Study On Phytoplankton Community Succession And Its Response To Global Changes In Marginal Seas Of The Western Pacific Ocean

With the aggravation of environmental changes caused by global warming and other human activities,the study of variations of phytoplankton communities in marginal seas and their driving factors,as well as their response to future environmental changes,has become a hot topic.The East China Sea(ECS)and the South China Sea(SCS)are both important marginal seas in the Western Pacific.In terms of the relationship between phytoplankton community succession and global change,both the ECS and the SCS are in their infancy.In this study,the data of phytoplankton photosynthetic pigments and corresponding environmental parameters in the ECS and SCS for more than ten years were collected and collated.The succession patterns of phytoplankton communities in these two marginal seas and their responses to global changes were analyzed from the realized niche perspective.The main results were as follows:(1)Based on 11 cruises in the ECS from 2006 to 2012,seasonal and spatial patterns of phytoplankton in the ECS were investigated to understand the response of phytoplankton biomass and community composition to environmental factors.The results of CHEMTAX analyses on group-specific phytoplankton composition were consistent with those of microscopy and flow cytometry observations,revealing three patterns of seasonal variability.Canonical correspondence analysis(CCA)and generalized additive models(GAMs)were used to resolve the spatiotemporal variations of major phytoplankton groups and their relationships to month,temperature,salinity,nutrients,mixed-layer depth and bottom depth.Monsoon forcing drove the distributional patterns of environmental factors and was critical to explaining phytoplankton dynamics at the seasonal scale.Compared to autumn and winter,significantly higher chlorophyll a concentrations were observed during spring and summer,associated with the spring bloom and the Changjiang(Yangtze)River plume,respectively.Diatoms dominated biomass over the ECS,especially during the summer months influenced by the Changjiang(Yangtze)River plume,whereas dinoflagellates were especially important during spring blooms.GAMs analysis showed the differences in their responses to environmental variability,with a clear mid-range salinity optimum 31)and a more pronounced temperature effect for dinoflagellates.The photosynthetic bacteria,Prochlorococccus and Synechococcus,both increased strongly with warming,but Prochlorococcus showed stronger sensitivity to variations in physical environmental parameters,while Synechococcus was more responsive to chemical(nutrient)variability,with broader tolerance of low salinity conditions(2)A series of niche models were established to study the response of diatom and dinoflagellate biomass in the ECS to various environmental changes based on 2815 samples randomly collected from 23 cruises spanning 14 years(2002-2015).The results show that the interaction of temperature change and eutrophication is key in driving diatom-dinoflagellate dynamics in the ECS.Diatoms and dinoflagellates responded differently to temperature,nutrient concentrations and ratios,and their interactions.Diatoms preferred lower temperature and higher nutrient concentrations,while dinoflagellates were less sensitive to temperature and nutrient concentrations,but tended to prevail at low phosphorus and high N:P ratio conditions.These different traits of diatoms and dinoflagellates resulted in the fact that both the effect of warming resulting in nutrients decline as a consequence of increasing stratification and the effect of increasing terrestrial nutrient input as a result of eutrophication might promote dinoflagellates over diatoms.We predict that conservative forecasts of environmental change by the year 2100 are likely to result in the decrease of diatoms in 60%and the increase of dinoflagellates in 70%of the surface water of the ECS,and project that mean diatoms should decrease by 19%while mean dinoflagellates should increase by 60%in the surface water of the coastal ECS.Our findings reveal that dinoflagellate blooms will be more frequent and intense,which will affect coastal ecosystem functioning.(3)We used decadal-scale studies of pigment-based phytoplankton groups and environmental conditions in the SCS to test whether realized traits could explain the biogeographic patterns of phytoplankton variability.We estimated the mean and breadth of the phytoplankton realized niches based on responses of the group-specific phytoplankton composition to key environmental factors,and we showed that variations of major phytoplankton groups in this system can be explained by different adaptive trade-offs to constraints imposed by temperature,irradiance,and nutrient concentrations.Differences in the patterns of trade-offs clearly separated the dominant groups from one another and generated four sets of realized traits that mirrored the observed biogeographic distribution patterns.The phytoplankton realized niches and their associated traits that we characterized in the present study could help to predict responses of phytoplankton to changes in environmental conditions in the SCS and could be incorporated into global biogeochemical models to anticipate shifts in community structure under future climate scenarios.(4)Based on a decadal-scale database,a novel approach for selecting optimum niche numbers and niche boundaries,and use of multivariate methods of data reduction,ordination,and classification,we created a niche habitat classification scheme relating phytoplankton community composition to fifteen niches comprised of three niche dimensions.We show that although practical considerations limit the number of niches,a synecological analysis of the data produced a niche scheme useful for predicting the composition of phytoplankton communities at the class level.The niche scheme supported general ecological theories,such as physical and metabolic pathways of warming effects on plankton and theoretical resource competition models for phytoplankton communities.The niche scheme also challenged some concepts based on autecological studies on two phytoplankton functional groups,diatoms and Prochlorococcus.Our findings suggest that the most informative approach to predicting the consequences of climate change on phytoplankton communities is to use synecology and Occam's Razor,i.e.,keep the number of functional groups and niches small.Our approach can be easily extended to other groups of organisms in specific regions and represents an important improvement of using the concept of niche classification scheme for the purpose of ecological forecasting.