CO_2浓度升高对浮游藻类元素化学计量值的影响

The effect of CO_2 enrichment on elemental stoichiometry of phytoplankton under different trophic status

通过梅梁湾和东太湖的四季原位实验,研究CO_2浓度升高对不同营养水平淡水生态系统中浮游藻类C、N、P元素计量值的影响.实验设置了270、380和750 ppm共3个CO_2浓度,分别代表工业革命前、当前和IPCC预测的21世纪末的CO_2浓度.结果表明梅梁湾水体营养盐浓度在四季均高于东太湖水体营养盐浓度,但梅梁湾原位实验中浮游藻类C、N、P含量却普遍低于东太湖原位实验中浮游藻类营养元素含量,并且前者在季节上变化更大.CO_2浓度升高使梅梁湾原位实验中浮游藻类C∶P比明显增加,N∶P比略有增加,这种增加归因于藻细胞内C、N含量的升高,而东太湖浮游藻类化学计量值对CO_2浓度变化的响应不显著.因此浮游藻类元素化学计量值对CO_2浓度变化的响应程度与水体营养盐的绝对浓度无关,而与浮游藻类的生长是否受营养盐限制有关,只有当藻类生长受到水体营养盐浓度限制时,CO_2浓度升高才会显著改变其元素组成.

Seasonal in situ microcosm experiments were carried out in Lake Taihu to assess the seasonal effects of CO2 on the carbon：nutrient stoichiometry of phytoplankton in freshwater ecosystems with different trophic status. The experimental treatments were three concentrations of CO2, 270, 380 and 750 ppm, which represent the level for preindustrial, present and the prediction for the end of this century, respectively. Although nutrient concentrations were higher in the Meiliang Bay than those in the East Taihu, the later one has generally higher major nutrient contents of phytoplankton. Meanwhile, the nutrient contents of phytoplankton showed seasonal variation in the Meiliang Bay, while relatively stable in the East Taihu. The rise of CO2 could largely increase the C：P ratio of phytoplankton and slightly increase the N：P ratio of phytoplankton of microcosms in the Meiliang Bay, but did not have significant impact on those two stoichiometric values in the microcosms of the East Taihu. The rise of elemental ratios were due to the increase of C and N content in phytoplankton under high CO2 treatments. Rising CO2 concentration is likely to have a more significant impact on particulate elemental ratios if phytoplankton is in nutrients-limited status, regardless of the nutrient concentrations of surrounding waters.