Abstract
Bottle assays and large-scale fertilization experiments have demonstrated that, in the Southern Ocean, iron often controls the biomass and the biodiversity of primary producers. To grow, phytoplankton need numerous other trace metals (micronutrients) required for the activity of key enzymes and other intracellular functions. However, little is known of the potential these other trace elements have to limit the growth of phytoplankton in the Southern Ocean. This study, investigates whether micronutrients other than iron (Zn, Co, Cu, Cd, Ni) need to be considered as parameters for controlling the phytoplankton growth from the Australian Subantarctic to the Polar Frontal Zones during the austral summer 2007. Analysis of nutrient disappearance ratios, suggested differential zones in phytoplankton growth control in the study region with a most intense phytoplankton growth limitation between 49 and 50°S. Comparison of micronutrient disappearance ratios, metal distribution, and biomarker pigments used to identify dominating phytoplankton groups, demonstrated that a complex interaction between Fe, Zn, and Co might exist in the study region. Although iron remains the pivotal micronutrient for phytoplankton growth and community structure, Zn and Co are also important for the nutrition and the growth of most of the dominating phytoplankton groups in the Subantarctic Zone region. Understanding of the parameters controlling phytoplankton is paramount, as it affects the functioning of the Southern Ocean, its marine resources and ultimately the global carbon cycle.
Highlights
The circumpolar Subantarctic Zone (SAZ) is an important biome of the global ocean which separates the High Nutrient Low Chlorophyll (HNLC) Southern Ocean from the mostly Low Nutrient Low Chlorophyll (LNLC) subtropical water from the Indian, Pacific, and Atlantic Oceans (Bowie et al, 2011a)
Previous studies from the SAZ-Sense project have demonstrated that the E-SAZ, despite greater chlorophyll a (Chla) was less productive than the W-SAZ at the time of observation (Cavagna et al, 2011; Westwood et al, 2011; Table A1 in Appendix)
Distribution of phytoplankton size classes and dominant groups accorded with microscopic observations, pigment size class, and CHEMTAX calculations done during the SAZ-Sense voyage
Summary
The circumpolar Subantarctic Zone (SAZ) is an important biome of the global ocean which separates the High Nutrient Low Chlorophyll (HNLC) Southern Ocean from the mostly Low Nutrient Low Chlorophyll (LNLC) subtropical water from the Indian, Pacific, and Atlantic Oceans (Bowie et al, 2011a). The SAZ region forms a “belt” of important carbon sequestration accompanied by a phytoplankton biomass characterized by a low seasonality (Banse, 1996; Metzl et al, 1999; McNeil et al, 2001) It is still unclear what parameters mostly constrain phytoplankton growth and how the dynamics of the SAZ region will be affected in the future. The recent SAZ-Sense project took advantage of the natural variability in the SAZ region around Tasmania to study the impact of predicted future changes that will affect its functioning. This project focused on parameters likely to control phytoplankton biomass and carbon fixation, such as macro- (N, P, and Si) and micronutrients (essential trace metals), grazing and cell lysis promoted by the microbial loop, and as well, carbon export. The scientific rationale behind this study is summarized in Bowie et al (2011a) and most of the results have been published in a special issue in Deep-Sea Research II (Table A1 in Appendix)
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