Temperature Effects On The Nitrogen-to-phosphorus Stoichiometry Of Phytoplankton And Droop Equation

IPCC predicted that the sea surface temperature is expected to increase by 3-5?by the end of the 21th century,thus the study of the impacts of warmer ocean on the growth of phytoplankton is critical to a better understanding of ecosystem functions of the future ocean.We proposed that increasing temperature has different effects on the growth of phytoplankton between the nitrogen-limited condition and the phosphorus-limited condition.In this study,we conducted semi-continuous experiments on three typical species Emiliania huxleyi,Phaeodactylum tricornutum and Platymonas subcordiformis.The results showed:(1)The chlorophyll a content per cell of all species positively correlated with temperature under both nitrogen-limited condition and phosphorus-limited condition,indicating the nutrient-independent effect of temperature on chlorophyll synthesis then potentially phytoplankton can have higher chlorophyll a to higher temperature.The cell size of Platymonas subcordiformis decreased with temperature,but increased with the growth rate.Emiliania huxleyi had the largest cell size at 20?,which is the optimum temperature for its growth.The cell size of Phaeodactylum tricornutum Bohlin declined with the growth rate,possibly due to the lowest optimum temperature.(2)The Droop equation was used to describe the effect of limiting nutrient concentration on the growth rate,the coefficients of equation(?max and Qmin)for different temperature treatments,different limiting nutrient and different species were obtained using R software.The results showed significant temperature effect on ?max and Qmin:a)The Qmin(N)of Platymonas subcordiformis under the nitrogen-limited condition increased with temperature;the Qmin(P)was the lowest at 15? under the phosphorus-limited condition,above 20 ?,the Qmin(P)increased with temperature.?max increased with temperature under both the nitrogen-limited condition and the phosphorus-limited condition,and this pattern was not affected by the ambient nutrient concentration.b)Under the nitrogen-limited condition,the Qmin(N)of Emiliania huxleyi increased with temperature,while the Qmin(P)did not show any correlation with temperature under the phosphorus-limited condition.?max increased with temperature under both the nitrogen-limited condition and the phosphorus-limited condition.c)The Qmin(P)of Phaeodactylum tricornutum under the nitrogen-limited condition increased initially and then decreased with temperature,the Qmin(N)is the opposite.?max increased with temperature under both the nitrogen-limited condition and the phosphorus-limited condition.(3)The changing temperature has profound effect on the optimal cellular nitrogen to phosphorus ratios.The Qmin(N)/Qmin(P)of all species fell between 10 and 50.The Qmin(N)/Qmin(P)of Platymonas subcordiformis and Emiliania huxleyi increased with temperature,but the ratio was lowest at 20? to Phaeodactylum tricornutum.The different nitrogen to phosphorus ratios can be interpreted as the adaptation to the changing environment.We predicted that the Refield ratio in the future warmer ocean will change with temperature.