of nutrients showed an increase in DIN and decreases in DIP and DSi in average concentrations of the three seasons (May, August and October, the same below); DIN/DIP increased continually, while DSi/DIN declined significantly, leading to the shift from DIN limitation to DIP or DSi limitation. Anthropogenic activities, e.g., domestic wastewater discharge and marine culture, were important factors relating to those changes in nutrients concentrations and ratios. Seasonally, compared to spring and autumn, summer showed a gentle increase in DIN and a sharp decrease in DSi, which was possibly related to the situation that more significant decline of precipitation in summer weakened the transportation of N and Si from land to inshore waters, while the gentle decrease of DIP in summer was possibly associated with more P released from sediment due to low dissolved oxygen conditions.
Phytoplankton abundance and chlorophyll a first decreased and then increased in average values during 1950s~2010s. Dinoflagellates and small size diatoms increased in phytoplankton composition. The increases of DIN and DIN/DIP were important factors associating with the above historical changes of phytoplankton assemblages. Seasonally, phytoplankton abundance in spring first increased and then decreased, while that in summer first decreased and then increased, inflection points of which were both observed around 1990s. More observations were necessary to verify the seasonal variations of historical changes of phytoplankton abundance.
This study elaborated temporal and spatial variations of nutrients and phytoplankton assemblages in the Bohai Bay. The results innovatively showed that the Bohai Bay could be partitioned into western and eastern sea areas with 15~20 m depth as the separation, which were mainly affected by terrestrial and marine factors, respectively. It suggested that the further seawater quality improvement should be conducted focusing on the western Bohai Bay and terrestrial N inputs reducing. Furthermore, this study analyzed the historical changes of nutrients and phytoplankton assemblages based on previous investigations, and found their changes exhibited seasonal variations and different influencing factors, which would provide more understandings to the evolution characteristics of the Bohai Bay under anthropogenic activities and climate change.