| Yesso scallop(Patinopecten yessoensis)is an important cultured shellfish of the North Yellow Sea(NYS)in China.However,with the rapid development of aquaculture,the frequent occurrence of Yesso scallop’s mass mortality has led to huge production losses.As the major food source of Yesso scallop,phytoplankton are closely related to the yield,nutritional ingredients and even toxin contents of Yesso scallop.Previous studies have shown that nutrient limitation in the surface seawater led to a shift in phytoplankton community structure from diatoms to dinoflagellates in the NYS with frequent occurrence of harmful dinoflagellate blooms,which resulted in a subhealthy state of Yesso scallop owing to the degradation of food structure.The "seed bank" of dinoflagellate cysts accumulated in the sediments can provide early warning signals of harmful algal blooms.In the present study,phytoplankton and dinoflagellate cysts were used as research objects.By using 18S rDNA sequencing and microscopic observation,the temporal and spatial variations of environmental factors in the water and sediment of Yesso scallop farming waters on the Zhangzi Island of the NYS were investigated,the dynamics of phytoplankton communities in different water layers and dinoflagellate cysts in the surface sediments were analyzed,and the relationships between the two and environmental factors were explored.The major results were listed as follows:1.Water stratification and mixing alter phytoplankton assemblages in the bottom-seeding farming watersThe variation characteristics of environmental factors and phytoplankton communities in different water layers of the bottom-seeding farming waters were analyzed from March 2021 to January 2022.According to the vertical distribution of temperature,vertical stratification was observed from June to August and disappeared in March,October and the following January with vertical homogeneity.Nutrient limitation,affected by the thermocline,mainly occurred in the stratification period with phosphate and silicate limitation.18S rDNA high-throughput sequencing results showed that Dinophyceae was the most dominant population with high percentages in the stratification(42.91%)and mixing period(59.63%).While Diatomea dynamic was mainly regulated by phosphate,its relative abundance was lower in the phosphate-limited stratification period(4.98%)and higher in the nutrient-supplemented mixing period(8.78%).Moreover,Noctilucales and Dictyochophyceae were dominant in the stratification period,with their average abundances in the water column of 7.32%and 21.19%,respectively.The dominant genera of diatoms in the stratification period were Thalassiosira,Pseudo-nitzschia and Guinardia,while those in the mixing period were Thalassiosira,uncultured and Porosira.It is worth noting that Protoperidinium was dominant in the mixing period.The network analysis results showed that the numbers of nodes and edges,the values of avgK and avgCC,and the percentage of positive correlation were higher while the average path distance was shorter in the stratification period than those in the mixing period.Further,the niche breadth of phytoplankton communities was significantly higher in the stratification period than that in the mixing period(p<0.001).2.Differences in dinoflagellate cysts in the surface sediments of different culture modesThe variation characteristics of nutrient concentrations and dinoflagellate cysts in surface sediments of the suspended(site F)and bottom-seeding(site D)farming waters were analyzed from June to July 2022.A total of six categories 53 species and one undetermined species of cysts were identified from the surface sediments.The cyst density at site F increased with the increasing TN and TP concentrations(p<0.05),while the TN and TP concentrations and cyst density at site D were higher and less changed over time.At site D,the cyst community was mainly consisted of Biecheleria brevisulcata,with an average density of 108.34 cysts/g DW during the sampling period.It is worth noting that a higher density of paralytic shellfish poisoning-producing cysts,such as Alexandrium catenella,A.minutum and Gymnodinium catenatum,was detected at site F in July compared with site D.In addition,Scrippsiella trochoidea and Protoperidinium spp.cysts were dominant at sites F and D in July.In summary,the phytoplankton community in the bottom-seeding farming waters showed the features of "excessive dinoflagellates and insufficient diatoms",which might be detrimental to the scallop growth.Especially in the nutrient-limited stratification period,low abundance of diatoms and high abundance of Noctilucales might be one of the reasons for the mass summer mortality of Yesso scallop.In addition,the density of harmful cysts in the suspended farming waters increased with the increasing nutrient concentrations,which has the risk of lagging scallop growth and increased shellfish toxicity.These results have guiding significance for the construction of Green Culture Technology System for shellfish in the NYS. |
