Temperature and nutrients drive distinct successions between diatoms and dinoflagellates over the past 40 years: Implications for climate warming and eutrophication.

Diatoms and dinoflagellates are two typical functional groups of phytoplankton, playing important roles in ecosystem processes and biogeochemical cycles. Changes in diatoms and dinoflagellates are thought to be one of the possible mechanisms for the increase in harmful algal blooms (HABs), due to changing hydrological conditions associated with climate change and human activities. However, little is known about their ability to adapt to changing ocean environments, thus making it difficult to know whether and how they are adapting. By analyzing a 44-year monitoring dataset in the central Bohai Sea during 1978-2021, we found that the abundance ratio of diatoms to dinoflagellates showed a decreasing trend seasonally and ecologically, indicating that the phytoplankton community underwent distinct successional processes from diatom dominance to diatom-dinoflagellate co-dominance. These processes exhibited varying responses to temperature, nutrient concentrations and ratios, and their interactions, of which temperature primarily drove the seasonal succession whereas nutrients were responsible for the ecological succession. Specifically, diatoms showed a preference for lower temperatures and higher DIP concentrations, and were able to tolerate lower DIN at lower temperatures. In contrast, dinoflagellates tended to prevail at conditions of warming and high N/P ratios. These different traits of diatoms and dinoflagellates reflected the fact that warming as a result of rising temperature and eutrophication as a consequence of nutrient input would favor dinoflagellates over diatoms. Moreover, the increasing dominance of dinoflagellates indicated that dinoflagellate blooms were likely to become more frequent and intense in the central Bohai Sea.

Phycotoxins in scallops (Patinopecten yessoensis) in relation to source, composition and temporal variation of phytoplankton and cysts in North Yellow Sea, China.

The North Yellow Sea is a major aquaculture production area for the scallop Patinopecten yessoensis. In this study, the temporal and spatial variation of phycotoxins in scallops, phytoplankton, and their cysts were analyzed during a survey conducted from June 2011 to April 2012 around Zhangzi Island. The study area is a semi-enclosed epicontinental sea surrounded by the Shandong Peninsula, the Liaodong Peninsula and the Korean Peninsula. The three main results of the study were as follows: (1) The saxitoxin-group toxins, okadaic acid and analogues, and pectenotoxins were the major phycotoxin residues found in scallops; (2) Six kinds of toxic microalgae were identified, Protoperidinium spp., Gonyaulax spp., and Alexandrium spp. were the dominant taxa; Seven types of potential marine toxin-producing dinoflagellates, A. tamarense, A. catenella, Dinophysis fortii, G. catenatum, Gambierdiscus toxicus, Azadinium poporum, and Pseudo-nitzschia pungen were identified as the primary source of phycotoxins and were present at relatively high density from June to October; and (3) azaspiracids and domoic acid might be new potential sources of toxin pollution. This study represents the first assessment to phycotoxins around Zhangzi Island in the North Yellow Sea.

[Shrimp community structure and its relationships with environmental factors in the Yellow Sea in summer].

Based on the data collected from the bottom trawl survey in Yellow Sea, August, 2014, the community structure of shrimp assemblage and its relationships with environmental factors in summer was examined by using index of relative importance, ecological diversity indices and multivariate statistical analysis. A total of 20 shrimp species were captured, belonging to 3 orders, 10 families, 16 genera. The relative abundance of shrimp of all stations ranged from 13 to 45047 g · h(-1) and its mean value was 6838 g · h(-1). The dominant species was Crangon affinis, and the common species was Eualus sinensis. The rare species were Metapenaeopsis dalei, Palaemon gravieri and Oratosquilla oratoria. The ranges of Shannon diversity index (H) , Pielou' s evenness index (J) and the Margalef' s species richness index (D) of all stations of the shrimp community structure were 0.007-1.538, 0.101-1.138 and 0.006-0.947, respectively, and the mean values of H, Jand D were 0.391, 0.374 and 0.298, respectively. MDS and Cluster analyses revealed that two clusters of Group I named cold water mass group and Group II named coastal group, which were bounded by the 45 m isobaths, were identified for all the sampling stations. Significant difference was detected by ANOSIM analysis between Group I and Group II. BIOENV analysis indicated that bottom temperature and bottom salinity were the most important environmental factors for structuring the spatial distribution of the shrimp assemblage. Cold water mass group accounted for absolute advantage and Yellow Sea cold mass had a decisive influence on the distribution pattern of the shrimp community in the Yellow Sea in summer.

[Size-fractionated biomass of phytoplankton in the Yangtze River estuary and its adjacent waters in winter and spring].

Based on the multi-discipline investigation in 28th February-10th March and 30th May-4th June, 2005, this paper studied the distribution of size-fractionated phytoplankton in the Yangtze River estuary and its adjacent waters, and the environmental factors that affected this distribution. The results showed that in winter, the average chlorophyll a concentration was 1.28 mg X m(-3), and the region with high concentration chlorophyll a was located near the river mouth. Small-sized phytoplankton (SSP, <20 microm) contributed 66.7% to the total phytoplankton biomass, but big-sized phytoplankton (BSP, >20 microm) was dominant in the plume system. In spring, the average chlorophyll a concentration in inner river and outer estuary was 0.67 and 6.03 mg x m(-3), respectively, and the maximum of chlorophyll a concentration appeared in the waters between 122.5 degrees E and 123.0 degrees E. The SSP dominated in the plume system, especially in the algal bloom frequently occurring area which located at south of survey area, contributed 83.5% to the total phytoplankton biomass. The vertical profile of size-fractionated chlorophyll a showed that with the increase of water depth, the SSP and BSP became equal in the offshores. Among the environmental factors in the Yangtze River estuary and its adjacent waters, salinity and nutrients were the main factors affecting the distribution and composition of size-fractioned phytoplankton biomass in winter and spring.

[Phytoplankton assemblage in Yangtze River estuary and its adjacent waters in winter time].

Water samples were collected from Yangtze River Estuary and its adjacent waters in 28th February-10th March, 2005, and the species composition of phytoplankton was analyzed by Utermöhl method. A total of 130 taxa (including 25 uncertain species) which belong to 67 genera of 5 phyla were identified. The phytoplankton community was mainly composed of Bacillariophyta, followed by Dinophyta. There were also a few species belonging to Chrysophyceae, Cyanophyceae and Chlorophyceae. The dominant species were Paralia sulcata, Skeletonema costatum, Thalassiosira rotula, Bleakeleya notata, Coscinodiscus radiatus and Thalassiosira excentrica. The cell abundance of phytoplankton ranged from 0.1 to 90.0 cells x ml(-1), with an average of 10.1 cells x ml(-1). Horizontally, the cell abundance was relatively high in inshore and low in offshore; while vertically, it was high in surface water and decreased slightly with increasing water depth. The cell abundance and chl a concentration of phytoplankton positively correlated with the concentrations of nitrate, nitrite, ammonium, phosphate and silicate in water, but negatively correlated with water salinity. The Shannon-Wiener diversity index and Pielou evenness index were higher in the center of survey area but lower in northeast part and inshore area.