Seasonality of phytoplankton growth limitation by iron and manganese in subantarctic waters

Brief description

This record describes data products produced by Latour et al. (2023) from Marine National Facility data collected on board RV Investigator. The data products were used in research looking at the seasonality of phytoplankton growth limitation by iron and manganese in subantarctic waters.

Phytoplankton indirectly influence climate through their role in the ocean biological carbon pump. In the Southern Ocean, the subantarctic zone represents an important carbon sink, yet variables limiting phytoplankton growth are not fully constrained. Using three shipboard bioassay experiments on three separate voyages, Latour et al. evaluated the seasonality of iron (Fe) and manganese (Mn) co-limitation of subantarctic phytoplankton growth south of Tasmania, Australia. They observed a strong seasonal variation in a phytoplankton Fe limitation signal, with a summer experiment showing the greatest response to Fe additions. An autumn experiment suggested that other factors co-limited phytoplankton growth, likely low silicic acid concentrations. The phytoplankton responses to Mn additions were subtle and readily masked by the responses to Fe. Using flow cytometry, they observed that Mn may influence the growth of some small phytoplankton taxa in late summer/autumn, when they represent an important part of the phytoplankton community. In addition, Mn induced changes in the bulk photophysiology signal of the spring community. These results suggest that the importance of Mn may vary seasonally, and that its control on phytoplankton growth may be associated with specific taxa.

For more details please refer to the cited article (download link provided below): Latour, P, Strzepek, RF, Wuttig, K, van der Merwe, P, Eggins, S, Bach, LT, Boyd, PW, Ellwood, MJ, Pinfold, TL, Bowie, AR. 2022. Seasonality of phytoplankton growth limitation by iron and manganese in subantarctic waters. Earth and Space Science Open Archive [Preprint]. DOI: 10.1002/essoar.10511502.1

Lineage

Researchers performed three field bioassays experiments onboard RV Investigator, in the subantarctic zone, to study iron (Fe) and manganese (Mn) co-limitation of phytoplankton growth.

The first experiment (spring) was performed at Process Station 2 (45.44°S, 153.31°) during IN2018_V04 and the two other experiments (summer and autumn) were conducted at the Southern Ocean Time Series (SOTS) station (46.80°S, 141.884°E) during IN2019_V02 (autumn) and IN2020_V08 (summer).

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 reproduce the light penetrating the surface ocean. Incubations bottles were sampled at the end of the experiment (day = 7) for macronutrient concentrations, measured through segmented flow analysis, photophysiology, measured using Light-induced Fluorescence Transients Fast Repetition Rate (LIFT-FRR) fluorometer and community composition using flow cytometry. Flow cytometry samples 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 7 days of incubation, a portion 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.

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).