Effects Of Irradiance, Nitrate And Phosphate On Growth Of Phytoplankton In Changjiang Estuary

There were many studying papers about effects of environmental factor on phytoplankton growth,but basically lacked for the contents about growth of picophytoplankton and heterotrophic bacteria,especially those papers discussing the interaction of irradiance and nutrients were seldom,furthermore,the conclusions were drawn mostly by the way of field observation of algae distribution or mathematical modal,and the in situ physiological and ecological incubations were scarce.This paper was based on field observation and in situ incubation experiments controlled by coupling between different phosphate or nitrate concentrations and different levels of irradiances in Changjiang Estuary and its adjacent sea in summer and autumn of 2005 to 2007,and variations of chlorophyll a concentration,nutrients uptake,growth of picophytoplankton and heterotrophic bacteria were studied,the results were as follows:1.In summer the concentrations of chlorophyll a were basically less than 2μg/l in Changjiang estuary,and two high values of chlorophyll a lied in south and north of 122-123°E sea areas,which could reach 11.9μg/l;in autumn,due to the limitation of surface water temperature and light intensity,the concentrations of chlorophyll a offshore was low,same with that inshore,which were mostly less than 2μg/l.2.The addition of phosphate could accelerate the uptake of phosphate,nitrate, nitrite and silicate obviously under high irradiance in the station A in summer,but the addition of nitrate would restrain the uptake of nutrients and increase of chlorophyll a and pH value;The phytoplankton in statjion A had a long time of slow-growth in autumn,which was different from summer,then exponential growth rapidly,and changes of nutrients uptak was the same;The phytoplankton in the station B in summer has a marked scarceness of phosphate,and the addition of phosphate could have much more obvious acceleration to nutrient uptake or increase of chlorophyll a, but the restraining of nitrate addition was not happened;In a word,phytoplankton firstly assimilated NO₃-N and NO₂-N,not NH₄-N,and phytoplankton growth in summer was P-limited which was due to great consumption of phosphate by algae self, and N-limitation was not existed;Nutrients limitation of phytoplankton was not obvious,it was light or temperature limitation in autumn.3.The low irradiance could make nutrients uptake and increase of chlorophyll a and pH value slow,but picophytoplankton growth in summer was not slowed down under low irradiance,and the peak value was much less than that under high irradiance;In autumn the peak of picophytoplakton under low irradiance was close to or little higher than that under high irradiance;There was a relation of competition between phytoplankton and heterotrophic bacteria in the station A and B in summer, but the relation between the two planktons was interdependent in autumn.4.In the dark the phosphate and ammonium were released in the station A and B in summer,and the nitrite of the station A was also increased,however,the ammonium of sampling location of A was not increased in autumn;The biomass of phytoplankton and heterotrophic bacteria were decreased,and the addition of phosphate or nitrate didn't affect the release rates of nutrients with linear increase and the growth rates of phytoplankton with exponential decrease,which showed irradiance was a significant factor to phytoplankton growth,only it was abundant, could phytoplankton growth be accelerated by the addition of nitrate or phosphate.; The heterotrophic bacteria density was still increased in the dark in autumn,but the extent of increase was much little than that under high or low irradiance.5.Carbon contribution of Nano+Micro in the first time of incubation in the station A were 20%,45%,respectively in summer and autumn,and the value could reach more than 90%when chlorophyll a concentration reached the peak under whatever high or low irradiance,however,at the end of incubation although carbon contribution of Nano+Micro in summer was biggest,but the dominant character was less obvious,carbon contribution of Bact was biggest in the dark both in summer and autumn;In the station B carbon contribution of Bact,Euk and Nano+Micro were all about 30%in the first time of incubation,and carbon contribution of Euk and Nano+Micro in groups of P addition were both increased when chlorophyll a concentration reached the peak under high irradiance,and that in groups of N addition was not basically changed because the time of chlorophyll a peak was the first day of incubation,however,under low irradiance carbon contribution of Nano+Micro could reach more than 60%,but that of Euk was decreased;At the end of incubation carbon contribution of heterotrophic bacteria in the station B was biggest whatever under high,low irradiance or in the dark which was different from that of the station A.