Phytoplankton Size Structure And Grazing In Coastal Waters And Biological Regulation Of PH In Estuaries

The relationships between phytoplankton,microzooplankton and bacteria are regulated by the environmental factors,such as sea water temperature,salinity,nutrient level and light intensity in the ocean.The contrasting environmental factors in Yellow Sea(YS)and South China Sea(SCS),sea waters and estuarine waters,contribute to the local characters of ecosystems.These characters directly determine the material cycle and energy flow in the ocean.The relationships between these characters are not fully understood and there are few comparisons of them in the large ecosystems.In order to compare the different effects of environmental factors on plankton community,track the coupling relationships between phytoplankton growth and microzooplankton grazing during phytoplankton blooms,and explore how can the phytoplankton maintain growth in the carbon limitation condition during the phytoplankton blooms,we collected historical data in the YS and SCS,conducted experiments in the SCS cruise and estuarine waters.This study includes these 4 aspects:1.The regulations of nutrients on size structure of phytoplankton community and zooplankton grazing activity,affect food chains and carbon cycle in the ocean.The historical data in the YS(in March and May)and SCS(in May)showed that as the water column went through low to high temperature,weak to strong stratification and high to low nutrients from the YS to the SCS,Chl-a,primary production and bacterial biomass decreased;In contrast,bacterial production,microzooplankotn grazing and its size preference increased from the YS to SCS.The increasing grazing activity and decreasing f-ratio from the YS to the SCS suggest the roles of regenerated nutrients increased in supporting the community and more important roles played by bacteria in the carbon flow in the oligotrophic SCS than in the eutrophic YS.These variabilities force the classical food chain dominated community in the eutrophic waters to the microbial loop dominated in the oligotrophic waters.As nutrients deplete,temperature and grazing activity increase from the YS to the SCS,the increasing ratio of integrated bacterial production to integrated primary production indicates communities change from autotrophy to heterotrophy and waters change from the carbon sink to carbon source.2.Phytoplankton size structure is regulated by nutrient,light and microzooplankton grazing in the surface and DCM layer water in the ocean.The experimental and historical data in the SCS showed that grazing rates were higher in the daytime than in nighttime by the DCM microzooplankton,but were lower by the surface community in daytime.In addition,microzooplankton in the surface layer preferred large phytoplankton(>5 ?m),but the DCM microzooplankton did not have such preference.The abundance of >70 ?m zooplankton was higher in nighttime than in daytime in the euphotic zone,indicating the diurnal migration of the large size zooplankton affect the small size zooplankton.The total grazing rates in the surface layer were not much different under incubation with nutrient additions,but grazing on smaller phytoplankton was promoted.The addition of nutrients caused the DCM phytoplankton assemblage to grow more rapidly and to reach a higher biomass level than the surface phytoplankton assemblage.At the same time,the DCM phytoplankton assemblage became dominated by large phytoplankton.These results suggest that the DCM layer can be a linking zone for phytoplankton to be exported to higher trophic levels in oligotrophic oceans due to not only the more rapid response of large phytoplankton to higher irradiance and pulses of nutrients,but also the low grazing activity in the DCM layer.3.This study examined the roles of bottom-up and top-town control on phytoplankton size fractions during simulated phytoplankton blooms in estuarine waters.We conducted three sets(river water,sea water and mixed water)of five-day dilution experiments to estimate the simultaneous determination of microzooplankton grazing(m),phytoplankton growth(?)and their coupling during the entire phytoplankton blooms in Pearl River Estuary.Nutrient addition increased the maximum Chl-a,lengthened the duration of phytoplankton blooms.Generally,m in all nutrient addition treatments increased during the first 2 or 3 days and then decreased.In contrast,in sea water and mixed water treatments,phytoplankton bloomed and ? increased.At the end of incubations,the previously dominated picophytoplankton and nanophytoplankton communities switched to microphytoplankton dominance.The consumption ratio of phytoplankton by microzooplankton(m/?)increased as phytoplankton specific net growth rate decreased,indicating as the growth of phytoplankton decreased during the phytoplankton bloom,the role of top-down control increased.Furthermore,in contrast to microphytoplankton and nanophytoplankton,there was a significantly negative relationship between m/? and picophytoplankton size fraction,indicating that only picophytoplankton is under severe top-down control,while bottom up control also plays an important role in regulating larger size phytoplankton.Microphytoplankton exhibited the highest growth rate,while the m/? of picophytoplankton was the highest,suggesting that the evolution of dominant sizefraction during phytoplankton blooms in eutrophic water is a result of the simultaneous effects of bottom-up(nutrients stimulation)and top-down control(microzooplankton grazing).4.Phytoplantkn intensive growth consumes dissolved inorganic carbon(DIC)and promotes high pH in the water.How phytoplankton buffers the carbon limitation during phytoplankton blooms to drive the high pH is the objective of this study.Three sets(low,moderate and high salinity waters)of five-day mesocosm incubation experiments in the Delaware Bay and Pearl River Estuary showed that during phytoplankton growth in mesocosms,pH kept increasing even when concentrations of chlorophyll a stopped increasing or even started to decrease at the end of incubation;DIC disappeared with increasing pH,but the disappeared DIC was more than the production of particulate and dissolved organic carbon(POC and DOC)could account for.This indicates that a certain amount of DIC is missing(missing DIC).Missing DIC was the largest as the maximum pH was the highest in the low salinity mesocosms.At the end of the incubation,total alkalinity(TA)largely decreased.The decrease in TA and the increase in missing DIC indicate carbonate precipitation as a result of increased pH.Because phytoplankton only directly use CO2 as the carbon source for photosynthesis,a mechanism must have existed which provides CO2 to maintain phytoplankton growth at high pH.Here we hypothesized the phytoplankton regulated carbon form switching mechanism in which high pH driven by high biomass phytoplankton growth results in carbonate precipitation,releasing CO2,and once CO2 is utilized by phytoplankton,pH continues increasing.A model based on the observed DIC,pH and TA was built to test this hypothesis.The model output agreed well with our observations,demonstrating that released CO2 maintains phytoplankton growth drives the high pH.Due to higher buffering capacity in high salinity waters,the efficiency of switch mechanism decreased from low to high salinity waters in an estuary.