Description: During the 13 day Southern Ocean Iron RE-lease Experiment (SOIREE), dissolved iron concentrations decreased rapidly following each of three iron-enrichments, but remained high (〉1 nM, up to 80% as FeII) after the fourth and final enrichment on day 8. The former trend was mainly due to dilution (spreading of iron-fertilized waters) and particle scavenging. The latter may only be explained by a joint production-maintenance mechanism; photoreduction is the only candidate process able to produce sufficiently high FeII, but as such levels persisted overnight (8 hr dark period) -ten times the half-life for this species- a maintenance mechanism (complexation of FeII) is required, and is supported by evidence of increased ligand concentrations on day 12. The source of these ligands and their affinity for FeII is not known. This retention of iron probably permitted the longevity of this bloom raising fundamental questions about iron cycling in HNLC (High Nitrate Low Chlorophyll) Polar waters.

Description: Phytoplankton community composition was assessed using chemotaxonomic pigments (including all chlorophyll types, accessory and photoprotective pigments) measured with high performance liquid chromatography (HPLC). Water samples from 7 depths spanning the euphotic zone were collected using a CTD-rosette during the five Lagrangian experimental cycles conducted during Salp Particle expOrt and Ocean Production (SalpPOOP), from October to November 2018 in the vicinity of the Chatham Rise (New Zealand). Depths varied according to experimental cycle, as it was adjusted based on PAR profiles to span the depth to 0.1% of surface irradiance. Chemotaxonomic pigments of phytoplankton in the water column were filtered onto GF/F filters (2L), then immediately flash-frozen in liquid nitrogen, and shipped frozen to Instituto Español de Oceanografía, Centro Oceanográfico de Gijón, where they were extracted for High-performance liquid chromatography (HPLC) analyses following established protocols (doi:10.4319/lom.2014.12.46). For this study, 2 profiles from each experimental cycle were analyzed, and vertically integrated to obtain areal estimates for each cycle. Pigment contributions were normalized by total chlorophyll a (Chl a) to account for potential losses and low yield, and are thus presented as integrated percentages per cycle. These proportions do not always sum to 100 because ancillary pigments/other compounds that cannot be positively identified are not included.

Description: Water samples from 7 depths spanning the euphotic zone were collected using a CTD-rosette during the five Lagrangian experimental cycles conducted during Salp Particle expOrt and Ocean Production (SalpPOOP), from October to November 2018 in the vicinity of the Chatham Rise (New Zealand). Depths varied according to experimental cycle, as it was adjusted based on photosynthetically active radiation (PAR) profiles to span the depth to 0.1% of surface irradiance. The physiology of the phytoplankton community was evaluated using the Mini-Fire fast repetition rate fluorometer (FRRF), using approximately 5ml per depth. Fv/Fm and reoxidation of Qa-, the first quinone acceptor of PSII, were evaluated using the Mini-Fire FRRF, which provided indications of phytoplankton stress most likely caused by iron-limitation following the procedure outlined in Gorbunov and Falkowski (2020 doi:10.1002/lno.11581).

Description: During the Southern Ocean iron release experiment (SOIREE) in February 1999, the composition and dynamics of the microbial food web were studied. SOIREE was a mesoscale experiment with four infusions of Fe into the patch to elevate Fe concentrations inside the patch. During the 13 d experiment, samples were collected from the mixed layer inside and outside the patch for the enumeration of bacteria, picophytoplankton, phyto and heterotrophic nanoflagellates, ciliates, and for estimation of bacterial production and microzooplankton grazing. Inside the patch, bacterial numbers remained constant throughout SOIREE although bacterial production increased three-fold. A strong relationship between the increase in bacterial and primary production suggested that dissolved organic carbon and nitrogen, rather than Fe, potentially limited bacterial growth. The picophytoplankton population, was composed solely of eukaryotic cells and increased three-fold over the first 7 d of the experiment before decreasing to initial concentrations of approximately 4000 cells/ml. In contrast to the bacterial and picophytoplankton populations, the nanophytoflagellate population increased six-fold in numbers and 23-fold in biomass. This resulted in a three-fold increase in carbon flow through the microbial food web inside the patch by the end of the experiment. The increased carbon flow resulted in a small increase in total microzooplankton biomass. Ciliate abundances tripled and biomass, doubled; however, the ciliate population only contributed 3-10% of the microzooplankton biomass, which was dominated by the heterotrophic nanoflagellate population. The heterotrophic nanoflagellate numbers decreased three-fold by the end of the experiment; however, there was no significant change in biomass throughout the experiment. The changes in the dynamics and structure of the microbial food web during the SOIREE experiment suggest that microzooplankton grazing controlled the bacterial and possibly the picophytoplankton populations. In contrast, the nanophytoflagellates were initially controlled by the Fe concentration, with microzooplankton having an impact on the population towards the end of the experiment. The addition of Fe to a small patch of the Southern Ocean had a considerable impact on the microbial components of the food web, even though the overall importance of the microbial pathways decreased as a result of Fe addition.

Description: Here we present data of phytoplankton pigment and flow cytometry (FCM) data in the water column (CTD_HPLC_FCM_May2022.xlsx) and dilution grazing experiments (DILEX_HPLC_FCM_Conc_May2022.xlsx), and derived apparent growth rates for specific pigment markers and picophytoplankton groups assessed by FCM (APPARENT_GROWTHRATES_May2022.xlsx). Samples and experiments were collected and carried out in subantarctic waters across the Campbell Plateau region southeast of New Zealand. We distinguish between a subset of 'Experimental' stations (n = 8), where serial dilution experiments were conducted and a subset of additional 'Biomass' stations (n = 7), where depth-resolved physico-chemical and biological measurements (e.g., nutrients, size-fractionated chlorophyll a (SF-Chla), optical microscopy, FCM and HPLC) were also undertaken throughout the euphotic zone. 'Experimental' stations were used to characterize phytoplankton group-specific growth and grazing rates and associated carbon production and microzooplankton consumption dynamics in surface-mixed layer waters (10 m) On-plateau (n = 5) and Off-plateau (n = 3) with 'Biomass' stations providing a broader context of the environmental conditions and phytoplankton biomass and taxonomic composition across the region.