Phytoplankton's role in the biological pump during the growth season across the Atlantic Southern Ocean

Flynn, Raquel

Phytoplankton's role in the biological pump during the growth season across the Atlantic Southern Ocean

Thesis / Dissertation

2023

Abstract

Southern Ocean phytoplankton growth is seasonally co-limited by light and iron availability. In winter, iron and macronutrients such as nitrate are supplied to surface waters during deep mixing events. Sea ice melt and increased solar radiation in spring drive rapid stratification of surface waters, alleviating phytoplankton from light limitation and allowing them to consume the nutrients supplied in winter. In the framework of the “new production paradigm”, phytoplankton growth fuelled by upwelled nitrate (“new production”) can be equated to atmospheric CO2 removal, while growth fuelled by ammonium and urea (“regenerated production”) results in no net CO2 drawdown. As such, once phytoplankton begin assimilating the nitrate supplied from the subsurface, the upper Southern Ocean ecosystem starts removing atmospheric CO2. As the spring/summer growth season progresses, mixed-layer iron concentrations decline, leading to phytoplankton growth being predominantly fuelled by regenerated nutrients, with a concomitant decrease in CO2 removal. The role of phytoplankton in the Southern Ocean's CO2 sink remains poorly understood, particularly early in the growth season (i.e., spring) and in regions where thick sea-ice conditions persist year-round (e.g., Larsen C Ice Shelf; LCIS). For the research described in this thesis, field sampling was undertaken near the LCIS during summer 2018/2019 and across the Atlantic Southern Ocean in spring 2019. Rates of net primary production (NPP) and nitrogen (N; as nitrate, ammonium, and urea) uptake were measured in both seasons to determine the dominant N source supporting phytoplankton growth and to quantify carbon export potential. In spring, coincident rates of iron uptake were also measured, allowing for an assessment of the iron requirements of different phytoplankton size classes. In contrast to expectations that large diatoms dominate the open Southern Ocean spring bloom, the biomass and rates of NPP and N uptake were dominated by nanoplankton (2.7-20 µm), particularly the diatom Chaetoceros spp., which employs a “boom-and-bust” life-cycle. It appears that Chaetoceros were able to grow so rapidly because of their low iron and light requirements, as well as their ability to form long spiney chains that reduce grazing pressure. The Chaetoceros bloom was relatively short-lived (a few weeks) and its senescence, driven by iron limitation and increased grazing pressure, drove a massive export event that accounted for roughly a third of the carbon exported over the entire spring/summer growth season. In contrast to spring, the late-summer phytoplankton community in the Weddell Sea relied strongly on regenerated N, with ammonium and urea consumption fuelling 53 ± 8% of their growth at LCIS. These high rates of regenerated production coincided with an elevated ammonium supply rather than low iron availability as sea-ice melt led to non-limiting iron concentrations in surface waters. The high ammonium concentrations partially inhibited nitrate uptake and thus, biological carbon export, particularly near LCIS. Here, the phytoplankton community shifted from Phaeocystis Antarctica- to diatom-dominated as the growth season progressed, coincident with an increase in upper water-column stratification. This shift may have further decreased carbon export as P. Antarctica can fix up to 50% more carbon per mole of phosphate consumed than diatoms. To further investigate the role of small (<20 µm) phytoplankton in nutrient and carbon cycling in the summertime Weddell Sea, the 15N/14N of four groups sorted by flow cytometry (Synechococcus, picoeukaryotes, nanoeukaryotes, and cryptophytes) was measured. Phytoplankton growth fuelled by subsurface nitrate produces particulate organic N (PON) that is high in 15N/14N relative to growth fuelled by ammonium. Using biomass 15N/14N as a measure of new- versus regenerated N dependence, the contribution of each phytoplankton group to carbon export potential can be estimated. Synechococcus generally relied on nitrate less than the other phytoplankton groups although interestingly, its nitrate dependence was highest at LCIS and increased with increasing seawater temperatures and ammonium concentrations. Conversely, the high LCIS ammonium concentrations may have partially inhibited nitrate consumption by the picoeukaryotes, nanoeukaryotes, and cryptophytes. These groups relied most heavily on nitrate at the Antarctic Peninsula and in the Weddell Gyre even though total productivity was significantly lower in these regions than at LCIS. The flow cytometry-15N/14N data yield a higher (although still overlapping) mean estimate of new- relative to total production (i.e., the proportion of exportable carbon) than the 15N-tracer-derived uptake rates (56 ± 18% versus 47 ± 15% of productivity fuelled by nitrate). This discrepancy is likely due to the different time-scales captured by the two methods, with PON isotopes integrating over days to weeks and the rate experiments quantifying N uptake at the time of sampling only. Additionally, flow cytometric sorting may have excluded certain phytoplankton groups that relied more heavily on regenerated N (e.g., P. Antarctica). It is thus important that the results of either method be interpreted in the appropriate context. The work presented in this thesis shows how seasonal shifts in the phytoplankton community drive changes in biological carbon export in the Southern Ocean. In contrast to previous suggestions, the novel measurements described herein indicate that although summer is the period of maximum biomass accumulation in the Southern Ocean, it may not be the season of highest carbon export. This finding can be explained by the increasing reliance of phytoplankton on regenerated N as the season progresses. In ice-adjacent waters, this shift is largely due to elevated ammonium availability rather than iron limitation, which may (partially) inhibit nitrate uptake. By contrast, in the open Southern Ocean, iron limitation drives the seasonal shift towards regenerated production because of the high iron requirement of nitrate uptake. The work detailed in this thesis also indicates that different phytoplankton groups are better adapted to different physicochemical conditions (e.g., P. Antarctica to deep mixed-layers versus diatoms to recently-stratified waters), with the dominance of one group over another acting to strengthen or weaken the Southern Ocean's biological pump. Understanding the drivers of phytoplankton community composition is essential if we are to predict how phytoplankton will respond to a changing climate, and the implications of their response for the Southern Ocean's biological pump.

Keywords

Oceanography

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PhD / Doctoral

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