Brief description
Southern Ocean phytoplankton growth is limited by low iron (Fe) supply and irradiance, impacting the strength of the biological carbon pump. Unfavourable upper ocean conditions such as low nutrient concentrations can lead to the formation of deep chlorophyll or biomass maxima (DCM/DBM). While common in the Southern Ocean, these features remain under-studied due to their subsurface location. To increase our understanding of their occurrence, we studied the responses of phytoplankton communities from a Southern Ocean DCM to increasing light, Fe, and manganese (Mn) levels. The DCM communities were light- and Fe-limited, but light limitation did not increase phytoplankton Fe requirements. The greatest physiological responses were observed under combined Fe/light additions, which stimulated macronutrient drawdown, biomass production and the growth of large diatoms. Combined Mn/light additions induced subtle changes in Fe uptake rates and community composition, suggesting species-specific Mn requirements. These results provide valuable information on Southern Ocean DCM phytoplankton physiology.Lineage
Maintenance and Update Frequency: notPlanned
Statement: We performed a field bioassay experiment onboard RV Investigator in polar waters of the Southern Ocean (55.47°S, 138.34°E) to study the response of phytoplankton communities from a deep chlorophyll maximum (DCM) to changes in iron (Fe), manganese (Mn) and light conditions. Unfiltered trace metal clean seawater was collected using a trace metal rosette and incubated with Fe and/or Mn additions in deck-board incubators. Neutral-density mesh bags were used to alter the light conditions. Two light treatments were used: a low light treatment (1% incident irradiance) to reproduce the light conditions at the DCM (located at 87m) and a high light treatment (12.4% of incident irradiance) to stimulate shoaling up to 40m. Incubations bottles were sampled at the end of the experiment (day = 10) for multiple parameters: chlorophyll-a/particulate organic carbon/biogenic silica concentrations. Macronutrient concentrations were measured onboard through segmented flow analysis. Photophysiology was studied using a Light-induced Fluorescence Transients Fast Repetition Rate (LIFT-FRR) fluorometer. Flow cytometry samples (for community composition) were fixed at sea and analysed back onshore using an Aurora Cytek flow cytometer (Cytek Biosciences). In addition, carbon and iron uptake rates were measured through radioisotopes additions: after 10 days of incubation, 300 ml of the incubated water was dispensed into smaller bottles and spiked with 14-carbon (NaH14CO3; specific activity 1.85 GBq mmol-1; PerkinElmer, USA) and 55-iron (55FeCl3 in 0.1 M Ultrapure HCl; specific activity 30 MBq mmol-1; PerkinElmer) solutions. The spiked bottles were incubated for 24h under initial conditions before sequential filtration though 20, 2 and 0.2 µm polycarbonate filters. Iron and carbon uptake rates were determined by measuring disintegrations per minute (DPM) on a liquid scintillation counter (PerkinElmer Tri-Carb 2910 TR) after incubation of the filters in Ultima Gold liquid scintillation cocktail at least 24h prior to analysis (PerkinElmer).
To study the effect of metal additions and light on the tested response variables, linear mixed effect models were performed using R. Models were fit with the ‘lme’ function of the ‘lme4’ package, using maximum likelihood. The variability between replicates (or bottle effect) was included as a random effect in all analyses. Using the ‘drop1’ function with a Chi-squared test (null hypothesis of independence), the best model fit was selected by sequentially eliminating variables among the fixed effects: Mn, Fe, light and in some cases size class (Fe/carbon uptake) or gated population (flow cytometry). To normalize model residuals, data were log10 transformed before analyses. Iron and Mn addition treatments were treated as two separate factors, each possessing two levels (‘True’ for addition or ‘False’ for no addition). When treatment effects were suggested by the model fit (with more than two factor levels), we performed pairwise comparisons using the ‘emmeans’ package with the Tukey method.
Notes
CreditWe would like to acknowledge the officers and crew of the RV Investigator (CSIRO Australian Marine National Facility) for the deployment of all the instruments during the SOLACE voyage (IN2020_V08), and the hydro-chemistry team who performed the macronutrient analyses onboard. We acknowledge Terry L. Pinfold for his help with the flow cytometry. This work was funded by the Australian Antarctic Program Partnership (AAPP; ASCI000002) and through the Antarctic Climate & Ecosystems Cooperative Research Centre (ACE CRC).
Credit
We acknowledge the use of the CSIRO Marine National Facility (https://ror.org/01mae9353 ) in undertaking this research.
We acknowledge the use of the CSIRO Marine National Facility (https://ror.org/01mae9353 ) in undertaking this research.
Issued: 11 10 2023
Data time period: 2020-12-29 to 2021-06-15
Subjects
AQUATIC ECOSYSTEMS |
BIOGEOCHEMICAL CYCLES |
BIOSPHERE |
Biogenic silica |
Carbon uptake rates |
Concentration of chlorophyll-a per unit volume of the water body |
Concentration of nitrate and nitrite {NO3 and NO2} per unit volume of the water body |
Concentration of phosphate {PO4} per unit volume of the water body |
Concentration of silicate {SiO4} per unit volume of the water body |
EARTH SCIENCE |
ECOLOGICAL DYNAMICS |
ECOSYSTEM FUNCTIONS |
Flow cytometry |
Functional absorption cross-section of Photosystem II |
Global / Oceans | Global / Oceans | Southern Ocean |
Iron uptake rates |
PHYTOPLANKTON |
PLANKTON |
Particulate organic carbon |
Photochemical efficiency of Photosystem II |
biota |
deep chlorophyll maximum |
iron |
light |
manganese |
photophysiology |
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Other Information
(DATA ACCESS - browse and download available files)
uri :
https://data.imas.utas.edu.au/attachments/93561671-c14f-4207-92e1-3b24d419855b/
(SUPPLEMENTARY - Rmarkdown code and results [PDF])
Identifiers
- DOI : 10.25959/6S84-RM14
- global : 93561671-c14f-4207-92e1-3b24d419855b
