Photosynthetic response of Southern Ocean phytoplankton under iron and light limitations : bioassay experiments

[摘要] ENGLISH ABSTRACT: The Southern Ocean (SO) is of significant interest in the understanding of the global carboncycle and therefore many studies have been conducted to determine the limiting factorscontrolling the biological pump within the region. During photosynthesis phytoplankton requirevarious nutrients such as NO3, PO4, inorganic carbon and the micronutrient Fe. The SO is aHigh-Nutrient Low-Chlorophyll region, therefore no macronutrient limitation is experienced byresident phytoplankton but instead the micronutrient Fe is a significant limiting factor withinthese waters due to limited inputs. Due to deep mixed layer depths, ice cover, low sun anglesand cloud cover throughout parts of the year, light is also considered a limiting factor in theSO. Fe and light limitation cause a decrease in photosynthetic efficiency and therefore adecrease in carbon fixation capabilities. During this study we conducted five bioassayshipboard incubation experiments during two cruises along the Greenwich meridian betweenSouth Africa and the ice edge, SOSCEx during March and SAFePool during January toFebruary, in which we varied Fe concentrations and light levels to determine the effects of Feand light limitation or co-limitation within resident phytoplankton. Spatial and temporalvariations in phytoplankton response were studied to determine varying effects of limitationacross water masses and different stages of bloom decline within the study area. Thecombined addition of Fe and light gave the largest increase in biomass, photosyntheticcapacity and nutrient uptake. In support of the hypotheses tested changes in thephotosynthetic apparatus led to changes in the photosynthetic efficiency and growth of the SOphytoplankton, as a result of variations in Fe and light availability. Variability was also observedin the response of phytoplankton to Fe and light amendments due to spatial and temporalvariation in resident phytoplankton communities. It was therefore concluded that both Fe andlight are significant controls in the resident phytoplankton photosynthetic apparatus,photosynthetic capabilities, organic carbon fixation and therefore the biogeochemical cycleswithin the Atlantic sector of the SO.

[发布机构] Stellenbosch University